Magnetic Resonance Imaging of the Myocardium, Coronary Arteries, and Anomalous Origin of Coronary Arteries

Cheong, Benjamin Y. C.; Angelini, Paolo

doi:10.1007/978-1-4471-2828-1_13

Benjamin Y. C. Cheong MD, FRCP(Edin)^4,5 &
Paolo Angelini MD, FSCAI^6,7,8

Part of the book series: Cardiovascular Medicine ((CVM))

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4 Citations

Abstract

Over the past decade, cardiovascular magnetic resonance imaging (CMRI) has emerged as an important, non-invasive imaging modality. In certain aspects, CMRI has become the gold standard in the assessment of ischemic and nonischemic cardiomyopathy. It has relatively high spatial and temporal resolution, and it is also reproducible and does not involve ionizing radiation. Therefore, it is an ideal modality for long-term imaging follow-up. Cardiovascular magnetic resonance imaging can assess ventricular function, myocardial ischemia, and viability in a single setting. There is also a growing interest in tissue T₁ and T₂ imaging, which can be used to characterize tissues with various disease pathologies.

Although coronary multi-detector computed tomographic angiography is now widely used to assess the coronary arteries, magnetic resonance imaging coronary angiography still has an important role in the evaluation of suspected anomalous coronary artery origin, especially in the younger population.

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Magnetic Resonance Imaging of the Myocardium, Coronary Arteries, and Anomalous Origin of Coronary Arteries

Myocardial Perfusion and Late Gadolinium Enhancement Imaging in Cardiovascular Magnetic Resonance to Assess Coronary Artery Disease

Cardiovascular Magnetic Resonance Imaging of Coronary Arteries

Keywords

Magnetic Resonance Imaging of the Myocardium

Over the past two decades, significant progress has been made in magnetic resonance imaging (MRI). Because of advances in hardware and software, MRI has become an important diagnostic tool in everyday clinical practice. This modality is unique because it generates native, pathology-specific images that reflect the composition of tissues. Compared to other imaging modalities, MRI has important advantages: it is noninvasive, requires no ionizing radiation, relies on no geometric assumptions, and can depict any part of the human body in any plane. In addition, MRI has high spatial and temporal resolution, high contrast, and a high signal-to-noise ratio. Under certain conditions, it can detect disease even without use of a contrast agent.

Magnetic resonance imaging can be used to diagnose diseases in almost any organ. For example, MRI can evaluate anatomy and locate tumors in different tissue systems and can also assess organ function (i.e., left ventricular [LV] systolic and diastolic function [1–3]). In evaluating brain function [4], MRI can determine which segment of the human brain is responsible for thought, speech, movement, and sensation. Moreover, MRI angiography can show an artery’s course, size, and acute pathologic characteristics and can determine whether a stenosis is present with or without contrast [5–7]. By assessing perfusion, MRI can determine the significance of an underlying stenosis in coronary artery [8] and neurovascular diseases and can provide absolute quantification of blood flow [9]. Diffusion-weighted MRI can show whether a tumor is benign versus malignant and can monitor the progress of treatment [10]. In addition, magnetic resonance spectroscopy [11] can evaluate myocardial metabolism.

A number of clinicians use MRI not only as a diagnostic modality but also to provide treatment. For instance, interventional MRI enables physicians to localize a tissue site for targeted treatment and to evaluate the treatment result in the same session, as in closure of atrial septal defects [12]. In the same manner, a recently introduced method—MRI-guided, high-intensity focused ultrasound—facilitates noninvasive tumor embolization therapy of uterine fibroids [13].

Historical Perspective

The Fourier transform, named after the nineteenth-century French mathematician Jean Baptiste Joseph Fourier, is a mathematical function that was initially applied to heat transfer and vibrations [14]. In MRI, the Fourier transform allows an image to be reconstructed from a set of encoded MRI signals; it changes these signals from a frequency/time domain to a frequency/amplitude domain. In the early twentieth century, the Larmor equation, derived by Sir Joseph Larmor, stated that the precession frequency (ω) is equal to the gyromagnetic ratio (γ) multiplied by the magnetic field strength (B₀). In 1946, Edward Purcell and Felix Bloch independently described an interesting phenomenon: when certain nuclei are placed in a magnetic field, they absorb energy in the radiofrequency (RF) range of the electromagnetic spectrum, and that energy is re-emitted when the nuclei relax back into their baseline state. Both of these scientists were awarded the Nobel Prize in Physics in 1952, and the phenomenon was termed nuclear magnetic resonance (NMR). In the early 1970s, Dr. Raymond Damadian noted that the relaxation times of tumor tissues were longer than those of normal tissues. In 1973, Professor Paul Lauterbur published the first magnetic resonance image; it showed a 4.2-mm-diameter test tube that contained two water-filled capillary tubes. The image was produced by joining a weak gradient magnetic field with a stronger main magnetic field for spatial localization of the capillary tubes. That same year, Sir Peter Mansfield also described how NMR signals could be mathematically analyzed and, subsequently, how MR gradients could be used to acquire spatial information. The name NMR was subsequently changed to MRI to avoid the word “nuclear” [14]. In 2003, the Nobel Prize in Physiology or Medicine was awarded to both Paul Lauterbur and Peter Mansfield.

During the brief history of MRI, substantial progress has been made in this field. In 1981, it took 150 s to obtain a single cardiac image measuring 4 × 4 × 10 mm. In contrast, as of 2013, 25-phase cine MRI using steady-state free precession (SSFP) with parallel imaging can be performed in only three to four heartbeats (approximately 3–4 s in a patient with a heart rate of 60 beats/min), with an average temporal resolution of 40 ms and a spatial resolution of 1.8 × 1.8 × 8 mm in any commercially available 1.5 Tesla (T) scanner.

Basic Principles

The following is a brief description of how an MRI image is formed. For a comprehensive review of MR physics, readers are referred to the many excellent available review articles [15–19]. A large proportion of the human body consists of fat and water. In fact, 2/3rds of the human body’s weight is due to water. Therefore, hydrogen is the most abundant atom in the human body. Unlike the other elements, hydrogen has only a single proton in its nucleus. The hydrogen proton behaves like a tiny magnet, spinning on its axis, with a north–south pole. Magnetic resonance imaging makes use of this unique property of hydrogen. When placed in a steady magnetic field, these protons will become magnetized, align with the main magnetic field (either parallel or anti-parallel), and precess along the axis of the field according to the Larmor frequency. When energy is added, in the form of a RF pulse oriented perpendicularly to the main magnetic field, the magnetic vector will be deflected, tipping the net magnetization to the transverse plane. When the RF pulse is switched off, magnetization will return to its initial direction via a process called relaxation, and this will cause a signal or current to be induced in the MRI receiver coil per Faraday’s Law. Images can then be created from these signals. After the RF pulse is switched off, magnetization will return to the longitudinal orientation; this phenomenon is known as T₁ or spin–lattice relaxation. The rate of relaxation is different for each tissue, and the T₁ time constant is defined as the time required for longitudinal relaxation to return to 63 % of its original value. When the RF pulse is switched off, transverse magnetization is at its maximum, and the protons are all in phase. However, immediately after the RF pulse is switched off, proton dephasing will begin; this is known as T₂ relaxation or spin-spin relaxation. The T₂ time constant is defined as the time it takes for transverse magnetization to decay to 37 % of its original value.

Because each tissue type has its own inherent T₁ and T₂ relaxation properties, each tissue has a different appearance on MRI. To generate MRI images, protons in the body must be localized to specific parts of an image by means of magnetic field gradients. The resulting signal intensity is subsequently plotted on a gray scale, and the cross-sectional images are reconstructed to allow clinician interpretation.

Indications

With improvements in coil design, magnetic gradients, and software, as well as the increased use of high-field MRI scanners, cardiovascular MRI (CMRI) has emerged as the gold standard for evaluating ischemic and nonischemic cardiomyopathy [1]. Surface echocardiography is noninvasive, relatively inexpensive, portable, and patient friendly, so it remains the first-line imaging modality for assessing cardiac function. However, by using different imaging sequences, CMRI can provide a comprehensive evaluation of the cardiovascular system. Cardiovascular MRI can be used to assess systolic and diastolic function, detect valvular stenosis and quantitate regurgitation, and measure myocardial iron levels. In addition, it can evaluate underlying coronary artery disease (CAD) by means of a vasodilator stress perfusion study or a dobutamine wall-motion assessment to determine any underlying ischemic burden or visualize the coronary arteries to reveal stenosis. Furthermore, nonischemic and infiltrative cardiomyopathies, such as arrhythmogenic right ventricular (RV) dysplasia/cardiomyopathy (ARVD/C), myocardial sarcoidosis, and amyloidosis, can be detected by CMRI. Myocardial viability can be determined by using either delayed-enhancement (DE) MRI or low-dose (LD) dobutamine CMRI to measure the recovery of myocardial contractility. Moreover, in appropriately selected patients, CMRI can provide high-quality imaging of structures outside the heart, including the entire thoracoabdominal aorta, the cerebral circulation, the pulmonary veins, and the peripheral vascular tree. Cardiovascular MRI can be performed with or without gadolinium (Gd) contrast agents, thereby avoiding the radiation and potential nephrotoxicity caused by the iodinated contrast agents used in computed tomography (CT) [20, 21]. To help guide clinicians in the use of CMRI, appropriateness criteria and expert consensus reports have been published by several professional societies [2, 3]. Table 13.1 lists the appropriate indications for CMRI [2, 3].

Table 13.1 Appropriate indications for cardiovascular magnetic resonance imaging

Full size table

Cardiomyopathies

Cardiomyopathies are defined as diseases of the myocardium associated with cardiac dysfunction. This family of diseases can be subdivided into ischemic cardiomyopathy (due to CAD) and nonischemic cardiomyopathy (not due to CAD). Cardiomyopathies were originally classified by the World Health Organization in 1980; this classification system was then updated in 1996 [22]. A newer classification scheme has also been recently introduced by the European Society of Cardiology [23]. When CAD, hypertension, congenital heart disease, and significant underlying valvular disease are not the cause of ventricular dysfunction, the remaining subtypes include hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy, ARVD/C, restrictive cardiomyopathy, and unclassified cardiomyopathy.

Ischemic Cardiomyopathy (Coronary Artery Disease)

Coronary artery disease is the leading cause of death worldwide [24]. Coronary angiography is traditionally regarded as the gold standard for the evaluation of significant CAD. Nevertheless, coronary angiography is an invasive procedure that involves radiation and can have uncommon, but potentially serious, complications [25]. Computed tomographic angiography (CTA) is also being increasingly used in the evaluation of CAD. Although highly sensitive and specific, CTA still requires iodinated contrast agents and radiation exposure. Therefore, careful patient selection and preparation is needed to ensure that CTA yields high-quality diagnostic data while minimizing radiation exposure [2]. Other established methods for the assessment of CAD include stress echocardiography [26] and single-photon emission CT (SPECT) [27].

In a single session, CMRI can accurately evaluate LV systolic function, myocardial perfusion, and myocardial viability, therefore offering a “1-stop shop” for the comprehensive evaluation of underlying CAD. In addition, CMRI can also provide prognostic information, and a negative CMRI result predicts low short- to medium-term event rates [28–30].

Myocardial Perfusion

Since the early 1990s, investigators have used CMRI for the evaluation of myocardial perfusion [31]. Positron emission tomography (PET) and SPECT are currently considered the standards for myocardial perfusion imaging in the evaluation of patients with suspected CAD, as both of these methods have a high sensitivity (89 and 88 %, respectively) and specificity (86 and 74 %, respectively) for detecting such disease [32]. However, nuclear examinations have certain limitations, including attenuation artifacts in SPECT, the use of ionizing radiation, and relatively limited resolution. In a vasodilator CMRI perfusion study, the in-plane resolution is at least 3 × 3 mm, allowing visualization of perfusion defects in both the endocardium and epicardium. In a canine model, Lee and colleagues [33] detected a linear relationship between the relative regional flows derived from CMRI and those derived from microspheres; the correlation between both ^99mTc-sestamibi and ²⁰¹Tl relative regional flows and microsphere relative regional flows plateaued with increasing flow. Most CMRI centers now assess myocardial viability by performing DE-MRI after the stress-rest myocardial perfusion study. This enables the clinician to distinguish perfusion defects that occur during stress and rest, suggesting critical ischemia (where the same area is viable), versus the fixed-perfusion defects during stress and rest that represent myocardial scarring (coincide with the presence of DE-MRI) [34].

According to the “ischemic cascade” theory, when a significant coronary artery stenosis is present, a perfusion abnormality will occur before diastolic dysfunction and regional wall-motion abnormalities arise; these developments are followed by electrographic ST-segment changes and, finally, the onset of chest pain [35]. Vasodilators such as adenosine and regadenoson act on the A_2A receptor. They increase myocardial blood flow (MBF) in normal coronary arteries by causing smooth muscle relaxation and vascular dilation; however, coronary arteries that have a significant stenosis show little or no increase in blood flow on exposure to these vasodilators. The resulting flow heterogeneity produces a steal phenomenon [36], which manifests as a reversible perfusion defect that occurs during stress perfusion and resolves during rest perfusion if critical coronary stenosis is not present.

When thallium or sestamibi is used in SPECT, signal intensity in the myocardium depends on both myocardial tissue perfusion and cell viability, as cells must be viable to extract the agent into the intracellular space. Therefore, in necrotic myocardium, the signal intensity will remain low regardless of whether or not blood flow has been restored to the region. Most of the Gd-chelates used in the United States are extracellular agents [37]; therefore, they rapidly diffuse from the intravascular space to the interstitial space. When these standard extracellular contrast agents are used, about half of the compound leaks out into the interstitial space during the first pass. Consequently, the myocardial signal intensity depends on the tissue blood volume, perfusion, the extravascular compartment, and the degree of capillary permeability. Despite these limitations, MRI measurements of myocardial perfusion correlate well with microsphere measurements, which are currently the gold standard in perfusion assessment [38].

Figure 13.1 shows the perfusion protocol used at the St. Luke’s Hospital/Texas Heart Institute. During peak stress (approximately 1 min after administration of a 400-mcg intravenous bolus of regadenoson or 3 min after administration of a 140 mcg/kg/min intravenous infusion of adenosine) [36], Gd is injected through a peripheral vein at a rate of 4–6 mL/s; the dosage commonly ranges from 0.05 to 0.1 mmol/kg [39–41]. The dose of Gd should be kept low to maintain a linear relationship between image intensity and contrast concentration [42]. Furthermore, if the Gd dose is too high, T₂* effects could predominate, interfering with perfusion analysis. Figure 13.2 shows a CMRI perfusion study in which adenosine was used as the vasodilator, along with the corresponding angiographic image.

Because Gd is a T₁-shortening agent, the T₁-weighted gradient-recalled echo (GRE) sequence is the one most commonly used in clinical practice and described in the literature. With this technique, an inversion recovery or a saturation recovery pre-pulse is applied first to provide T₁ weighting. Other sequences, including the hybrid GRE-echo planar imaging and balanced SSFP sequences, have also been investigated. Although the latter 2 sequences have fast imaging speeds and high signal-to-noise and contrast-to-noise ratios [43–45], they are more prone to motion artifacts than is the GRE sequence. In the short-axis orientation, at least three slices covering the basal, mid, and apical thirds of the left ventricle are acquired during stress and rest perfusion. Despite the fact that the 3 LV slices are acquired in different cardiac phases, there appear to be no variations in coronary and MBF over the cardiac cycle. More slices can be covered if a faster imaging sequence is used or if 2 R-R intervals are used instead of 1 R-R interval.

Typically, the in-plane spatial resolution with a 1.5 T MRI scanner is approximately 2–3 × 2–3 mm, allowing detection of subendocardial hypoperfusion. Recently, though, the 3 T MRI scanner has been gaining popularity, primarily because it has a higher signal-to-noise ratio than the 1.5 T device. In addition, use of the 3 T MRI scanner in conjunction with new acceleration techniques, such as the broad-use linear acquisition speed-up technique (k-t BLAST) and sensitivity encoding (k-t SENSE), can reduce imaging time, improve LV coverage, and/or further improve spatial resolution [46–48].

Evaluation of CMRI perfusion data is commonly performed qualitatively. A perfusion abnormality is characterized by a black or dark gray area of myocardium at the peak bolus; the signal reduction should also persist longer than the first pass of Gd through the LV cavity [41]. In a recent meta-analysis of the diagnostic performance of vasodilator perfusion MR in detecting CAD, this method was determined to have a sensitivity of 89 % (95 % confidence interval [CI], 88–91 %) and a specificity of 80 % (95 % CI, 78–83 %) [49]. The reference standard was a ≥50 % diameter stenosis on coronary angiography, and all data were analyzed at the patient level. Limitations of this meta-analysis included the use of coronary angiography as the standard—so that the functional significance of the stenosis was not evaluated—and the variability in the Gd dose used in different studies.

The Magnetic Resonance Imaging for Myocardial Perfusion Assessment in Coronary artery disease Trial (MR-IMPACT), published in 2008, was a multicenter, multivendor trial that compared the diagnostic performance of perfusion CMRI with that of SPECT and coronary angiography (≥50 % stenosis) [40]. The study cohort comprised 234 patients, divided into five groups; each group received a different Gd dose. In the group that received the optimal Gd dose, 0.1 mmol/kg, the diagnostic performance of perfusion CMRI was similar to that of SPECT, as evaluated by receiver-operating characteristic (ROC) analysis. However, when the entire SPECT study population was considered, the diagnostic performance of perfusion CMRI at the 0.1 mmol/kg Gd dose was better than that of SPECT for the evaluation of 1- to 3-vessel disease. No difference in diagnostic performance was noted, though, if the comparison was made between perfusion CMRI and electrocardiography (ECG)-gated SPECT only. The MR-IMPACT II study, a similar but larger study with 515 patients, compared both the sensitivity and the specificity of perfusion CMRI versus SPECT in CAD detection [41]. Perfusion CMRI had a higher sensitivity than SPECT (0.67 vs 0.59, respectively) but a lower specificity (0.61 vs 0.72, respectively). The results of these two studies suggest that perfusion CMRI is a safe alternative to SPECT.

In a recently reported study, more than 600 patients with suspected angina underwent both vasodilator perfusion CMRI and ECG-gated SPECT, followed by invasive angiography [50]. The sensitivity, specificity, positive predictive value, and negative predictive values were 86.5, 83.4, 77.2 and 90.5 %, respectively, for CMRI and 66.5, 82.6, 71.4, and 79.1 % for ECG-gated SPECT. The sensitivity and negative predictive values were significantly better for CMRI than for SPECT (P < 0.0001). As Fig. 13.3 shows, CMRI also performed better than SPECT in various other categories, as indicated by ROC analysis.

The use of semi-quantitative analysis, rather than qualitative analysis, could make the assessment of perfusion abnormalities a more objective process [8, 51]. Indices that could be measured for this purpose include the peak signal intensity, up-slope, time to peak signal, mean transit time, and area under the curve. Of these, the up-slope is the most commonly used parameter. Furthermore, the stress and rest up-slope values can be normalized with the blood pool during stress and rest (Fig. 13.4). In animal studies, these semi-quantitative indices have correlated well with flow measured by microspheres [52].

Furthermore, MBF can be quantified in mL/min/g in perfusion CMRI. For background information regarding the absolute quantification of MBF, readers are referred to several excellent previously published review articles [8, 11, 43, 53]. Models have been developed that use the myocardial and LV blood pool time-intensity curve to estimate absolute MBF. One method for MBF quantification is the Fermi function deconvolution (Fig. 13.5), in which the maximum amplitude of the transfer function reflects the absolute MBF. Therefore, perfusion can be calculated by “deconvoluting” the arterial input function from the transfer function. The arterial input function may be obtained by using the dual-bolus technique [54]. In a recent study in which a 3 T scanner was used to assess healthy volunteers during CMRI, the MBFs at rest and during stress on CMRI were 1.0 ± 0.8 and 3.0 ± 1.6 mL/min/g, respectively; these values correlated well with those obtained by ¹³N-ammonia PET, which were 0.8 ± 0.2 for the MBF at rest and 3.0 ± 1.1 mL/min/g during stress [55].

Myocardial Viability

Left ventricular function is among the most important prognosticators in patients with CAD. Therefore, assessment of myocardial viability before revascularization is a conceptually sound and logical approach for evaluating myocardial dysfunction, because ventricular function should improve after revascularization in patients with viable myocardium [56, 57]. In the setting of ischemic cardiomyopathy with significant CAD and LV systolic dysfunction, data suggest that revascularization for patients with myocardial viability confers a survival benefit [58–60]. In a meta-analysis of >3,000 patients, ²⁰¹thallium SPECT, dobutamine echocardiography, and F-18 fluorodeoxyglucose PET were used to assess myocardial viability; patients with viable myocardium who underwent revascularization had a significant survival advantage when compared to similar patients who underwent medical therapy (annual mortality rate, 3.2 and 16 %, respectively; P < 0.0001), but in patients with no evidence of myocardial viability, no difference in mortality was observed in those who underwent revascularization versus medical therapy (although there was a trend toward increased mortality in the revascularization group) [58]. A more recent meta-analysis of myocardial viability assessment, as determined by dobutamine stress echocardiography, SPECT, and PET, also showed that revascularization in patients with viable myocardium consistently provided a survival advantage when compared to medical therapy alone [61]. In addition, successful revascularization has been shown to improve global and regional systolic function [62] and LV remodeling [62], and it has also been shown to reduce the occurrence of adverse outcomes such as myocardial infarction (MI), heart failure, and unstable angina [63].

A variety of CMRI methods can be used to assess myocardial viability. The DE-MRI technique is relatively easy to perform, is reproducible, and provides high spatial resolution. Myocardial viability can also be evaluated by using CMRI spectroscopy, an ion transport agent (e.g., manganese), necrosis-specific agents (e.g., gadophorins), or a low dose of dobutamine, which determines contractile reserve.

In healthy myocardium, cell membranes are intact, so extracellular Gd agents cannot enter the intracellular space, which forms approximately 80–85 % of the imaging voxel. In irreversible myocardial injury, myocardial cell membranes become disrupted, allowing Gd to enter the intracellular space. As a result, the Gd concentration and the volume of distribution increase (Fig. 13.6) [64]. When the DE-MRI sequence is performed, a certain amount of time is allowed to pass after the R wave of the electrocardiogram before an 180° inversion pulse is applied. T₁ relaxation will then occur. The MRI operator will input the inversion time (i.e., the time required for normal myocardium to reach the “null” point), after which data acquisition is begun. Areas with irreversible injury (i.e., myocardial scar) will appear bright with high signal intensity; this is referred to as hyperenhancement. In contrast, normal, viable myocardium will have low signal intensity and will appear dark [64, 65]. The typical in-plane resolution of DE-MRI is 1.5–2 × 1.5–2 × 6–8 mm on a 1.5 T MRI scanner; this resolution is easily 20× higher than that of SPECT. When used to determine infarct size in an animal model, DE-MRI showed excellent agreement with tetrazolium chloride staining, the gold standard [66] (Fig. 13.7). In a seminal study by Kim and colleagues [67], DE-MRI was able to identify dysfunctional but viable myocardium and predict functional recovery after coronary revascularization. In that study, approximately 80 % of the dysfunctional segments with fully viable myocardium showed improvement in functionality after revascularization, whereas only 10 % of the segments with 51–75 % hyperenhancement/scar showed functional recovery (Fig. 13.8). The above-mentioned findings have been confirmed by other studies [68, 69].

Typically, 0.1–0.2 mmol/kg of Gd is administered as a contrast agent for DE-MRI. Data are usually acquired after 10–15 min of Gd administration, allowing adequate wash-in and wash-out of the contrast agent from the extracellular space of the myocardium. The input of the inversion time is critical to achieve adequate “nulling” of the myocardium. The correct inversion time can be found by using the inversion-time scout sequence, available from all major MRI vendors. Because of the introduction of the phase-sensitive inversion sequence for DE-MRI, choosing the exact nulling time has become less important [70]. The grading of DE-MRI is usually qualitative and is determined by the transmural extent of DE: 0 = fully viable, thin subendocardial = 1 to 25 %, dense subendocardial = 26 to 50 %, near-transmural = 51 to 75 %, and transmural = 76 to 100 % [67].

Regarding the study by Kim and colleagues [67], it is important to note that even in dysfunctional myocardium with no scarring, contractility improved in only approximately 78 % of segments. Some of the limitations of viability assessment have been elegantly summarized in a recent article by Schinkel and colleagues [61]. The usefulness of viability assessment may be limited for the following reasons: (1) a large amount of scar tissue adjacent to viable myocardium could limit functional recovery [71]; (2) in a severely dilated heart that has undergone significant remodeling, improvement may not be possible, despite the presence of viable myocardium [72]; (3) hibernating myocardium, in which metabolism has been significantly downregulated, may no longer respond to revascularization; (4) revascularization may have been incomplete or may have failed, or new myocardial injury may have occurred during revascularization; and (5) imaging must be performed at the correct time, because it could be as long as 1 year before functional recovery is seen [73].

Follow-up CMRI studies performed after revascularization consistently show only moderate functional recovery of segments that had 25–76 % DE [67, 69, 74]. In these segments, LD dobutamine CMRI is better able to predict functional recovery [74, 75]. In a recent study by Glaveckaite and colleagues [74], LD dobutamine performed better than DE-MRI at predicting functional recovery in segments with 26–75 % DE (P = 0.043), as shown by ROC analysis. However, the combination of DE-MRI and LD dobutamine was not found to provide more information than LD dobutamine alone. In general, DE-MRI offered similar, if not better, sensitivity than LD dobutamine MRI did in predicting functional recovery; however, LD dobutamine MRI offered better specificity than DE-MRI did. Finally, the investigators also found that the predictive value of DE-MRI did not increase when the thickness of the viable myocardium in each segment was also considered [74].

During an acute MI, the “no-reflow” phenomenon may be visualized with coronary angiography. This phenomenon results from obstructions in the microcirculatory system, which includes vessels <200 μm in diameter [76]. This microvascular obstruction (MO) can be imaged by DE-MRI after Gd administration (Fig. 13.9) and can be detected during an acute MI, especially an ST-segment–elevation MI (STEMI), with or without percutaneous intervention. Such obstruction is especially common when the patient is elderly, the thrombus burden is large, the baseline coronary flow is poor, or reperfusion is delayed. When visualized with DE-MRI, MO is seen as regions of low signal intensity surrounded by hyperenhancement. Although DE-MRI is used for this purpose, there is no consensus regarding the optimal imaging sequence to use for MO or the appropriate timing of imaging after Gd administration. For example, some studies suggest that images should be acquired a few minutes after Gd administration (early) [77, 78], whereas others suggest the use of a standard imaging time of 10–15 min after Gd administration (late) [77–81]. Other imaging methods include a first-pass perfusion sequence [78, 80–82] and cine SSFP imaging [83]. In 1 study, the median mass of MO was found to be greatest on first-pass perfusion, followed by early and late DE-MRI (4.7, 2.3, and 0.2 g, respectively) [78].

Microvascular obstruction is common after an acute MI, even after successful reperfusion, and patients with MO have a larger infarct size than those without MO [79, 80, 82, 84]. In addition, Kaplan-Meier analysis has indicated that survival is worse in patients with MO [79, 80, 84]. The presence of late/persistent MO is a significant prognosticator for major adverse cardiovascular events [79, 80], indicating that more aggressive therapy and vigilant clinical follow-up are needed for patients who have MO.

Data from multiple studies suggest that besides playing a major role in the assessment of myocardial viability, DE indicates a worse prognosis in patients with ischemic or nonischemic cardiomyopathy. In a study of 857 patients, the presence of DE was an independent predictor of death/cardiac transplantation in the patients with and without CAD (Fig. 13.10) [28]. Similarly, in a cohort of patients with suspected CAD who underwent DE-MRI, significant survival differences were found between the patients with DE and those without DE [85]. In addition, the presence of DE had the highest hazard ratio (HR) when compared to other traditional clinical factors in predicting cardiac mortality and major adverse cardiac events; patients whose percentage of DE was in the lowest tertile had a 7-fold increase in major adverse cardiac events. In a recent multicenter observational study, more than 1,500 patients underwent routine CMRI, which included DE-MRI and LV ejection fraction (LVEF) measurements. In this study, significant predictors of all-cause mortality included older age, the presence of CAD, and a greater number of DE segments [86]. In addition, patients with preserved LV systolic function (LVEF ≥50 %) who had >4 segments with DE had significantly poorer survival than patients who had ≤4 DE segments. The authors of the above-mentioned studies have provided two possible explanations as to why DE is associated with poorer outcomes in patients with CAD: first, myocardial scarring could act as a substrate for ventricular tachyarrhythmia [87, 88], and second, the presence of DE could be a marker of significant underlying atherosclerosis [85].

Two recent articles described the use of DE-MRI for patient risk assessment to determine the appropriate use of implantable cardioverter-defibrillator (ICD) therapy. In the study by Klem and colleagues [89], 137 patients underwent CMRI, and 104 of these patients had an ICD placed. The investigators found a significant increase in the primary endpoint event rate (all-cause death, sudden cardiac death [SCD], or ICD discharge) when the amount of DE was >5 % of the LV mass; however, the event rate then plateaued with further increases in DE. In addition, DE of >5 % was the only significant independent predictor of SCD or ICD discharge, with an HR of 4.8 (95 % CI, 1.7–13.7; P = 0.0004). The authors then used an LVEF cut-off of 30 % to assess the effect of DE % on the event rates of SCD and ICD discharge; they obtained the following results: (1) LVEF >30 %, DE ≤5 %, 3-year event rate = 6 %; (2) LVEF >30 %, DE <5 %, 3-year event rate = 25 %; (3) LVEF ≤30 %, DE ≤5 %, 3-year event rate = 11 %; and (4) LVEF ≤30 %, DE >5 %, 3-year event rate = 35 %. These results suggest that even in patients with a low LVEF (≤30 %), the presence or absence of significant DE could affect the occurrence of these adverse outcomes. A similar trend was also seen in all-cause mortality. The second study to assess the use of DE-MRI for risk stratification followed up 257 patients after ICD implantation; both the percentage of DE (HR, 1.8; 95 % CI, 1.1–2.8; P = 0.02) and the number of transmural segments (HR, 1.4; 95 % CI, 1.2–1.7; P = 0.001) were independent predictors of the need for ICD therapy in this study [90].

Area-at-risk Assessment

Area-at-risk (AAR) is defined as the region of myocardium that becomes ischemic during an acute coronary occlusion [91]. The T₂-weighted imaging used in CMRI is sensitive to water-bound protons, and it has been postulated that this method can be used to detect myocardial edema, regardless of whether the edema is caused by inflammation or coronary occlusion [92]. From data collected by T₂-weighted imaging and DE-MRI, the following parameters can be computed: (A) AAR = myocardial edema volume / volume LV mass; (B) percentage of infarct size = volume infarct / volume LV mass; (C) myocardial salvage = (A) – (B); and (D) myocardial salvage index = (C) / (A) (Fig. 13.11) [93]. In a study by Ugander and colleagues [94], measurements of the AAR myocardium after coronary occlusion taken by T₂ mapping were in good agreement with those taken by the reference standard of microsphere blood flow assessment (R² = 0.96) in a canine model. In several publications, T₂-weighted imaging has been used to assess myocardial edema or myocardial salvage during acute coronary syndrome [93, 95–98].

To define the prevalence and significance of myocardial edema in patients with non-STEMI, Raman and colleagues [97] analyzed data from 88 patients who had non-STEMI and adequate CMRI studies before coronary angiography. These data included results from triple inversion T₂-weighted edema imaging and DE-MRI. The treating physicians were blinded to the CMRI results. All CMRI data were analyzed in a semi-quantitative fashion. Revascularization was required for significantly more patients with myocardial edema than for patients without edema (88 % vs 26 %, respectively; P <0.0001). When compared to patients without myocardial edema, those with myocardial edema had a higher risk of having a cardiovascular event or dying within 6 months after the index event (HR, 4.5; 95 % CI, 1.0–20.0; P = 0.05).

Eitel and colleagues [93] analyzed the CMRI results of more than 200 patients with STEMI to determine the prognostic significance of the myocardial salvage index in reperfused STEMI. For each patient, CMRI was performed within 4 days of the index event. The analyzed parameters included myocardial edema, infarct size, myocardial salvage index, and standard LV functional parameters. Higher myocardial salvage index values correlated with shorter symptom-to-percutaneous intervention times (i.e., symptom-to-balloon time). In multivariable regression analysis, only the myocardial salvage index was an independent and significant predictor of the study endpoint, which was death, reinfarction, or new congestive cardiac failure within 6 months of the index event (HR, 0.93; 95 % CI, 0.91–0.96; P <0.001). This means that major adverse cardiovascular events are less likely to occur in patients with a higher myocardial salvage index value than in patients with a lower value. In a second publication, Eitel and colleagues [99] provided the results of a follow-up study of the above-mentioned patients (median follow-up period, 18 months). The investigators found that the incidence of major adverse cardiovascular events or death was significantly lower in patients with a median myocardial salvage index of ≥48.3, which indicates a larger degree of myocardial salvage (P <0.0001 for both comparisons). In this study, the percentage of DE was the most significant predictor of major adverse cardiac events—specifically, a composite of death, reinfarction, and new congestive heart failure (HR, 1.06; 95 CI%, 1.03–1.09; P <0.001). Although the myocardial salvage index was not found to be a significant predictor of major adverse cardiac events, it was a significant independent predictor of death (HR, 0.93; 95 CI%, 0.91–0.96, P <0.001). In contrast, traditional prognosticators, such as age, diabetes, and percentage of DE, were not significant predictors of death. In addition, infarct size and myocardial salvage were noted to have a strong inverse correlation (r = −0.86; P <0.001).

In a study of 137 patients with acutely reperfused STEMI, the myocardial salvage index was determined to be the most significant predictor of both early ST-segment resolution (B coefficient = 0.61; P <0.0001) and adverse remodeling, defined as a ≥15 % increase in LV systolic volume on the 4-month follow-up CMRI (odds ratio, 0.64; 95 % CI, 0.49–0.84; P = 0.001) [100].

Larose and colleagues [98] studied 101 patients with acute STEMI who underwent CMRI 12 h after primary percutaneous intervention and 6 months after the index event. The patients were followed up for 2 years. Data from CMRI were used to quantitatively examine LV function and assess DE-MRI of the entire left ventricle; first-pass perfusion and T₂-weighted images were obtained in the apical, mid, and basal left ventricle. The myocardial salvage index was also computed. All of the above-mentioned variables were significant predictors of LV systolic dysfunction at 6 months on univariate analysis, but the percentage of DE measured during the acute STEMI was the best independent predictor in multivariable analysis (odds ratio, 1.33; 95 % CI, 1.09–1.78; P = 0.002).

The use of DE-MRI for prognostication has been well established in multiple publications [28, 85, 86]. In the assessment of novel myocardial perfusion strategies, the myocardial salvage index could prove to be a better prognosticator than the percentage of DE because the final infarct size depends on various factors, including the collateral blood flow, duration of ischemia, and extent of AAR. There is a close association between DE size and AAR; the myocardial salvage index corrects the infarct size to the amount of AAR [100]. The myocardial salvage index could be useful in assessing the safety and efficacy of new reperfusion strategies.

The technical limitations of T₂-weighted imaging are well summarized by Wince and colleagues [101]. One such limitation is that the contrast-to-noise ratio is low between edematous and normal myocardium in conventional T₂-weighted imaging. Also, wall-motion abnormalities associated with acute myocardial injury can lead to “slow blood flow,” creating a high signal that can be mistaken for edema. In addition, incomplete dark-blood preparation due to reduced cardiac motion and misregistration between the preparation and imaging phase in hypokinetic myocardium can result in a higher signal intensity than that in normal myocardium. Finally, in most studies, the area of edema is arbitrarily defined as a region with a signal intensity that is >2 standard deviations above that of a remote region. Improved imaging sequences [102, 103] and quantitative T₂ measurements [104] may overcome some of these limitations.

Nonischemic Cardiomyopathy

Cardiovascular MRI has emerged as the gold standard for the assessment of nonischemic cardiomyopathy [1–3]. Surface 2-dimensional (2D) echocardiography remains the first-line, noninvasive imaging modality used for patients who present with cardiomyopathy. However, 2D echocardiography has a few potential limitations. First, patient body habitus could affect image quality, especially in patients who are obese or who have chronic obstructive pulmonary disease. Second, the apex may not be readily visible (i.e., in patients with apical ballooning). Third, 2D echocardiography has limitations regarding the accurate assessment of the right ventricle, which is of great importance when one is assessing diseases such as ARVD/C. Fourth, 2D echocardiography relies on geometric assumptions for chamber and mass quantification. Fifth, 2D echocardiography has a limited ability to characterize certain tissues, including fat, fibrosis, and scar.

The following sections provide a brief summary of the role CMRI plays in the diagnosis of various cardiomyopathies commonly encountered in day-to-day clinical practice.

Imaging Techniques

Cine Imaging

Steady-state free-precession cine imaging is the most widely used imaging sequence for functional assessment by means of CMRI, especially for the right ventricle. Parallel imaging can also be used in conjunction with SSFP imaging to reduce the imaging time and enhance patient comfort, to improve the spatial resolution, or both; these benefits come at the expense of a slightly reduced signal-to-noise ratio.

Spin Echo

Tissue characterization is commonly performed with the turbo/fast spin echo sequence, using T_1- or T₂-weighting. A double inversion recovery preparation scheme is commonly used to suppress the blood signal. Briefly, an initial nonselective 180° pulse is applied to the entire imaging volume. Then, a second 180° pulse is applied to the slice of interest immediately after the first 180° pulse to restore the longitudinal magnetization; note that magnetization outside the slice of interest will remain inverted. The acquisition sequence is started after an inversion time that corresponds to the time when blood from outside the imaging slice crosses zero during recovery after the 180° inversion pulse; therefore, the blood will generate no signal. With the application of another inversion pulse to suppress the fat signal, a triple inversion recovery preparation scheme will have been achieved.

For the detection of myocardial edema, the T₂-weighted spin echo sequence is especially useful when long T₂ water-bound protons are used as the contrast-generating mechanism, resulting in high signal intensity in edematous tissue [92]. To evaluate fatty infiltration or replacement, as would be needed for the assessment of ARVD/C, a fat saturation sequence is employed. Examples of these sequences include short inversion-time inversion recovery (STIR), spectral presaturation with inversion recovery (SPIR), spectral attenuated inversion recovery (SPAIR), and the principle of selective excitation technique (ProSet).

Spin echo images have certain limitations. For example, the contrast-to-noise ratio between normal and abnormal myocardium is low; the image quality is affected by background field inhomogeneity; and in areas of reduced myocardial contraction, slow blood flow could result in a bright signal that could be mistaken for myocardial edema. The presence of a significant arrhythmia can also affect image quality. In addition, through-plane motion can cause a loss of the posterior/inferior wall signal, and the use of phase array coils can cause signal intensity variability.

Dixon Technique

Another method of imaging fat (and water) is the Dixon technique [105]. This chemical-shift–based water-fat separation technique was originally described by Dixon in 1984 [106]. Protons in water and fat have different resonance frequencies, and phase shifts occur because of these frequency differences. As a result, fat and water can be separated by using the improved multi-echo Dixon technique, which is less susceptible to background magnetic field inhomogeneity than are traditional fat-suppression techniques. This method can be incorporated into contrast-enhanced or cine imaging to achieve a high signal-to-noise ratio with a shorter imaging time, especially when parallel imaging is used [107]. Some clinical applications of the multi-echo Dixon technique include detecting fatty infiltrates in MI, imaging cardiac tumors (such as lipomas), and evaluating ARVD/C (Fig. 13.12) [105].

Delayed-enhancement MRI

The DE-MRI sequence is commonly performed approximately 10–15 min after Gd enhancement. Different types of nonischemic cardiomyopathy can produce different enhancement patterns, such as mid-myocardial, subepicardial, and transmural (Fig. 13.13). A DE pattern alone may not be specific enough to diagnose a disease, but when considered in conjunction with the patient’s clinical presentation, symptoms, and extravascular findings (e.g., hilar adenopathy in sarcoidosis), DE-MRI findings can help physicians arrive at the correct clinical diagnosis.

T₁ Imaging

In certain pathological conditions, such as CAD, infiltrative diseases (e.g., amyloidosis), and cardiac hypertrophy (e.g., systemic hypertension, HCM), fibroblasts proliferate and extracellular collagen deposition and fibrosis increase with structural and biomedical remodeling [108]. In a rat model, myocardial fibrosis has been associated with ventricular remodeling, increased myocardial stiffness, and a reduction in systolic function [109]. Fibrosis can be arrhythmogenic because it can lead to electrical conduction disturbances. In humans, the interstitial/extracellular space accounts for approximately 20–25 % of the total heart volume [110]. With fibrosis, the extracellular space increases. Endomyocardial biopsy is the gold standard for evaluating myocardial fibrosis; however, it is an invasive procedure with known morbidity and mortality hazards. Furthermore, such a biopsy could result in sampling error.

In CMRI, there is T₁ shortening after Gd administration. The signal change is related to the extent of the extracellular space. When irreversible injury is present, the intracellular spaces will be directly exposed to Gd. As a result, there will be an apparent increase in the extracellular space and volume of distribution, increasing the signal strength after Gd administration when an appropriate imaging sequence is used (e.g., DE-MRI). Similarly, in an infiltrative cardiomyopathy, such as amyloidosis, the expansion of extracellular space could result from fibrosis or amyloid protein accumulation [111]. T₁ mapping provides a more objective assessment of the extracellular space and, therefore, of fibrosis [112]. The extracellular volume fraction (ECF) can be obtained by using the formula (ΔR_{1 Myocardium}/ΔR_{1 Blood}) × (1-hematocrit), where R₁ is the reciprocal of T₁ (T₁ is the longitudinal relaxation time constant) and ΔR₁ represents the difference between R₁ before and after contrast [113].

T₂ Imaging

Both inflammation (as occurs in myocarditis) and ischemia-reperfusion injury (as occurs in acute coronary syndrome) can lead to myocardial edema [114]. In the initial stages of edema, an Na⁺/K⁺ pump dysfunction leads to the intracellular accumulation of Na⁺, resulting in an influx of water due to an increase in osmotic pressure. With a prolonged insult, capillary membrane damage leads to interstitial edema due to intravascular water leakage. Interstitial edema can cause systolic and diastolic dysfunction and can promote myocardial stiffness and fibrosis [115]. Discriminating between intra- and extracellular myocardial edema by means of noninvasive imaging is difficult.

The T₂-weighted imaging sequence is used clinically to detect myocardial edema, which produces a high signal intensity with this approach. Recently, some improved versions of T₂-weighted imaging have been described. For example, the T₂-prepared SSFP sequence has been shown to have fewer artifacts and higher accuracy than the traditional fast spin echo sequence [103], and a hybrid sequence has been described that combines the advantages of SSFP and turbo spin echo imaging [102]. In addition, quantitative T₂ mapping to detect myocardial edema has been shown to increase objectivity and improve accuracy while also addressing the potential limitations of traditional spin echo imaging sequences [104].

T₂* Imaging

Serum ferritin is commonly measured to determine body iron stores; however, it is not a good indicator of the myocardial iron level. Hepatic iron overloading leads to a reduction in signal intensity, and the T₂ value is reduced when iron is present. Initially, the spin echo T₂ sequence was used to quantify tissue iron content, but the gradient-echo T₂* sequence has shown greater sensitivity to iron deposition [116]. Using the gradient-echo sequences to measure T₂* has the advantage of a shorter acquisition time; therefore, it is possible to produce a motionless image by using a single breath-hold with ECG-gating. Furthermore, the gradient-echo sequence is more sensitive to the paramagnetic effect of iron because it does not require the 180° refocusing pulse used in spin echo sequences to correct for field inhomogeneities [117]. However, the quality of bright-blood gradient-echo images can be affected by noise, motion, and blood artifacts [118]. In bright-blood images, the blood signal spills into the myocardium, potentially affecting T₂* quantification. To reduce blood-signal contamination of the myocardium, a double inversion recovery prepulse sequence can be applied to suppress the blood signal, giving rise to a black blood image when a gradient-echo sequence is used [119]. With this method, the myocardium will appear more homogeneous and have greater border definition than it does when the traditional bright-blood sequence is used. He and colleagues [117, 118] used this optimized black-blood preparation gradient-echo sequence so that the T₂* value could be obtained by using a monoexponential model fit (SI = ke^−TE/T2*; where SI = signal intensity, k = constant, and TE = echo time). In contrast, in certain cases, the bright-blood T₂* value could be better estimated by using either the truncation model or the offset model [118, 119]. These measurements were usually made in a full-thickness region of interest that was placed in the mid-ventricular septal region to avoid susceptibility artifact [118].

In 12 hearts obtained from siderotic cardiomyopathy patients after death or cardiac transplantation for end-stage heart failure, midventricular septal iron concentrations and CMRI measurements of R₂* (calculated as 1/T₂*) were both highly representative of mean global myocardial iron [120]. The same study also confirmed that iron deposition is higher in the epicardium than in the endocardium; therefore, for T₂* measurements, a large, full-thickness section of the mid-septum should be chosen as the region of interest.

Nonischemic Cardiomyopathies

Myocarditis

Myocarditis is defined as inflammation of the myocardium. The presentation of myocarditis can vary from nonspecific symptoms to acute hemodynamic instability and death [121]. It has been reported that in some patients myocarditis can progress to dilated cardiomyopathy [122] or ARVD/C [123]. Myocarditis can be caused by an infection (viral, bacterial, or fungal) or can occur secondary to hypersensitivity or immunological conditions.

In patients with suspected myocarditis, CMRI can be used to evaluate global and regional function; wall thickness and mass; pericardial effusion; myocardial edema, which can be detected by characterizing the tissue in T₂-weighted images; increases in volume of distribution caused by hyperemia and capillary leak, which can be detected by early Gd enhancement (the global relative early enhancement ratio, gRE); and myocardial necrosis and fibrosis, which can be detected by using DE. Table 13.2 outlines the proposed Lake Louise Consensus Criteria for using CMRI for diagnosing myocarditis [92].

Table 13.2 Lake Louise consensus criteria for the diagnosis of myocarditis by cardiovascular magnetic resonance imaging

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In a study of 25 patients with clinically diagnosed myocarditis, the sensitivity, specificity, and diagnostic accuracy were determined for three CMRI sequences: (1) the values for the T₂ signal intensity ratio between myocardium and skeletal muscle were 84, 74, and 79 %, respectively; (2) the values for GRE were 80, 68, and 75 %, respectively; and (3) the values for DE-MRI were 44, 100, and 71 %, respectively. When positive results were obtained by any two of the three sequences in a single patient, the values were 76, 96, and 85 %, respectively [124]. In another study of 42 patients with myocarditis, the T₂ signal intensity ratio was elevated in 57 % of the patients, the GRE was elevated in 31 % of the patients, and DE was present in 64 % of the patients. Furthermore, two of these three criteria were present in 76 % of the patients [125]. The fact that not all patients with myocarditis have DE probably reflects the inability of DE-MRI to reveal diffuse myocardial changes. In DE-MRI, myocarditic lesions typically affect the lateral free wall and the inferior wall, epicardially or mid-myocardially [124, 126], although multifocal or diffuse distribution has also been reported (Fig. 13.14) [92]. Patients with myocarditis have also been shown to have increases in myocardial ECF by T₁ imaging [127].

In a group of 87 patients with active myocarditis who underwent CMRI and endomyocardial biopsy, the most common findings were parvovirus B19 (56 % of patients), human herpesvirus 6 (18 % of patients), and a combination of both viruses (17 % of patients) [128]. Coxsackie B virus and Epstein-Barr virus were infrequently diagnosed (1 % of patients for each virus). Delayed enhancement was seen in the epicardium of the lateral wall in patients who were found to have parvovirus B19; these patients tended to have a relatively benign clinical course on follow-up evaluation. In contrast, DE was commonly found in the anteroseptal region, intramurally, in patients with human herpesvirus 6; the clinical course of these patients was more variable than that of patients with parvovirus B19. For patients affected by both viruses, DE was present in a large area of the anteroseptum, predominantly mid-myocardially. On follow-up examination, these patients were found to have symptoms of persistent congestive cardiac failure and no significant improvement in LVEF. Significant predictors of an impaired LVEF included the presence of DE in the septum, the amount of DE, the end-diastolic volume at presentation, and combined infection with parvovirus B19 and human herpesvirus 6.

Published studies of the use of CMRI for the diagnosis of myocarditis have several limitations. The study cohorts are generally small, and most of the studies involve single-center experiences. Also, different studies use different imaging sequences and diagnostic criteria. Moreover, only a limited number of reports mention histological confirmation of the diagnosis [92].

Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy

Arrhythmogenic RV dysplasia/cardiomyopathy is a heritable form of cardiomyopathy characterized by fibrofatty replacement that primarily affects the right ventricle. At a later stage, the left ventricle can also be affected, thereby worsening the prognosis. The disease is thought to be caused by dysfunctional desmosomes [129]. In 2010, the original 1994 Task Force criteria for the diagnosis of ARVD/C were modified [130]. The diagnosis is fulfilled by the presence of 2 major criteria, or 1 major plus 2 minor criteria, or 4 minor criteria from different categories. These categories include global or regional dysfunction and structural alterations, tissue characterization of RV myocardium, repolarization abnormalities, depolarization/conduction abnormalities, arrhythmias, and family history. The major and minor CMRI criteria proposed in the updated recommendations include regional RV wall-motion abnormality and either impaired RV function or increased indexed end-diastolic volume (Table 13.3) [130, 131], whereas the major and minor criteria previously recommended included global dilation of the right ventricle, reduced RV ejection fraction, regional wall-motion abnormalities (including localized RV aneurysms), and segmental RV dilation.

Table 13.3 Revised 2010 task-force criteria for the diagnosis of arrhythmogenic right ventricular dysplasia/cardiomyopathy by cardiovascular magnetic resonance imaging

Full size table

With the use of a state-of-the-art SSFP sequence, CMRI is inherently suited for the detailed evaluation of wall-motion abnormalities and global RV function, and this method has high temporal resolution and relatively high spatial resolution in comparison with other noninvasive imaging modalities [1]. Quantitative values obtained by means of CMRI are useful for both diagnosis and follow-up evaluation. Imaging of the right ventricle remains challenging because of the limited wall-thickness. The presence of significant ventricular ectopy and tachyarrhythmia also affects accurate ECG-gating. In the proposed 2010 criteria, tissue characterization of the RV free wall is accomplished by endomyocardial biopsy (which is considered the gold standard), not by CMRI [130]. However, as described in the section “Imaging techniques” of this chapter, several CMRI sequences are designed specifically to evaluate fatty and fibrous replacement of the right ventricle. The presence of fat in the right ventricle is not itself specific for ARVD/C, because this finding is commonly associated with aging [132].

To evaluate the presence of fat in the RV myocardium, the typical imaging protocol includes T₁- and T₂-weighted turbo spin echo sequences, with and without fat saturation; images are acquired in both the axial-oblique orientation (parallel to the RV free wall) and the transverse/axial orientation. With the routine use of phased-array surface MRI coils for cardiac imaging, rather than the body coil, the anterior coils can be used alone in order to avoid wrap-around artifacts. In addition, to minimize signal interference and improve image quality, a saturation band can be applied to the anterior chest wall. Fatty infiltration occurs predominately at the epicardial border and extends toward the endocardium in finger-like projections [133]. The LV free wall and the interventricular septum can also be affected. In addition to the technique described above, the multi-echo Dixon technique has also been used to evaluate fatty infiltration [105].

For the accurate quantitative analysis of LV and RV function, high-spatial-resolution cine imaging is performed in the axial-oblique and short-axis orientations by using the state-of-the-art SSFP sequence, which provides high contrast between blood and myocardium. The areas frequently affected by ARVD/C include the RV inflow tract, RV outflow tract, and RV apex; these regions are known as the “triangle of dysplasia” [134]. Patients with ARVD/C may also have heavy trabeculation and dilation of the RV outflow tract (in comparison to the LV outflow tract) [135]; these findings are shown well by SSFP imaging. In patients with ARVD/C, the RV free wall can be thinned and can have a bulging or scalloped appearance, suggesting aneurysmal dilation. In a study of familial ARVD/C, 38 family members of patients who had been diagnosed with ARVD/C and a desmosome mutation underwent genetic testing and CMRI. Of the 25 family members who also had a desmosome mutation, 15 (60 %) of them had RV free wall crinkling that worsened during systole, known as the “accordion” sign, and 8 (32 %) of them reported having cardiac symptoms [136].

When the RV myocardium is imaged, DE-MRI is performed approximately 10–15 min after 0.1–0.2 mmol/kg of Gd is administered [137]. Because the right ventricle is a thin-walled structure, we at our institution commonly acquire DE-MRI images both in systole (the shortest trigger-delay the scanner would allow) and in traditional mid-diastole to optimize visualization of the RV myocardium; in our experience, acquiring the image during systole does not result in significant motion artifact. In addition, because the RV myocardium is much thinner than the LV myocardium, the spatial resolution of the DE-MRI sequence has to be optimized for the evaluation of fibrosis replacement in order to reduce partial volume effects; this potentially increases the imaging time and breathing artifact. The inversion time that should be used for the DE-MRI sequence is the same for both the left and right ventricles [138]. In a study by Tandri and colleagues [137], the presence of DE was highly correlated with the existence of fibrofatty changes in endomyocardial biopsies and inducible ventricular tachycardia, as revealed by electrophysiologic testing. In addition, an increasing amount of DE was found to be correlated with a reduction in the RV ejection fraction and an increase in the RV end-diastolic volume. Figure 13.15 shows the results for a patient with ARVD/C.

Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy is a genetic disorder of the myocardium that affects 1 in 500 people and is the most common cause of sudden death in youth [139]. Surface echocardiography is the first-line imaging modality for HCM because it is patient friendly and provides information about both systolic and diastolic function and the degree of LV outflow tract (LVOT) obstruction at rest and during provocation maneuvers. However, CMRI can often provide a more precise evaluation of the extent and distribution of hypertrophy. In addition, CMRI with a combination of phase-contrast imaging and ventricular volumetric assessment can be used to ascertain other important aspects of HCM such as LVOT obstruction and the degree of associated mitral regurgitation. Recent reports indicate that DE-MRI can also provide important prognostic information concerning patients with HCM.

Hypertrophic cardiomyopathy is typically diagnosed when the patient has unexplained hypertrophy and an LV wall thickness of ≥15 mm with nondilated ventricular chambers [140]. One advantage of CMRI in the context of HCM diagnosis is the ability to image in any plane with no foreshortening or echo dropout, which is especially helpful for visualizing the LV apex. Furthermore, CMRI can accurately quantify LV mass without requiring any geometric assumptions [1, 3]. In a study that compared the diagnostic abilities of CMRI and echocardiography in HCM patients, echocardiography underestimated the maximal LV wall thickness in 10 % of the patients when compared to CMRI (24 ± 3 mm vs 32 ± 1 mm, respectively), and CMRI measurements of the basal anterolateral wall were higher than those taken by echocardiography (17 ± 8 vs 13 ± 6 mm, respectively) [141]. In a study of 333 consecutive HCM patients, CMRI showed that 77 % of the patients had hypertrophy in both the basal anterior wall and the adjacent basal septum [142]. In addition, 12 % of patients had hypertrophy localized in the apex, inferoseptum, or anterolateral wall; this hypertrophy was underestimated or not detected by echocardiography [142]. There are several variants of HCM: asymmetric septal HCM, with and without obstruction; symmetric/concentric HCM; apical HCM (Yamaguchi’s disease); mid-ventricular HCM; mass-like HCM; and noncontiguous HCM [143].

Our institution was among the first to describe the use of DE-MRI in the evaluation of HCM [144]. Delayed enhancement is present in 75–80 % of patients with HCM. Potential causes of HCM include plexiform fibrosis, expanded interstitial spaces, and replacement fibrosis due to underlying ischemia (Fig. 13.16) [64, 145–147]. The presence of DE has been shown to correlate with increased wall thickness [142, 148, 149] and regional wall-motion abnormalities [148]. From the CMRI findings in a small series of HCM patients, researchers determined that although patterns of DE distribution can be seen at almost any location that does not follow a coronary artery distribution (such as subendocardial, mid-myocardial, transmural, and multifocal DE patterns), certain DE patterns, such as RV septal and transseptal, are associated with a high risk of sudden death [145].

In several recent studies, DE-MRI was used to evaluate the risk of future clinical events and mortality in patients with HCM. The annual mortality rate for these patients was reported to be 1–5 % [150]. In a study that followed up 217 consecutive HCM patients for 3.1 ± 1.7 years, 63 % of the patients were found to have DE [149]. Multivariable analysis showed that for every 5 % increase in the amount of DE, there was a 15 % increase in the risk of reaching the combined primary endpoint of cardiovascular death, unplanned cardiovascular hospital admission, sustained ventricular tachycardia or ventricular fibrillation, or appropriate ICD discharge (HR, 1.15; 95 % CI, 1.01–1.30; P = 0.03). In addition, the researchers also determined that the probability of having congestive heart failure increases as the amount of DE increases. Another study followed up 220 HCM patients (60 % were asymptomatic, 11 % had chest pain, 21 % had New York Heart Association functional class I/II symptoms, and 8 % had class III symptoms) for a mean period of 3 years. For these patients, the presence of any DE was an independent prognosticator of cardiac death, which included SCD, congestive heart failure, and aborted SCD (HR, 4.81; P = 0.035), whereas the presence of one or two traditional clinical predictors of SCD was not clinically significant [151].

In a study that used CMRI and Holter electrocardiographic monitoring, HCM patients with DE were shown to have significantly more premature ventricular complexes, couplets, and nonsustained ventricular tachycardia than HCM patients without DE, and the presence of DE in these patients was an independent predictor of nonsustained ventricular tachycardia (relative risk, 7.3) [152]. Rubinshtein and colleagues [153] followed up 424 HCM patients for 43 ± 14 months. The 239 (56 %) patients with DE on CMRI had more episodes of nonsustained ventricular tachycardia and a higher frequency of ventricular extrasystoles per 24-h period than did the HCM patients without DE. During the study period, SCD occurred in four patients, and appropriate ICD discharge occurred in four other patients; all eight of these events occurred in patients with DE. Nevertheless, the positive predictive value of DE for SCD or ICD discharge in HCM patients with DE was low (3.3 %). In these cases, myocardial fibrosis could be acting as a substrate that promotes reentrant ventricular tachyarrhythmia. Prospective trials are still needed to determine whether using DE-MRI to risk-stratify HCM patients and to determine whether they need ICD implantation would better promote patient survival than using established traditional risk factors, especially as approximately 75–80 % of patients with HCM are reported to have some degree of DE. The presence of DE/fibrosis could contribute to an increase in myocardial stiffness, leading to diastolic dysfunction and symptoms of heart failure, even in patients with normal systolic function. The amount of DE and the rate of DE increase between two CMRI examinations have both been shown to correlate with a worsening New York Heart Association functional class in HCM patients [154].

Hypertrophic cardiomyopathy patients with severe symptoms that are refractory to medical therapy, as well as dynamic LVOT obstruction, associated septal hypertrophy, and systolic anterior motion of the mitral valve leaflet, can be considered for a surgical myectomy or, possibly, alcohol septal ablation (ASA) [140]. Figure 13.17 shows the DE-MRI results 2 days after ASA. Delayed-enhancement CMRI is inherently suited to evaluating the extent of ASA. If imaged early, patients with ASA are invariably found to have MO. At the same time, patients can be assessed for the presence or absence of a residual LVOT gradient and systolic anterior motion of the anterior mitral valve leaflet. In a study that compared the long-term outcomes of 91 consecutive HCM patients who underwent ASA with the outcomes of 40 HCM patients who underwent a traditional septal myectomy (mean follow-up period, 5.4 years), the patients who underwent ASA were found to have lower 1-, 5-, and 8-year survival rates. Furthermore, in multivariable analysis, ASA was found to be an independent predictor of the composite endpoint of cardiac death and aborted SCD (HR, 6.1; 95 % CI, 1.4–27.1; P = 0.02) [155]. In a similar study, however, Sorajja and colleagues [156] found no apparent differences in the survival rate among patients who underwent ASA, individuals in a comparable general population, or age- and gender-matched patients who underwent a myectomy.

Sarcoidosis

Sarcoidosis is an inflammatory disease characterized by the formation of noncaseating granulomas, which can form in multiple organs. The prevalence of cardiac sarcoidosis varies among reports but could be as high as 50 % in the United States [157, 158]. In 13–50 % of cases, sarcoidosis-related death may be cardiac in origin [159, 160], and SCD may account for to 65 % of the deaths that result from cardiac sarcoidosis [161].

Cardiac MRI can be used to detect several morphological abnormalities present in patients with cardiac sarcoidosis [162]. During acute inflammation, T₂-weighted double-inversion spin echo images reveal focal areas of high signal intensity, with and without myocardial thickening. Cine CMRI can be used to detect regional wall-motion abnormalities in locations that have sarcoid infiltration. In advanced cases, focal myocardial thinning and even focal aneurysmal dilation have been detected, especially in the basal septum. In patients with pulmonary involvement, pulmonary hypertension can occur; the associated CMRI findings can include enlarged pulmonary arteries, RV dilation, and hypertrophy. Furthermore, the findings of mediastinal and hilar adenopathy in axial spin and gradient-echo images may suggest pulmonary sarcoidosis.

Many authors have described the use of DE-MRI for evaluating patients with suspected or confirmed cardiac sarcoidosis [163–167]. This condition has a predilection for the interventricular septum, especially the basal septum [163, 167]; however, DE may be present in almost any myocardial segment [165]. Delayed enhancement may be mid-myocardial, perhaps extending into the subepicardium or subendocardium, or may be transmural, possibly extending into the adjacent right ventricle. In addition, DE may be multifocal (Fig. 13.18) [163, 165, 167].

Using the guidelines provided by the Japanese Ministry of Health and Welfare as the standard for diagnosing cardiac sarcoidosis, the sensitivity and specificity of CMRI in the diagnosis of this condition were found to be 100 and 78 % (n = 58), respectively, by Smedema and colleagues [165] and 75 and 77 % (n = 21), respectively, by Ohira and colleagues [164]. Patients in both studies had documented extravascular sarcoidosis, and they presented for evaluation of suspected cardiac sarcoidosis. The study by Smedema and colleagues did not require endomyocardial biopsy, and the criteria for diagnosis of cardiac sarcoidosis by CMRI included a combination of wall-motion abnormalities and DE. In the study by Ohira and colleagues, only two patients underwent endomyocardial biopsy, and the criteria for diagnosing cardiac sarcoidosis by CMRI included an increased signal intensity in T₂-weighted imaging and/or the presence of DE. In another study in which DE-MRI was used to evaluate ten patients with cardiac sarcoidosis (diagnosed by either clinical criteria [nine patients] or biopsy [one patient]), the sensitivity of DE-MRI was 100 %; four of the ten patients also had regional wall-motion abnormalities. In addition, these patients underwent ²⁰¹Tl and ⁶⁷Ga scintigraphy; the detection rates for these techniques were only 50 and 20 %, respectively [166]. All the above-mentioned studies had the same limitations: a small sample size and few histologically confirmed cases of cardiac sarcoidosis.

Patel and colleagues [167] evaluated the ability of DE-MRI to identify cardiac involvement in 81 patients with biopsy-confirmed extracardiac sarcoidosis (biopsy specimens were all obtained from the RV side of the interventricular septum). Endomyocardial biopsy or autopsy results for 15 patients (19 %) revealed histologic evidence of cardiac sarcoidosis in four patients, all of whom also had DE. Of the remaining 11 patients whose pathology findings were negative for cardiac sarcoidosis, six patients (55 %) had DE. The DE was present in the left ventricle, not in the right ventricle, highlighting the possibility that a sampling error might have occurred during the biopsy. In patients with DE, the event rate (all-cause mortality, symptomatic bradycardia, or ventricular tachyarrhythmia) and the cardiac death rate were, respectively, 9 and 11.5 times higher than in patients without DE [167]. Because of the limited number of adverse events, larger prospective studies should be conducted to confirm whether CMRI and DE-MRI could be used to risk-stratify patients with cardiac sarcoidosis.

Left Ventricular Noncompaction

Left ventricular noncompaction (LVNC) is characterized by excessive and prominent trabeculations with deep recesses that communicate with the ventricular cavity but not with the coronary circulation [168]. This condition is denoted by many different names, including spongiform cardiomyopathy, honeycombed myocardium, LV hypertrabeculation, and isolated LV abnormal trabeculation. It may have been mentioned as early as the 1920s [169], and it is increasingly being described in the current literature, probably because of technological advances in noninvasive imaging modalities.

The exact etiology of LVNC is unknown. Many authors regard LVNC to be a developmental abnormality. Usually, during the second month of embryogenesis, the trabecular meshwork that constitutes the fetal myocardium undergoes intrauterine compaction. In LVNC, however, this compaction is believed to be arrested, resulting in the characteristic thin, compacted epicardial layer with an overlying noncompacted layer of trabeculations [170]. During normal embryogenesis, the trabecular layer of the developing LV myocardium is known to compact from the base to the apex, from the epicardium to the endocardium, and from the septal wall to the lateral wall [171]; this process explains the typical location of LVNC, as shown by CMRI (see below). Some authors have proposed that LVNC may be due to impaired myocardial growth, may represent an adaptation to different hemodynamic conditions, or may result from metabolic defects [172]. Although the American Heart Association classifies LVNC as a primary, genetically determined cardiomyopathy [173], the World Health Organization and the European Society of Cardiology regard LVNC as an unclassified cardiomyopathy [22, 23].

Left ventricular noncompaction is commonly associated with congenital heart disease. In a series of 202 patients who fulfilled the imaging criteria for LVNC, 24 patients also had congenital heart disease. Of these 24 patients, 11 had LVOT abnormalities (uni- or bicuspid aortic valve, aortic coarctation, aortic hypoplasia, or subaortic stenosis), 6 had Ebstein anomaly, and 2 had tetralogy of Fallot. Conversely, when the authors reviewed the records of patients with different congenital heart diseases at the same institution, they found that LVNC was most prevalent in patients with Ebstein anomaly (15 %; 6 of 40 patients), followed by coarctation of the aorta (3 %; 2 of 60 patients) and tetralogy of Fallot (2 %; 3 of 129 patients) [174]. The current discussion will be limited to patients who have LVNC without associated cardiac abnormalities.

Despite the increased diagnosis of LVNC by state-of-the-art noninvasive imaging, there is currently no consensus regarding the criteria to use for diagnosing LVNC by CMRI [175–177] or echocardiography [168, 178–180]. Because of the inherently high contrast-to-noise ratio, the state-of-the-art imaging sequence SSFP can be used to visualize the myocardial trabeculation and the blood pool present in LVNC patients. This approach offers high spatial and temporal resolution (Fig. 13.19). Furthermore, because no geometric assumptions are necessary, this sequence can produce accurate and reproducible LV quantitative data, including LV volumes and cardiac mass. Two echocardiographic reports have indicated that LVNC commonly involves the apical segment (the entire apex was regarded as 1 segment) [168, 181]. Furthermore, in correctly performed CMRI, there should be no ventricular foreshortening; this is particularly important in the detection of LV thrombus, a well-described complication in symptomatic LVNC with impaired LV systolic function. Black-blood imaging with the double-inversion recovery sequence can help detect abnormal areas with increased trabeculations by revealing slow flow. Although surface echocardiography is the first-line noninvasive imaging modality, CMRI more completely delineates the extent of trabeculation in patients with LVNC [182]. In addition, some findings that can mimic LVNC can be easily distinguished by CMRI; examples include false tendons, abnormal insertion of papillary muscles, and aberrant bands.

Table 13.4 summarizes the criteria commonly used for the diagnosis of noncompaction by means of surface echocardiography [168, 178, 180], and Table 13.5 presents the criteria used in recent publications to diagnose LVNC by means of CMRI [175–177]. In a CMRI study of healthy volunteers between 20 and 80 years old, the mean trabecular:compacted layer ratio during end-diastole was shown to be less than 1 in all cardiac segments except 16 (the apical lateral wall, 1.19) [183].

Table 13.4 Surface echocardiographic criteria commonly used to diagnose left ventricular noncompaction

Full size table

Table 13.5 Cardiovascular magnetic resonance criteria used to diagnose left ventricular noncompaction

Full size table

Using CMRI, Petersen and colleagues [175] noted that trabeculations occur most frequently in the apex and in the distal third of the left ventricle, followed by the mid-anterior, lateral, and inferior walls. Similarly, Grothoff and colleagues [177] noted that trabeculations are most frequently located at the apex and in the distal third of the left ventricle, followed by the mid-inferior, lateral, and mid-anterior walls. The basal septum is usually not involved.

Many investigators have observed DE in LVNC patients [184–189], although the location and extent of DE varies by report; DE can be subendocardial, mid-myocardial, subepicardial, transmural, or a combination of these, and it can affect both normal segments of the heart and areas with noncompaction [186, 188, 189]. There appears to be no association between DE and noncompaction of the LV segment [189]. In a case report regarding a patient who underwent cardiac transplantation because of LVNC with severe congestive cardiac failure, the near-transmural DE seen in the interventricular septum (free of trabeculations) corresponded to the presence of fibrosis, as shown by histologic analysis. Furthermore, diffuse DE was seen in the free wall of the left ventricle and in the associated noncompacted area; histologic examination revealed fibrosis in the trabeculated areas and mucoid degeneration in the endocardium [190]. In an original article by Jenni and colleagues [168], the hearts of seven LVNC patients were studied anatomically, either postmortem or after heart transplantation; none of the seven patients had other cardiac abnormalities. Histologic examination revealed ischemic lesions in the thickened endocardium and in the prominent trabeculations. Interstitial fibrosis was also detected in six of the seven patients.

The exact mechanism of DE in LVNC is largely unknown. In LVNC patients, diminished coronary flow reserve has been observed in noncompacted and compacted segments on PET evaluation, suggesting microvascular dysfunction [191]. Similar observations have been made in children with LVNC [192]. The reduction in flow reserve and the altered myocardial perfusion could be due to failure of the coronary microcirculation to grow in conjunction with the increasing LV mass [185]. The above-mentioned mechanisms could potentially contribute to myocardial ischemia and, ultimately, scarring. Microvascular dysfunction could lead to contractile impairment.

In a study by Kohli and associates [193], echocardiographic results from patients with LV systolic dysfunction were compared with results from 60 healthy volunteers (30 white and 30 black). Five of the volunteers (4 black and 1 white) fulfilled 1 or more of the echocardiographic criteria for LVNC (as listed in Table 13.4), raising the question of whether the imaging criteria for LVNC are too sensitive. A similar observation was made in a recent CMRI study of 323 participants in the Multi-Ethnic Study of Atherosclerosis who had no history of LVNC, cardiac disease, or hypertension and who showed no DE on CMRI examination [194]. Of these 323 participants, 140 (43 %) had a noncompaction:compaction (NC:C) ratio of >2.3 in at least 1 region, and 20 (6 %) had an NC:C ratio of >2.3 in >2 regions. An NC:C ratio of >2.3 was most frequently located in the apical anterior, apical septal, and apical lateral wall.

Cardiac Amyloidosis

Cardiac amyloidosis refers to the extracellular deposition of low-molecular-weight, insoluble amyloid fibrils in a β-pleated sheet configuration [195]. There are three common subtypes of amyloidosis: (1) AL amyloidosis, or primary amyloidosis, which is characterized by the accumulation of monoclonal immunoglobulin light chains and has the worst prognosis; (2) ATTR amyloidosis, which can be caused by either a transthyretin gene mutation, leading to hereditary amyloidosis, or by wild-type transthyretin deposits, resulting in senile systemic amyloidosis; and (3) systemic AA amyloidosis, which involves amyloid fibrils that arise from serum amyloid A protein and can complicate chronic inflammatory diseases [195].

The noninvasive-imaging findings that are commonly produced by surface echocardiography in patients with cardiac amyloidosis—such as biventricular hypertrophy, normal or small ventricular size, bi-atrial enlargement, and septal thickening [196]—can also be optimally evaluated by CMRI. A CMRI study by Fattori and colleagues [197] showed that the interatrial septum, right atrial free wall, and RV free wall were significantly thicker in patients with cardiac amyloidosis than in patients with HCM. In addition, approximately half of the patients with cardiac amyloidosis also had pericardial and or pleural effusions.

In a study by Maceira and associates [111], 30 patients who had histologically proven amyloidosis with cardiac involvement, diagnosed predominantly by echocardiographic criteria, were all assessed by late Gd enhancement CMRI. Twenty (69 %) of these patients had global and diffuse subendocardial DE after Gd-chelate administration that was evident in the right and left ventricles. This pattern of subendocardial DE has also been observed in other studies [198–200]. In Maceira’s study [111], the Gd washout from the myocardium and the blood was also faster than normal, probably because of the increased extracellular space and the increased distribution of Gd into the total amyloid load (the use of 0.1 mmol/kg of Gd could also have contributed to this finding). Therefore, the DE-MRI sequence produced similar myocardial and blood T₁ values in patients with amyloidosis, so the blood pool had a persistently low signal intensity, similar to that of the myocardium [111]. At our institution, we have noted that in patients who are subsequently confirmed to have cardiac amyloidosis, the “nulling-time” in the DE-MRI sequence necessary to suppress the myocardial signal is difficult to achieve, resulting in widespread patchy enhancement in the entire myocardium (Fig. 13.20). This finding is consistent with an increase in extracellular space due to amyloid protein deposition in the heart, rather than true myocardial fibrosis [198]. The same observation has been reported by other investigators [198, 201]. In a study of patients with amyloidosis, CMRI was performed in a subset of patients who had histologically proven cardiac sarcoidosis. Abnormal DE was found in 34 of the 35 patients (97 %); 21 of these patients (60 %) had global transmural DE, and 8 (23 %) had global subendocardial DE [198]. The location of DE matched the distribution of interstitial amyloid in the histologic specimens.

Endomyocardial biopsy is considered the gold standard for the diagnosis of cardiac amyloidosis [202]. A series of 33 consecutive patients who had congestive heart failure with a restrictive filling pattern underwent endomyocardial biopsy and a comprehensive CMRI examination [199]. The biopsy confirmed the diagnosis of cardiac amyloidosis in 15 of the patients. In this study, the typical DE pattern for cardiac amyloidosis was circumferential subendocardial with various degrees of extension into the adjacent myocardium. Using endomyocardial biopsy as the standard, the sensitivity, specificity, positive predictive value, and negative predictive value of DE-MRI were calculated to be 80, 95, 92, and 85 %, respectively [199]. In a recent study by Austin and colleagues [203], a subset of patients with suspected cardiac amyloidosis (38 of 47 patients) underwent endomyocardial biopsy and complete CMRI study, including DE-MRI. The diagnostic utility of DE was compared with that of low-voltage ECG (Carroll’s ECG criteria), abnormal deceleration time on Doppler echocardiography (≤150 ms), and the combination of ECG and deceleration-time criteria. The presence of DE had the highest diagnostic accuracy of the noninvasive imaging parameters tested. The sensitivity, specificity, positive predictive value, and negative predictive value of DE-MRI, using endomyocardial biopsy as the standard, were 88, 90, 88, and 90 %, respectively. These results are in agreement with those published by Vogelsberg and colleagues [199].

Besides using DE-MRI to evaluate suspected cardiac amyloidosis, various investigators have also relied on measured T₁ times and estimates of ECF due to extracellular space expansion to diagnose cardiac amyloidosis [111, 200, 204, 205]. Krombach and colleagues [200], using a 1.5 T MRI scanner, found that a patient with biopsy-proven cardiac amyloidosis had substantially higher myocardial T₁ times (1,387 ± 63 ms) than did a group of individuals without known myocardial disease (1,083 ± 33 ms). In another study, the T₁ times were longer in 19 consecutive patients with cardiac amyloidosis than in ten healthy control subjects (1,340 ± 81 vs 1,146 ± 71 ms; P <0.0001) [200]. A T₁ time of 1,273 ms was determined to be the best discriminatory value for diagnosing cardiac amyloidosis by ROC analysis, with a sensitivity of 84 % and a specificity of 100 %. Using a 3 T MRI scanner, Mongeon and colleagues [204] quantified myocardial ECF in 14 patients with biopsy-proven cardiac amyloidosis. The median ECF in amyloid patients was 0.49, whereas the median ECF in healthy subjects was 0.24 (P = 0.0001). The increase in myocardial ECF (reflecting the expansion of the extracellular matrix) is due to either amyloid protein accumulation in the interstitial space or fibrosis, both of which can lead to an increase in Gd uptake [111, 204]. In patients with cardiac amyloidosis, obtaining high-quality DE images could prove to be technically challenging because of the rapid myocardial uptake of Gd and the associated difficulty of selecting the correct inversion time for accurate “nulling” of the myocardium. The measurement of myocardial ECF could overcome this difficulty and provide an objective means to quantify the degree of amyloid deposition, fibrosis, or both. Another possible use of the ECF measurement could be disease monitoring after the treatment for amyloidosis has been administered.

Several recent studies have indicated that DE could be an adverse prognosticator for cardiac amyloidosis [203, 206, 207]. In a study by Ruberg and colleagues [206], which involved 28 patients with systemic AL amyloidosis, 19 patients were found to have DE. Although the presence of DE did not predict mortality (during the median follow-up time of 29 months), the amount of DE, expressed as a percentage of the LV mass, was the strongest independent predictor of the B-type natriuretic peptide level when compared to other LV quantitative factors. In another group of patients with AL amyloidosis, 23 (79 %) of the 29 patients who underwent CMRI had DE [207]. After 26 ± 13 months of follow-up, 14 of those 23 DE-positive patients had died, whereas none of the patients without DE had died (P = 0.0061). Potential mechanisms proposed by the authors to account for these adverse outcomes in patients with AL amyloidosis included systolic and diastolic dysfunction, underlying myocardial ischemia with perivascular and interstitial deposition of amyloid proteins, and endothelial dysfunction [207]. Like Ruberg and colleagues, Maceira and associates [208] did not identify DE as a significant predictor of mortality (only a trend was seen); however, the difference between the T₁ values of the subepicardium and subendocardium, as measured 2 min after Gd administration, was able to predict mortality with 85 % accuracy when a threshold value of 23 ms was used. The close T₁ values between the subepicardium and subendocardium probably reflect the underlying myocardial amyloid burden.

Takotsubo (Stress) Cardiomyopathy

Takotsubo cardiomyopathy (TCM), also known as stress cardiomyopathy, has been increasingly recognized as a differential diagnosis of acute coronary syndrome. Patients typically present with chest pain, ECG changes, and mild elevation of cardiac enzymes; these changes are often precipitated by emotional stress in the absence of significant CAD [209–211]. Classically, there is a “balloon-like wall-motion” abnormality that predominately involves the LV apex and the distal left ventricle [212]. However, different subtypes have been described, including “reverse TCM,” in which the base of the left ventricle has a wall-motion abnormality and the apex is hyperdynamic; the mid-LV variant, in which the base and apex of the left ventricle are spared; and the localized variant, which affects various segments of the left ventricle [209, 213]. Furthermore, although the left ventricle is usually affected, involvement of the right ventricle has also been well described [214–217]. The Mayo Clinic’s diagnostic criteria for TCM, as suggested by Hurst and colleagues [209], are outlined here with modifications: (1) transient wall-motion abnormalities of the LV apex and mid segment that involve more than a single vascular bed (although, as detailed above, wall-motion abnormalities can occur elsewhere), (2) absence of CAD or angiographic evidence of plaque rupture, (3) new ECG changes and elevation of myocardial biomarker levels, and (4) absence of pheochromocytoma and myocarditis.

Various CMRI sequences are helpful in the assessment of TCM. The cine SSFP sequence allows excellent morphological evaluation of both the left and right ventricles, especially the apices, and accurate quantitative assessment of biventricular function. The spin echo sequence with fat saturation can be used to evaluate myocardial edema. In addition, DE-MRI detection of myocardial fibrosis and necrosis is especially important in the differential diagnosis of acute coronary syndrome and TCM.

Eitel and colleagues [217] conducted 1 of the largest studies of TCM, which included 256 patients, 239 (93 %) of whom had CMRI data available. Two hundred seven patients (81 %) were postmenopausal women, 225 patients (88 %) presented with symptoms consistent with acute coronary syndrome, and 182 patients (71 %) had experienced a significant stressful event within the 48 h before presentation. Classical apical ballooning was detected by CMRI in 197 patients (82 %); in 81 patients, both the left and right ventricles were involved. In this study, patients with both RV and LV involvement (suggesting more extensive disease) were older, were hospitalized longer, and had lower LVEFs than patients without RV involvement (43.1 % vs 45.3 %; P <0.001). These findings are similar to those reported by other authors [214, 216]. Eitel and colleagues also detected evidence of myocardial edema in 162 of the 199 patients (81 %) who had dedicated imaging sequences performed; the location of myocardial edema was similar to the distribution of wall-motion abnormalities in the left ventricle. Furthermore, when the signal intensity threshold was limited to between 3 and 5 standard deviations, 22 patients (9 %) were shown to have focal or patchy DE, but the DE was not identified in follow-up CMRI examinations. Patients who were DE-positive tended to have higher cardiac troponin levels at presentation, although no significant difference was noted in the quantitative functional values between the DE-positive and DE-negative groups. Associated CMRI findings included pleural effusion (more often in patients with biventricular involvement), pericardial effusion (more often in patients with myocardial edema), and ventricular thrombus (in four patients) [217].

Myocardial edema has been observed in patients with TCM [217–222]. In T₂-weighted images acquired by CMRI, the majority of TCM patients appear to have an increase in signal intensity with a transmural distribution that corresponds to the location of wall-motion abnormalities [217, 219, 221]. In the study by Eitel and colleagues [217], 158 patients (62 %) underwent follow-up CMRI (median follow-up period, approximately 3 months). A significant reduction in the T₂ ratio (signal intensity of myocardium/signal intensity of skeletal muscle) was noted, indicating an improvement in myocardial edema. Similarly, Neil and colleagues [220] observed a reduction in T₂ signal intensity at the 3-month follow-up CMRI study; however, the signal intensity in patients with TCM was still higher than the signal levels in healthy control subjects. In contrast to the other authors referenced above, Neil and colleagues noted that although the T₂ signal intensity was greater at the apex than at the base (P <0.0001) in patients with typical TCM, these patients still had a higher T₂-signal intensity at the base than did age-matched control subjects [220]. Various mechanisms have been proposed to explain the presence of myocardial edema in TCM patients, including inflammation, transient ischemia, and increased wall stress. In addition, high intraventricular pressure could cause a perfusion abnormality that could subsequently lead to edema [217]. Figure 13.21 shows typical findings from a TCM patient.

It is not clear whether DE is associated with TCM. Some reports indicate that DE is absent in this condition [223, 224], whereas others indicate the opposite [217, 221, 222]. When DE has been detected at presentation, it has always been noted to resolve on follow-up CMRI examinations. Some authors have noted that the DE signal intensity is lower in TCM patients than in MI patients. Furthermore, the distribution of DE in TCM patients has been found to be either focal or patchy and located where wall-motion abnormalities are present [217, 222]. In a study by Rolf and colleagues [222], endomyocardial biopsy and CMRI results from patients with TCM were available from both the acute and recovery phases. The biopsy results revealed no signs of oncotic or apoptotic cell death in the acute phase [222, 225]. The amount of collagen-1, which reflects fibrosis, was significantly higher in DE-positive patients than in patients without DE. During the recovery phase, the amount of collagen-1 returned to normal in parallel with the resolution of DE, suggesting that these findings could represent reactive fibrosis rather than replacement fibrosis [222].

Chagas Disease

Chagas disease is caused by the parasite Trypanosoma cruzi. Symptoms are present during the acute phase, which lasts approximately 6–8 weeks. The majority of patients then enter a chronic yet asymptomatic period known as the indeterminate phase. Many years later, certain patients develop chronic symptoms, entering the chronic symptomatic phase. The manifestations of chronic Chagas disease mainly involve the cardiac and digestive systems. The most serious manifestation, Chagas cardiomyopathy, primarily presents as arrhythmia, cardiac failure, and thromboembolism [226].

In the acute phase, the T. cruzi organism can be detected by parasitologic tests, whereas in the chronic phase, serologic tests are used to detect antibodies against T. cruzi antigens [227]. Cardiac MRI can be used for morphologic evaluation of Chagas cardiomyopathy and for accurate quantitative assessment of the cardiac chambers. In multivariable analysis, left atrial volume, as determined echocardiographically, has been shown to be a powerful independent prognosticator for cardiac death or transplantation in patients with Chagas cardiomyopathy (HR, 1.04 per 1-mL/m² increase; 95 % CI, 1.018–1.056; P <0.001) [228]. Furthermore, LV systolic dysfunction has consistently been shown to affect the outcome in these patients [226, 229].

A recent CMRI study included 67 patients who were serologically positive for Chagas disease. The patients were divided into three groups: group I (n = 27) had no ECG or echocardiographic abnormalities, group II (n = 19) had only ECG abnormalities, and group III (n = 21) had both ECG and echocardiographic abnormalities [230]. Compared to the other groups, group III had the lowest LVEF, largest end-diastolic and end-systolic volumes, and largest atrial size. In addition, the amount of DE expressed as the percentage of the LV mass was the greatest in group III (12.4 %) versus group I (4.0 %) and group II (3.7 %). Wall-motion abnormalities and DE were more frequently observed in the apical segment and in the inferolateral walls, and the distribution of DE in this cohort was heterogeneous (27 % subendocardial, 14 % midwall, 23 % subepicardial, and 36 % transmural) [230]. Rochitte and colleagues [231] noted similar CMRI findings. In their study, patients with Chagas disease were also divided into three groups: (1) patients with indeterminate disease, (2) those with known Chagas cardiomyopathy, and (3) those with known Chagas cardiomyopathy and a history of ventricular tachycardia. When the three groups were compared, patients with indeterminate disease had the highest LVEF (65.7 ± 7.5 %) and the lowest amount of DE (0.9 ± 2.3 %), expressed as a percentage of the LV mass, whereas patients with Chagas cardiomyopathy and a history of tachycardia had the lowest LVEF (32.3 ± 12.7 %) and the highest amount of DE (25.4 ± 9.8 %), (P <0.001 for both comparisons). Delayed enhancement was predominantly located in the LV apex and in the inferior and inferolateral walls. De Mello and colleagues [232] noted that the relative risk of ventricular tachycardia in patients with Chagas cardiomyopathy was 4.1 when transmural DE was present in two or more segments, whereas the relative risk was 2.8 when the LVEF was ≤40 %.

In patients with Chagas cardiomyopathy, aneurysmal dilation is commonly seen at the apex or at the basal inferolateral wall; this can be seen well by using the state-of-the-art SSFP CMRI sequence. Any associated thrombus can also be easily detected by CMRI [233]. The LV apex (located at the terminal circulation of the left anterior descending [LAD] and right coronary artery [RCA]) and the basal inferolateral wall (located at the terminal circulation of the left circumflex artery and the RCA) are postulated to be the preferential sites of aneurysmal dilation because they may be especially prone to repeated ischemic insults due to chronic myocardial inflammation as various inflammatory mediators and cytokines are released [233].

Anderson-Fabry Disease

Anderson-Fabry disease (AFD) is an X-linked progressive disorder that is caused by a deficiency of the lysosomal enzyme α-galactosidase A, which leads to the accumulation of globotriaosylceramides (glycosphingolipids) [234]. Many organs can be affected, including the heart, kidneys, and brain. Enzyme replacement therapy (agalsidase alfa or beta) can stop the progression of the disease, and early treatment can prevent irreversible organ damage [234]. Cardiac manifestations of AFD include LV and/or RV hypertrophy, atrial dilation, valvular heart disease, conduction abnormalities, and diastolic dysfunction. Eventually, these patients can have congestive cardiac failure, an arrhythmia, or even an MI due to CAD [235].

The echocardiographic findings in AFD can be similar to those in symmetric HCM, making the differential diagnosis difficult. However, using DE-MRI, investigators have identified a DE pattern that appears to be specific to patients with AFD [236, 237]. Specifically, of the 13 patients with AFD who were imaged by De Cobelli and colleagues [236], 10 (77 %) had DE in the basal inferolateral wall (two patients also had DE that extended to the mid inferolateral wall, one patient also had DE in the mid inferior wall, and one patient also had DE in the apical lateral wall) [236, 237]. In a study of AFD patients by Moon and colleagues [237], CMRI revealed DE in half of the 18 males and half of the 8 females; in 92 % of these patients, DE occurred in the basal inferolateral wall. In addition, in both studies described above, the location of DE was mid myocardial, always sparing the subendocardium and the subepicardium. When Imbriaco and colleagues [238] compared normal healthy volunteers to HCM patients and AFD patients, the T₂ relaxation time was longest in the patients with AFD for all myocardial regions (measurements were obtained in the septum, apex, and lateral wall). The median T₂ values in the lateral wall were 52, 68, and 79 ms, respectively (P <0.001). The authors postulated that the prolonged T₂ relaxation time might be related to the myocardial glycolipid deposits. When 11 patients with AFD were treated with agalsidase beta for 45 months, the LV mass, LV wall thickness, and average T₂ relaxation times were significantly reduced after treatment [239]. No significant change in LVEF was noted, but the Mainz Severity Score Index (an objective assessment of the severity of AFD) significantly decreased after treatment. Another study of the effects of enzyme replacement therapy with agalsidase beta resulted in similar CMRI findings, but no change in LV systolic function was observed; however, the New York Heart Association functional class did improve during follow-up observation [240].

Iron Overload Cardiomyopathy

Iron overload cardiomyopathy (IOC) is defined as a systolic or diastolic dysfunction due to increased myocardial iron deposition [241]. In patients with iron overload, the transferrin saturation is >55 %, and the serum ferritin level is usually >200 μg/L in women and >300 μg/L in men [242]. Significant myocardial siderosis is generally defined as a myocardial T₂* value of <20 ms when assessed by CMRI with the T₂* imaging sequence [116, 243]. The causes of IOC can be divided broadly into two groups: those related to increased iron absorption (primary hemochromatosis) and those related to frequent blood transfusion therapy (secondary hemochromatosis). Anemia requiring blood transfusion can be hereditary, as in thalassemia and sickle cell disease, or can be acquired, as in myelodysplastic syndromes and myeloproliferative disorders [244]. The main organs that are affected by severe iron overload include the heart, endocrine glands, and hepatic system. Significant myocardial iron deposition causes cardiac failure and death at a young age. In fact, IOC is responsible for a third of all deaths in patients with hereditary hemochromatosis, is a major cause of death in patients with secondary hemochromatosis, and is the leading cause of mortality in patients with thalassemia major [245]. Without proper iron chelation therapy, most patients with IOC will develop a dilated phenotype, which is characterized by LV remodeling and LV systolic dysfunction, as well as LV and RV enlargement. In contrast, a minority of these patients (<10 %) will develop a restrictive phenotype, which is characterized by biatrial enlargement, a restrictive LV filling pattern, RV enlargement, and a preserved LVEF [245].

Echocardiography can be used to evaluate ventricular and atrial chamber size and to assess both systolic and diastolic function indices by using traditional and novel imaging parameters [246]. Chamber enlargement or LV systolic dysfunction may indicate advanced disease [116]. Echocardiography cannot detect the presence or absence of myocardial siderosis, much less quantify it. Serum ferritin also cannot be used to predict the myocardial iron content [116]. Tissue biopsy is considered the gold standard for the diagnosis of IOC. However, tissue biopsy is an invasive procedure, and the patchy distribution of myocardial siderosis may be missed by this technique.

Currently, CMRI is the only noninvasive means for quantifying myocardial siderosis. The Royal Brompton Hospital group pioneered the use of T₂* for the diagnosis of IOC [116]. Figure 13.22 shows an example of findings for a patient with both hepatic and myocardial siderosis. In the United Kingdom, the use of T₂* CMRI to identify IOC appears to have improved the survival of patients with thalassemia major because they received timely intensification of chelation therapy, thereby preventing myocardial iron overload [247]. Between 1980 and 1999, these patients’ rate of death due to iron overload was 7.9 per 1,000 patient-years, whereas between 2000 and 2003, it was 2.9 per 1,000 patient-years—a 71 % reduction (P <0.05). In the original CMRI T₂* assessment of patients with thalassemia major by Anderson and colleagues [116], all patients with ventricular dysfunction had a T₂* value of <20 ms, leading the authors to suggest that values above this threshold indicate the presence of substantial myocardial siderosis. Wood and colleagues [248] subsequently sought to confirm the utility of T₂* measurements for quantifying myocardial iron by using an iron dextran-loaded gerbil model. Myocardial and hepatic T₂* measurements were taken both in vivo and ex vivo. The investigators found a strong correlation between 1/T₂* and myocardial iron (wet weight) (r² = 0.94).

T₂* measurement can accurately quantify myocardial iron levels in IOC patients. This is important for assessing the risk of cardiac complications and tailoring the appropriate iron-chelating treatment [119]. Furthermore, T₂* measurement allows the early detection of myocardial siderosis, which may be reversible if aggressive chelation therapy is commenced early enough [243]. In addition, T₂* measurement can be used to monitor the chelating regime in these patients. Excellent review articles by Kremastinos and colleagues [245] and by Gujja and coauthors [246] provide management algorithms that are guided by CMRI T₂* measurements for patients with IOC.

In a recent CMRI study of more than 1,400 patients with thalassemia major, myocardial T₂* imaging was performed [249]. At the time of examination, the majority of patients had no arrhythmias or clinical signs of congestive heart failure. Of the patients with a T₂* value of <6 ms, 47 % developed heart failure and 14 % developed an atrial or ventricular arrhythmia within 1 year of scanning. Table 13.6 shows the risk of developing congestive heart failure or an arrhythmia in relation to the patient’s T₂* value [249]. In another study, 15 patients with thalassemia major underwent a combination chelating treatment with subcutaneous deferoxamine and oral deferiprone [243]. At 1-year follow-up evaluation, the myocardial T₂* values had increased from 5.7 ± 0.98 to 7.9 ± 2.47 ms (P = 0.010), and the LVEF had increased from 51.2 ± 10.9 % to 65.6 ± 6.7 % (P <0.001).

Table 13.6 Risk of congestive cardiac failure and arrhythmia in patients with thalassemia major, as assessed by cardiovascular magnetic resonance imaging using the T₂* sequence

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Magnetic Resonance Angiography of Coronary Artery Disease

According to the American Heart Association, more than one million inpatient diagnostic cardiac catheterizations were performed in 2009 [250]. The American College of Cardiology Foundation and the Society for Cardiovascular Angiography and Interventions estimates that the rate of normal findings or insignificant CAD varies from 20 to 39 % [251, 252]. Conventional coronary angiography is still the gold standard for assessing coronary artery stenosis. Nevertheless, angiography is an invasive procedure with a small, yet inherent risk of complications, including MI, stroke, arrhythmias, and death [251]. A noninvasive imaging method that could directly assess the integrity of the coronary lumen would be desirable.

Over the past decade, coronary CT angiography (CCTA) has been shown to be a robust noninvasive modality in the assessment of CAD. In a meta-analysis comparing 64-slice CCTA with diagnostic angiography, the patient-based pooled sensitivity, specificity, and positive and negative predictive values were 99, 89, 93, and 100 %, respectively; in a segment-based analysis, these same values were 90, 97, 76, and 99 %, respectively [253]. During CCTA, patients are exposed to radiation in the form of iodinated contrast agents. For high-quality results, the heart rhythm must be slow and regular.

Coronary magnetic resonance angiography (CMRA) was first described by Paulin and colleagues [254] using spin-echo sequences (1987) and by Wielopolski and colleagues [255] using gradient-echo sequences (1995). Since then, CMRA imaging has undergone significant improvement with respect to coverage, spatial resolution, and acquisition time, as well as the use of Gd-based contrast agents to further improve image quality. Table 13.7 shows a comparison between CMRA and CCTA.

Table 13.7 Comparison of coronary magnetic resonance angiography and coronary computed tomography angiography

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This review provides a brief synopsis of the basic technical aspects of CMRA, as well as some common indications for its use.

Technical Aspects

Because of its high spatial resolution (approximately 0.1–0.2 mm) and high temporal resolution (10–20 ms), cardiac catheterization with coronary angiography is considered the gold standard for evaluating CAD [256]. This approach can also allow therapeutic intervention in the same setting. Several technical challenges involved in noninvasive imaging of the coronary arteries with CMRA include the need for (1) a relatively high spatial resolution for evaluating stenosis; (2) a high temporal resolution and fast imaging time to “freeze” the fast-moving arteries; (3) methods for overcoming patient respiratory and cardiac motion, which affect the quality of the image; and (4) adequate coverage of the entire coronary tree. An excellent review of the technical aspects of CMRA has been published by Flamm and Muthupillai [257].

Imaging of epicardial coronary arteries is demanding because of their relatively small diameter (2–5 mm). The scan time for a 2D image is governed by the repetition time (ms) × the number of phase-encoding steps. Using a fixed field of view, one may increase the spatial resolution by increasing the number of phase-encoding steps and, therefore, prolonging the scan time. The introduction of parallel imaging techniques, such as SENSE, allows either a faster imaging time or increased spatial resolution [258]. In addition, a state-of-the-art phase-array coil, such as a 32-channel cardiac coil, allows the use of SENSE in two directions. Compared to the standard 5-channel cardiac coil, the 32-channel coil reduces the imaging time by almost 50 % while offering comparable imaging quality. Alternatively, it can improve spatial resolution at the expense of increased imaging time [259]. With a 1.5 T scanner, 32-channel coil, and Gd infusion, the imaging spatial resolution was 1.1 × 1.1 × 1.5 mm³ in a series recently reported by Nagata and colleagues [259]. With a 3 T scanner, 32-channel coil, and slow gadobenate dimeglumine infusion, Yang and associates [260] obtained a spatial resolution of 1.1 × 1.1 × 1.3 mm³. The rest period for coronary arteries is short, ranging from 66 to 333 ms (mean, 161 ms) for the left coronary artery (LCA) and from 66 to 200 ms (mean, 120 ms) for the RCA [261]. A coronary artery can move several centimeters during the cardiac cycle, and peak velocities of 2 cm/s have been documented during certain phases, such as rapid cardiac contraction and relaxation. As a result, high temporal resolution is necessary to obtain a motionless image [254]. In CMRA, the data are synchronized with the electrocardiogram and are usually acquired in the telediastolic phase (55–75 % of the R-R interval, or approximately −300 to −400 ms before the electrocardiographic R wave), when cardiac motion is usually minimal. In patients with a higher heart rate, the telesystolic phase may be used (25–35 % of the R-R interval, or approximately 200–300 ms after the R wave), when cardiac contraction is limited during isovolumic contraction. Because the RCA has a shorter rest period than the LCA [261], the trigger delay (selected time at which to start imaging during the cardiac cycle) is determined by performing a free-breathing cine SSFP sequence in the axial orientation involving a high number of cardiac phases. Both the trigger delay and the duration in which the RCA is relatively stationary are then determined at the imaging console [255].

To suppress respiratory motion, two approaches may be used. In one approach, different segments of the coronary arteries are imaged separately during different breath-holds after an initial low-resolution, 3-dimensional (3D) scan is performed in a single breath-hold [262]. This approach is termed volume coronary angiography with target scans. The other method for suppressing respiratory motion is respiratory gating, which uses a prospective real-time navigator to measure displacement of the coronary arteries due to breathing; the data are then accepted or rejected on the basis of displacement [263]. In this approach, a navigator pencil beam is usually placed on the dome of the right hemidiaphragm, along the liver-lung boundary in the foot-to-head direction, to monitor respiratory motion. An acceptance window is set, usually between 3 and 6 mm. Data will be accepted if they fall into this window; otherwise, the data will be rejected and remeasured in other cardiac cycles. The advantage of respiratory gating is that images are collected with free breathing, so a high spatial resolution and signal-to-noise ratio can be achieved. The disadvantages are the longer imaging time, sensitivity to irregular breathing patterns, and individual variations in the relationship between diaphragmatic and cardiac motion. As the motion of the heart is less than that of the diaphragm during breathing, a scale factor of 0.6 is commonly applied [264]. Some investigators also ask the patient to wear an abdominal belt to suppress diaphragmatic motion during breathing [259, 265].

In 2003, Weber and colleagues [266] introduced the 3D whole-heart imaging approach, using parallel imaging to reduce the scan time and an SSFP imaging sequence to improve the image quality. In the whole-heart approach, the entire heart is imaged from the origins of the coronary arteries to the cardiac base. Planning can easily be done on the basis of information obtained in the survey scan, thereby reducing the time required in the target-volume approach for coronary artery orientation planning. The trigger delay is determined by a high-temporal-resolution cine scan, as described above, and whole-heart data are acquired with a free-breathing technique. In the original report, the spatial resolution was 1 × 1 × 1.5 mm³, with a mean imaging time of 13:48 min (range, 7:18–20:18 min).

Whole-heart CMRA is currently the most widely used approach in coronary imaging [7, 260, 264, 267, 268]. Steady-state free-precession sequences are preferred with the 1.5 T scanner, which utilizes fully refocused gradients in each repetition time to maintain steady-state transverse magnetization. Therefore, the signal-to-noise and contrast-to-noise ratio is higher than with the traditional spoiled GRE sequence, as the GRE sequence has limited blood-inflow–related signal enhancement. However, a major challenge of using SSFP sequences in 3 T compared to 1.5 T imaging is sensitivity to increased B₀ and B₁ inhomogeneities [269]. With the increases in field strength, the specific absorption rate is also increased, so the repetition time must be longer. The imaging flip angle also needs to be higher to obtain sufficient contrast between the blood and myocardium. A gradient-echo sequence is the modality of choice in 3 T CMRA.

A magnetization-prepared, T₂-weighted sequence (T2-prep) is commonly used to accentuate the difference between the T₂ values of arterial (250 ms) and venous (35 ms) blood and myocardium (50 ms), thus suppressing signals from the myocardium and venous structures [270]. In addition, to suppress signals from surrounding fat, a spectrally selective fat-saturation pulse is applied before data acquisition.

Acquired Coronary Artery Disease

In 2001, Kim and colleagues [271] described a single-vendor, multicenter CMRA study in which a 1.5 T scanner without Gd contrast was used with a targeted approach and a respiratory navigator free-breathing technique. A total of 109 patients were imaged, and the results were compared to those of invasive coronary angiography (in which significant stenosis is defined as ≥50 % luminal narrowing). The prevalence of CAD was 59 %. The CMRA analysis was limited to the proximal coronary segments. The mean coronary coverage was approximately 6 cm for the LAD, 8 cm for the RCA, and 3 cm for the left circumflex artery. Based on the consensus of two expert readers at the core laboratory, the sensitivity, specificity, and accuracy of CMRA for detecting significant CAD were 93, 42, and 72 % respectively. For the detection of left main coronary artery (LMCA) or three-vessel disease, the sensitivity, specificity, and accuracy were 100, 85, and 87 %, respectively. The results were poorer for individual arteries. The limitations of the study included the long imaging time, the fact that only 84 % of the segments could be evaluated, and the limited coronary coverage.

Since Kim’s report was published, advances in hardware and software, as well as the introduction of the SSFP sequence, have significantly improved the ability of CMRA to detect CAD. Table 13.8 summarizes some selected recent publications about the use of CMRA with the whole-heart approach versus invasive angiography as the gold standard for diagnosing CAD [259, 260, 265, 267, 268]. Interestingly, the included publications predominantly concern the Asian population, whose relatively low body-mass index (mean, <25 kg/m²) could certainly affect image quality. As Nagel indicated in his editorial [272], the results of the CMRA studies included in Table 13.8 were comparable to those observed in the recently published CORE-64 CCTA study [273, 274]. In the original report of this study, 291 patients with an Agatston calcium score of <600 were analyzed by using invasive coronary angiography as the reference standard. The sensitivity, specificity, and positive and negative predictive values were 0.85, 0.90, 0.91, and 0.83, respectively [274]. The subsequent report concerned another 80 patients with Agatston calcium scores of ≥600 who were not included in the original report; in their cases, the sensitivity, specificity, and positive and negative predictive values were 0.94, 0.44, 0.93, and 0.50, respectively [273]. When both groups were combined, the overall sensitivity, specificity, and positive and negative predictive values were 0.88, 0.87, 0.92, and 0.81 [273].

Table 13.8 Diagnostic accuracy of coronary magnetic resonance angiography for significant coronary artery disease: results of selected studies

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For evaluating the coronary anatomy in patients with chest pain and an intermediate pretest likelihood of CAD, CCTA is probably the noninvasive imaging modality of choice, due to its short imaging time (from 5 to 6 s of breath-holding in a 64-detector CT scanner to 1 s of breath-holding in a 320-detector or dual-source CT scanner) [275]. Moreover, CCTA is preferred despite the need for ionizing radiation and iodinated contrast, as well as a slow, steady heart rate to achieve high-quality data and minimize radiation. Compared to CMRA, all CT scanners with 64 or more detectors have detector widths of 0.5–0.625 mm and, therefore, offer high isotropic spatial resolution. However, in patients who have a true allergy to iodine contrast or who have significant calcification, CMRA could play a role in the assessment of coronary stenosis [276]. In the American Heart Association’s 2008 scientific statement regarding CMRA and CCTA [277], the committee recognized that the majority of reports were published by experienced physicians at academic centers. The committee concluded that “the utility of coronary MRA in general practice has not been established, and multivendor trials have not been conducted.”

Most CMRA studies have been performed without Gd contrast administration. Hu and colleagues [7] evaluated the use of 0.2 mmol/kg of gadobenate dimeglumine injected at 2 cc/s during whole-heart CMRA with a 1.5 T scanner. The signal-to-noise and contrast-to-noise ratios were significantly higher in the contrast study than the noncontrast study, which were performed 1 week apart in each subject. In addition, the coronary artery vessel length was greater in the contrast study than the noncontrast study. Myocardial suppression was also improved with the application of a nonselective inversion pulse. Similarly, in 3 T CMRA, the use of Gd improves the signal-to-noise and contrast-to-noise ratios, especially when the GRE technique is the preferred imaging sequence [263, 268]. With the addition of an inversion pulse, the contrast between blood and background tissue is higher; with the higher spatial resolution, 3 T CMRA can assess smaller and more distal vessels (up to 1.5 mm in diameter) [263, 268].

Kawasaki Disease

Kawasaki disease (KD) is an acute necrotizing vasculitis of medium and small vessels that affects mostly infants and young children. It is characterized by fever, a rash, bilateral nonexudative conjunctivitis, erythema of the lips and oral mucosa, cervical lymphadenopathy, and changes in the extremities [278]. Organs that may be affected by this systemic disease include the heart, liver, lungs, and kidneys. Coronary artery aneurysm or ectasia develops in 20–25 % of untreated patients and can lead to myocardial ischemia, infarction, and death [279].

Because coronary artery aneurysm or ectasia is a major potential complication of KD, noninvasive imaging of the heart and coronary arteries is an important part of the evaluation and follow-up of KD patients [278]. The American Heart Association and other professional societies recommend surface echocardiography as the imaging method of choice for these patients [278]. Although invasive coronary angiography is the gold standard for evaluation of the coronary anatomy, CMRA has been shown to be a noninvasive alternative that reduces radiation exposure and the potential complications of coronary angiography. In six patients with KD, Greil and colleagues [280] performed targeted 3D segmented k-space free-breathing gradient echocardiography with navigator gating, as well as CMRA and conventional coronary angiography. Compared to coronary angiography, CMRA correctly identified all coronary aneurysms (n = 11), coronary stenoses (n = 2), and coronary occlusions (n = 2). In addition, there was close agreement between CMRA and conventional coronary angiography with regard to the proximal coronary artery diameter and the distance from the coronary ostium to the origin of the aneurysm.

A recent review has suggested that CCTA, rather than CMRA, may be the preferred imaging method for monitoring the evolution of KD over time, with regard to both aneurysmatic dilatation and mural thrombosis [281]

Magnetic Resonance Imaging of Congenital Coronary Artery Anomalies

In a frequently quoted analysis of 125,595 patients who underwent invasive coronary angiography at the Cleveland Clinic Foundation [282], the following incidences of congenital coronary artery anomalies (CAAs) were documented: LMCA, LAD, or RCA arising from the pulmonary artery, 0.01 %; LMCA arising from the right sinus of Valsalva (RSV), 0.018 %; LAD arising from the RSV, 0.03 %; and RCA arising from the left sinus of Valsalva (LSV), 0.108 %. More recent, unbiased CMRA studies of a large general population in adolescents revealed that 0.7 % of the subjects had important CAAs (see below) [283]. Machado and colleagues [284] were probably the first to report the use of MRI to visualize an anomalous RCA with T₁ spin-echo sequences. McConnell and associates [285] used 3D breath-hold, ECG-gated, segmented k-space, gradient-echo sequences to evaluate 15 patients with CAAs. All 15 patients had anomalous CAAs on invasive angiography, and CMRA correctly identified 14 of them. Post and colleagues [286] studied 37 patients by means of breath-hold 2D fast gradient-echo images; 19 patients were known to have anomalous coronary artery origins, and the remainder of the patients (the control group) had normal coronary origins. The CMRA images were reviewed blindly by two observers, and two other independent cardiologists reviewed the invasive angiograms separately. In the final consensus interpretation by all readers, CMRA had a sensitivity and specificity of 100 % in identifying the origins and proximal course of the anomalous arteries [286]. Because some patients may not be able to perform adequate breath-holding, current CMRA performed for CAA assessment is usually achieved with 3D gradient-echo sequences using a respiratory navigator technique, thus allowing the data to be acquired during free breathing. As the majority of clinical examinations are requested for suspicion of anomalous coronary origin without a concern for underlying arterial stenosis, coverage could be limited to the aortic root and proximal ascending aorta, with a relatively generous in-plane spatial resolution of 1–1.5 × 1–1.5 mm. With a navigator efficiency of 30–50 % and a cooperative patient, the dataset could be acquired in less than 5 min in most instances.

In the 2006 multi-group Appropriateness Criteria for CCTA and Cardiac MRI, the use of CMRA for assessment of suspected CAAs is graded as appropriate, with an average score of 8 (a score of 7–9 indicates an appropriate test for a specific indication) [2]. In addition, the 2008 American Heart Association scientific statement gives a Class IIa, evidence level B, recommendation for the use of CMRA or CCTA in evaluating CAAs [277]. This rating is presumably due to the relatively limited number of studies, which have mostly been confined to single centers.

Most CAAs diagnosed by CMRA are accidental findings that are discovered during the investigation of coronary atherosclerotic disease in the adult. The following section discusses (1) the general outline of the entity known as CAA (Table 13.9); (2) the use of MRI for screening young people to evaluate the risk of SCD; (3) the use of MRI for evaluating the severity of stenosis in some forms of CAA; and (4) the use of MRI versus CCTA versus intravascular ultrasonography or intravascular optical coherence tomography for imaging CAAs.

Table 13.9 Classification of coronary anomalies in human hearts

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General Outline of Coronary Artery Anomalies

Coronary artery anomalies are defined as coronary anatomic patterns that are outside the spectrum of normal [287, 288].

1.
“Normal” anatomic forms are generally defined as all the variants observed in more than 1 % of the general population.
2.
Clinically or prognostically “serious” CAAs are those that entail a high risk for SCD, especially in young persons and during exercise.
3.
Serious CAAs are distinguished by their capacity to produce ischemia and its consequences: chest pain, dyspnea, and syncopal equivalents, as well as sudden cardiac arrest or death [287, 288].
4.
In the diagnostic imaging of CAAs, the primary and fundamental objective is to properly identify the morphologic variants, especially those that have significant prognostic implications.

Classification of Coronary Artery Anomalies

Schemes for classifying CAAs are widely discussed in the current literature, but no final, widely recognized classification scheme has been agreed on. Table 13.9 shows a classification proposed by our group at the Texas Heart Institute [287], including a brief characterization of each form in terms of the demonstrated or likely mechanism of ischemia. Magnetic resonance angiography is quite able to identify most of these variants (Figs. 13.23–13.34).

Whereas 1–5 % of the general population has some type of CAA [287], less than 10 % of these “carriers” will experience ischemic consequences. The only CAAs that can potentially cause ischemic manifestations are those involving a mechanism of stenosis that varies in severity in each case. Here we will discuss the most important of these “potentially serious CAAs,” which occur in young persons and adults. Most other CAAs also need to be diagnosed properly because of the confusion that an incorrect diagnosis can generate.

Anomalous Coronary Artery Originating from the Opposite Sinus of Valsalva with an Intramural Course

In adults, the best characterized and most clinically serious type of CAA—the one that most frequently features the pathophysiologic mechanism of a high-risk CAA—involves an ectopic artery that originates from the opposite sinus of Valsalva (ACAOS) and has an intramural course inside the aortic root [287–291]. In this entity, typically either the RCA arises from the LSV (R-ACAOS), or the LCA arises from the RSV (L-ACAOS), and the affected artery pursues a preaortic course, running inside the aortic-wall media and crossing in front of the anterior aortic commissure before reaching its proper destination [289, 290]. This type of course has long been called “between the aorta and pulmonary artery, or interarterial” [287, 288]. Because of the recent discovery that this treacherous course is, however, always located inside the aortic wall and is essential to the nature of this entity, it has been suggested that previous names be changed to ACAOS and that the “intramural course” be underscored as the required common feature, which correlates with the prognosis and symptoms [287, 289–291]. At the embedded segment, the coronary artery is affected both in its internal circumference (which is typically smaller than that of the distal segment, leading to a characteristic but variable degree of hypoplasia, and is also laterally compressed, as shown in Figs. 13.30–13.34) and in its pulsatile cross-sectional area, which is further laterally compressed in systole and with exercise, while affected by aortic-wall phasic expansion during the cardiac cycle [289–291]. Unfortunately, such pulsatility of ACAOS vessels is poorly evaluated by CMRA protocols (which routinely show only end-diastolic compression and which lack the temporal and spatial definition for a similar evaluation), whereas the amount of stenosis that results from such mechanisms varies from case to case (Fig. 13.31). The intramural course of an LCA originating from the RSV is clinically and functionally much more important than that seen in cases of anomalous origin of the arteries from the opposite sinus with intraseptal, infundibular, or subpulmonic courses [287–290], which, by themselves, lack intrinsic stenotic effects (see Figs. 13.25, 13.26, and 13.28). Unfortunately, “intraseptal” anomalies are frequently confused with (intramural) ACAOS, partly due to the traditional label of interarterial or “between the aorta and pulmonary artery (or infundibulum).” Figures 13.25, 13.26, and 13.28 show the most common benign variants of anomalous origin of the LCA from the opposite sinus. The anterior, retroaortic, subpulmonic, or intraseptal courses are rarely associated with SCD [289–291]. The interarterial course in L-ACAOS is characterized by an LCA that arises either from the RCA or separately from the RSV, following a preaortic and intramural aortic course [283, 289–292], while the intraseptal variety of LCA arising from the RSV can be recognized easily on coronary angiography, CCTA, or CMRA because of the following specific features (Fig. 13.23a):

The LCA originates from the RSV—either with, or next to, the RCA—as in the case of an intramural course, but the LMCA trunk then travels anteriorly, inside the supraventricular crista and the upper or mid interventricular septum, and reaches the LAD at the proximal or mid trunk, where the LMCA becomes subepicardial, at the anterior interventricular sulcus (the proximal LAD provides the circumflex trunk).
The distal portion of the LMCA trunk courses inside the ventricular septum and tends to behave like a myocardial bridge: it may undergo mild systolic compression, and it usually provides a large first septal branch. Such an intraseptal course is generally considered benign and does not require treatment, although some patients with this condition have been erroneously sent to surgery with a confusing diagnosis. (This course is not “between the aorta and pulmonary artery” but runs “from the aorta to the RV infundibulum,” away from the aortic wall, as shown in Figs. 13.23 and 13.33).

Like CCTA, CMRA clearly allows a consistent and definitive diagnosis of an intraseptal course, ruling out the dangerous intramural variety (ACAOS). Other abnormal courses of ectopic coronary arteries (prepulmonic, retroaortic, or posterior to the atrioventricular valves) are also quite adequately and consistently identified by CMRA as generic types, even though this imaging technique cannot definitively document the cross-sectional area in a tomographic rendering (which indicates the severity of an individual case).

At the Texas Heart Institute, the Center for Coronary Artery Anomalies has recently been involved in a large, population-based screening study of 10,000 children [283]. This study includes generalized screening by a simplified CMRA protocol essentially designed to identify the prevalence of CAAs, cardiomyopathies, and high-risk electrocardiographic abnormalities in school children [283]. The protocol for this MRI-based screening study includes no intravenous line, Gd infusion, or sedation and involves a shortened acquisition time (approximately 5 min for studying the origin and proximal courses of the coronary arteries). In our initial 1,836 cases, the accuracy and consistency in identifying the proximal coronary anatomy (see the Introduction to this Section) has been in the range of 99 % [283]. Most types of CAAs can be clearly delineated by this method. However, in evaluating the severity of stenosis of the intramural course (a critical clinical necessity), CMRA does not have the space/time resolution necessary to quantify stenosis in such small arteries, which are phasically moving during the cardiac cycle (Table 13.7). Consequently, CMRA appears to be quite adequate and reliable in the first-stage screening of young populations for an increased risk of SCD (most such deaths in the context of CAAs occur during or after exertion in the young) [283, 291], but neither CMRA nor CCTA is precise enough to guide the clinical management of individual CAAs [291, 292]. The severity of an individual CAA seems to correlate with (1) previous symptoms (chest pain and/or dyspnea with exertion, syncope, cardiac arrest); (2) the results of stress testing (which has a low precision for quantifying risk in the young, due to a high incidence of false positive or negative results); and (3) the severity of stenosis of the ectopic coronary artery at the level of the intramural segment. For this purpose, more-predictive imaging can be performed by evaluating the cross-sectional area of the proximal ectopic vessel on CT-scan angiography [292]. For even more precision, one may (and probably should) in selected cases—involving patients with symptoms, stress-test abnormalities, syncope, or other high-risk characteristics—perform an invasive procedure such as intravascular ultrasonography [291] (Figs. 13.30 and 13.31) or optical coherence tomography (Fig. 13.32). These two techniques are able to provide tomographic cross-sectional images that are, respectively, 10 and 100 times more accurate than CMRA images [293–295].

Other Coronary Artery Anomalies: Imaging and Clinical Relevance

In addition to ACAOS, other CAAs may be found fairly often during screening or diagnostic CMRA in the adolescent or adult population. These entities are sometimes clinically significant but are more often benign. The three most frequent and/or important of them are coronary myocardial bridges (CMBs), coronary-cameral fistulas (CCFs), and anomalous left coronary artery arising from the pulmonary artery (ALCAPA).

Coronary Myocardial Bridges

Coronary myocardial bridges are frequently observed in anatomic, dedicated studies (involving microdissection and histologic imaging) [288], but they are also seen on CCTA, CT scans, or CMRA. In the human heart, a CMB is defined as a proximal coronary artery that is normally epicardial (lying on the surface of the myocardium) but that dips inside the myocardium, becoming compressed during systole (as shown also on angiography). This is a normal anatomic feature in many mammals and birds but is usually considered abnormal or potentially clinically significant in humans [287, 289].

Like CCTA, CMRA can properly identify the more severe forms of CMBs but not the milder forms, in which the bridging myocardium has a thickness of less than 1 mm. Clinical evaluation of the severity of individual CMBs cannot be carried out by using a morphologic, “still” imaging technique (even though these techniques offer the advantage of a tomographic approach that allows visualization of both the coronary vessel and the related myocardial bridge). Typically, CMRA allows only for imaging of the end-diastolic appearance of a CMB (but not phasic systolic narrowing of the coronary lumen, which can theoretically be assumed to impair coronary flow to some degree). Additionally, a routine CMRA study does not properly evaluate CMB thickness unless the tomographic plane is perpendicular to the target segment. Thickness, by itself, does not define the severity of a CMB, which is more likely related to the degree of systolic compression of the coronary lumen adjacent to the CMB, as well as the possibility of an excessive tendency to spasticity [290, 293].

Patients with hypertrophic obstructive cardiomyopathy have the highest probability of showing systolic blanching of the coronary branches (epicardially or intraseptally located). Magnetic resonance imaging is useful for identifying HCM (by the thickness of the septal wall and the free wall and by the pressure gradient at the subaortic stenosis), but it is not likely to identify systolic compression of the small septal branches, as shown by selective angiography. In HCM, such systolic compression could be the most relevant means of inferring the presence of myocardial fiber disarray with obstruction (a potentially important prognostic parameter for the risk of SCD) [291].

Coronary-Cameral Fistulas

Coronary-cameral fistulas are characterized by any type of abnormal coronary termination in any vascular thoracic structure. They are fairly frequent CAAs, which can be identified and evaluated with CMRA. These entities can be subdivided into two types, grossly based on their size [290].

1.
Smaller CCFs (grossly: having less than 500 mL/min of blood flow) most frequently include two subtypes: (1) those that drain into the proximal pulmonary artery and (2) those that drain into 1 or both ventricles. Both subtypes are generally small in diameter and involve multiple fistulous tracts. The CCFs that drain into the pulmonary artery are easily recognized with CMRA because of their multiple snake-like coronary side-branches that typically originate from 1 or 2 proximal coronary arteries. The multiple micro-CCFs to the ventricles originate from either coronary artery, or both, and drain into the right and/or left apical ventricular portions; they are generally difficult or impossible to recognize on routine CMRA studies. Both subtypes of small CCFs are relatively benign and do not usually require interventional treatment, except in the face of aneurysmal formation in rare coronary-to-pulmonary forms. The indication for CMRA of such CCFs is mainly related to the possible utility of a tomographic technique for better describing the extent and location of these entities on the pulmonary arterial wall, especially in planning catheter or surgical interventions.
2.
Large CCFs (more than 500 mL/min of blood flow) terminate into a thoracic cardiovascular cavity (at venous, pulmonary, and atrial or ventricular sites) [291]; indeed, blood flow through such fistulas could range from 500 mL to an amount that equals the total cardiac output, even in infants. In studying large-diameter, high-flow CCFs, the advantage of MRI (rather than CCTA) is the additional capacity to estimate the amount of blood shunting, valvular insufficiency, and reversible myocardial ischemia and/or scar tissue. No doubt, though, because of CCTA’s improved anatomic detail, this method is better for studying two critical aspects of CCF: (1) the site(s) of origin and the size and location of the nutrient branches arising from the fistulous tract (these branches must be spared during any interventional treatment) and (2) the size of ectasia/aneurysms, in addition to mural thrombus and/or coronary stenosis. Establishing the exact size of the fistulous loops’ lumen is particularly useful for planning coil embolization treatment of CCF and is best achieved with CCTA. Furthermore, the presence of distal fistulous-tract ostial stenosis (which is occasionally present in older CCF patients) is an important point to be clarified in view of the sometimes difficult decision about coil embolization versus surgical ligation of CCF [290]. The presence of stenosis favors coil embolization, because stenosis is associated with a decreased chance of coil distal embolization (into the pulmonary arteries) in the case of very large fistulas [290].

Anomalous Origin of the Left Coronary Artery from the Pulmonary Artery

In newborns or infants, anomalous origin of the LCA from the pulmonary artery (ALCAPA) is the fundamental entity that can be well diagnosed by CMRA [296–301]. This anomaly is frequently recognized clinically (based on a heart murmur, cardiomegaly, abnormal electrocardiogram, and heart failure) and is quite lethal. Typically, it is first diagnosed early after birth (“infantile-type” ALCAPA), because of the onset of an acute MI when the pulmonary artery pressure decreases, as is normally expected during the first few weeks after birth. Because of the presence of tachycardia, dyspnea, congestive heart failure, and/or shock, CMRA is rarely used in this context. Echocardiography may diagnose the essential features of ALCAPA, indicating the need for emergency surgical treatment of these fragile, unstable (and tachycardic) small patients.

When ALCAPA is discovered at a later age (“adult type”), CMRA may be an attractive means of imaging the anomaly and carrying out a functional study [296–301]. In addition to clearly defining the basic anatomic features of the anomaly, CMRA may be quite useful for assessing LV function (diagnosing reversible ischemia versus scar tissue in different myocardial segments and assessing the global ejection fraction), especially in view of the need for postoperative follow-up testing when recovery of ventricular function is variable in adults.

A particular detail that is useful to surgeons in planning a repair technique is the exact site of origin of the ectopic coronary artery. Usually, ALCAPA arises from the left posterior sinus of the pulmonary valve, but it could also arise from the more distal pulmonary main trunk, the proximal right or left pulmonary branches, the right sinus of the pulmonary valve, or the nonfacing sinus of the pulmonary valve (the anterior one), which may have a juxta-commissural location with an intramural proximal course. These features have possible implications regarding the choice of surgical technique used for repair.

Conclusions

In studying CAAs, MRI/MRA is commonly useful as a screening test in general populations at risk for SCD (such as athletes, military recruits, or patients without a previous diagnosis but with chest pain and/or a positive stress-test result). This modality is also useful as a secondary test in patients with an initial previous diagnosis of CAA, based on inconclusive coronary catheter angiography. Unfortunately, in some anomalies, CMRA does not have the necessary precision to indicate interventional treatment; in particular, it cannot establish the severity of stenosis of the luminal cross-sectional area of relatively small, possibly obstructed vessels, as in ACAOS.

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Authors and Affiliations

Advanced Cardiovascular Imaging (MR/CT), Department of Radiology, St. Luke’s Hospital/Texas Heart Institute, Houston, TX, USA
Benjamin Y. C. Cheong MD, FRCP(Edin)
Departments of Medicine, Cardiology, and Radiology, Baylor College of Medicine, Houston, TX, USA
Benjamin Y. C. Cheong MD, FRCP(Edin)
Department of Medicine, Baylor College of Medicine, Houston, TX, USA
Paolo Angelini MD, FSCAI
Department of Cardiology, St. Luke’s Hospital, Houston, TX, USA
Paolo Angelini MD, FSCAI
Center for Coronary Artery Anomalies, Texas Heart Institute, Houston, TX, USA
Paolo Angelini MD, FSCAI

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Benjamin Y. C. Cheong MD, FRCP(Edin)
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Paolo Angelini MD, FSCAI
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Correspondence to Benjamin Y. C. Cheong MD, FRCP(Edin) .

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Cardiology Division, President, Texas Heart Institute, Houston, Texas, USA
James T. Willerson
Cardiovascular Diseases, Mayo Clinic College of Medicine, Rochester, Minnesota, USA
David R. Holmes, Jr.

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Cheong, B.Y.C., Angelini, P. (2015). Magnetic Resonance Imaging of the Myocardium, Coronary Arteries, and Anomalous Origin of Coronary Arteries. In: Willerson, J., Holmes, Jr., D. (eds) Coronary Artery Disease. Cardiovascular Medicine. Springer, London. https://doi.org/10.1007/978-1-4471-2828-1_13

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DOI: https://doi.org/10.1007/978-1-4471-2828-1_13
Published: 27 January 2015
Publisher Name: Springer, London
Print ISBN: 978-1-4471-2827-4
Online ISBN: 978-1-4471-2828-1
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Magnetic Resonance Imaging of the Myocardium, Coronary Arteries, and Anomalous Origin of Coronary Arteries

Abstract

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Magnetic Resonance Imaging of the Myocardium, Coronary Arteries, and Anomalous Origin of Coronary Arteries

Myocardial Perfusion and Late Gadolinium Enhancement Imaging in Cardiovascular Magnetic Resonance to Assess Coronary Artery Disease

Cardiovascular Magnetic Resonance Imaging of Coronary Arteries

Keywords

Magnetic Resonance Imaging of the Myocardium

Historical Perspective

Basic Principles

Indications

Cardiomyopathies

Ischemic Cardiomyopathy (Coronary Artery Disease)

Myocardial Perfusion

Myocardial Viability

Area-at-risk Assessment

Nonischemic Cardiomyopathy

Imaging Techniques

Cine Imaging

Spin Echo

Dixon Technique

Delayed-enhancement MRI

T1 Imaging

T2 Imaging

T2* Imaging

Nonischemic Cardiomyopathies

Myocarditis

Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy

Hypertrophic Cardiomyopathy

Sarcoidosis

Left Ventricular Noncompaction

Cardiac Amyloidosis

Takotsubo (Stress) Cardiomyopathy

Chagas Disease

Anderson-Fabry Disease

Iron Overload Cardiomyopathy

Magnetic Resonance Angiography of Coronary Artery Disease

Technical Aspects

Acquired Coronary Artery Disease

Kawasaki Disease

Magnetic Resonance Imaging of Congenital Coronary Artery Anomalies

General Outline of Coronary Artery Anomalies

Classification of Coronary Artery Anomalies

Anomalous Coronary Artery Originating from the Opposite Sinus of Valsalva with an Intramural Course

Other Coronary Artery Anomalies: Imaging and Clinical Relevance

Coronary Myocardial Bridges

Coronary-Cameral Fistulas

Anomalous Origin of the Left Coronary Artery from the Pulmonary Artery

Conclusions

References

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