Myocardial Function and Viability

Abstract

Information on cardiac function is of substantial value in the assessment of patients suffering from a variety of cardiac diseases. Although cardiovascular MRI is considered the current standard of reference for functional imaging, information on global and regional myocardial function is routinely obtained by echocardiography or by levocardiography. The first CT approaches toward the assessment of cardiac function were developed as early as the late 1970s [1]. Cross-sectional imaging techniques like the dynamic spatial reconstructor [2] or electron beam CT [3] were successfully evaluated for their ability to visualize cardiac function. For all of these techniques, evaluation of left ventricular volumes, wall motion abnormalities, and contrast enhancement patterns was shown to be feasible. For several reasons, however, none of these techniques was considered a routine clinical tool.

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Information on cardiac function is of substantial value in the assessment of patients suffering from a variety of cardiac diseases. Although cardiovascular MRI is considered the current standard of reference for functional imaging, information on global and regional myocardial function is routinely obtained by echocardiography or by levocardiography. The first CT approaches toward the assessment of cardiac function were developed as early as the late 1970s [1]. Cross-sectional imaging techniques like the dynamic spatial reconstructor [2] or electron beam CT [3] were successfully evaluated for their ability to visualize cardiac function. For all of these techniques, evaluation of left ventricular volumes, wall motion abnormalities, and contrast enhancement patterns was shown to be feasible. For several reasons, however, none of these techniques was considered a routine clinical tool.

With the introduction of multidetector spiral CT in clinical routine, a new tool for assessing cardiac function has become widely available. Four-dimensional functional information is inherently available with each retrospectively ECG-gated cardiac CT data set, with no extra cost in terms of radiation exposure or contrast material delivery, so this information should be used as an adjunct to multidetector spiral CT coronary angiography. Several studies have proved that cardiac CT is a reliable tool for evaluating ventricular volumes and wall motio n . Moreover, recent studies have reported on the ability to assess myocardial perfusion and viability with cardiac CT.

This chapter explains the physiologic and technical basics of functional CT imaging, including the assessment of ventricular volumes and wall motion. The basics of CT myocardial perfusion imaging and the assessment of myocardial viability from contrast-enhanced late-phase CT are introduced.

The Physiologic Basis of Functional CT Imaging

There is a close correlation between the electrical and the mechanical events during the cardiac cycle (Fig. 15.1). The resolution of the imaging is dependent upon heart rate (Figs. 15.2 and 15.3).

Fig. 15.1

Correlation between electrical and mechanical events during the cardiac cycle . The RR interval is separated into the isovolumetric contraction phase (1), the ventricular ejection period (2), the isovolumetric relaxation phase (3), and the ventricular filling period (4). The different phases are separated by the opening and closing of the heart valves, events that mark the beginning or the ending of the isovolumetric contraction and relaxation phase. The aortic and pulmonary valves are opened during the ventricular ejection period; they are closed during the ventricular filling period, when the mitral and tricuspid valves are opened. Left and right ventricular function is well synchronized. Although end-systolic and end-diastolic volumes may differ, left and right ventricular volume changes (stroke volume) are similar

Fig. 15.2

(a) Normal heart rate distribution in a patient group undergoing cardiac multidetector spiral CT (n = 672) without additional beta-blockers for reducing the individual heart rate (Mahnken, unpublished data). Most patients present with a heart rate of approximately 65 beats per minute (bpm). Consequently, optimal temporal resolution should be achieved for heart rates ranging from 55 to 75 bpm. (b) Temporal resolution of different CT scanners that are used for cardiac CT. The continuous blue line represents the constant, heart rate–independent temporal resolution of a dual-source CT scanner, whereas the red line and the green line represent single-source CT scanners with gantry rotation times of 330 ms and 285 ms, respectively. Except for the dual-source CT scanner, temporal resolution has some bothersome maxima in the range of the most common heart rates, because the multisegmental image reconstruction techniques that are currently used achieve a temporal resolution that is shorter than the cardiac rest period (see Fig. 15.3). This connection is of particular interest because the reliability of ventricular volumes determined by CT depends on temporal resolution (see Fig. 15.10)

Fig. 15.3

The systolic and diastolic rest periods are heart rate–dependent. The duration of the cardiac rest periods shortens progressively with increasing heart rates. This effect is more pronounced with the diastolic rest period, but the systolic rest period also decreases in duration with increasing heart rate. Consequently a temporal resolution below approximately 100 ms is needed to depict the left ventricle and the myocardium without motion artifacts during end-systole and end-diastole. The velocity of myocardial motion varies, and some areas (e.g., close to the right ventricular groove) require an even higher temporal resolution to avoid motion artifacts. These effects are also known to affect the assessment of global and regional myocardial function (see Fig. 15.10)

Technical Basics of Functional CT Imaging

Determination of global and regional cardiac function requires image reconstruction at multiple cardiac phases from a retrospectively ECG-gated CT data set (Fig. 15.4). Standardized imaging planes are defined so that all myocardial segments are visualized from two perpendicular perspectives (Fig. 15.5).

Fig. 15.4

Determination of global and regional cardiac function requires image reconstruction at multiple cardiac phases from a retrospectively ECG-gated CT data set. For this purpose, the RR interval is separated in multiple phases (top row), covering the entire cardiac cycle. The number of phases multiplied by the temporal resolution should equal or exceed the length of the RR interval. Otherwise, gaps may occur, especially at low heart rates with long RR intervals. The middle row and the bottom row show an ECG taken from the same patient with the corresponding data sections used for functional image reconstruction. If the number of cardiac phases that are used for image reconstruction is too low, gaps will occur. For clinical routine, images should be calculated from at least 20 cardiac phases to exactly meet end-systole and end-diastole. For cine CT, calculating images from a large number of cardiac phases helps to avoid a potentially misleading staccato effect during wall motion analysis

Fig. 15.5

For cross-sectional imaging of the heart , standardized, double oblique imaging planes are used: a four-chamber view (4CV, top left); a two-chamber view (2CV, top right); a short-axis view (SA, bottom right); and a three-chamber view (3CV, bottom left). These imaging planes are defined in such a manner that all myocardial segments are visualized from two perpendicular perspectives. Except for the 3CV, all of these imaging planes are orthogonal to each other. The dashed line indicates the relation between SA view and the 4CV and 2CV. The dotted line describes the relation between the 2CV and 4CV. For calculation of the 3CV, images are reconstructed via the most basal plane of the SA view, through the left ventricular outflow tract. The bold arrows indicate a potential workflow sequence for calculation of the different imaging planes

Assessment of Ventricular Volume and Function

For the assessment of global ventricular function , end-systolic and end-diastolic images are reconstructed (Fig. 15.6), from which ventricular volu mes are calculated and may be used to calculate ejection fraction. True three-dimensional measurements are better suited for assessing complex geometric objects such as the cardiac ventricles, however, and most tools now provide threshold-based, semi-automated assessment of left and right ventricular volumes (Figs. 15.7 and 15.8).

Fig. 15.6

For the assessment of global ventricular function , end-systolic (left) and end-diastolic (right) images are reconstructed. Left and right ventricular volumes are traditionally calculated from 5- to 8-mm short-axis images (top) using the Simpson method—that is, summing the cross-sectional area (A) multiplied by the section thickness (S) from the base to the apex. The apex is defined as the most distal section, with the ventricular cavity visible throughout the entire cardiac cycle; the basal section is identified by the presence of at least 50% myocardium throughout the RR interval. For quantitative analysis, trabeculae and papillary muscles are often included with the blood pool. This approach is known to improve data reproducibility, as has been shown for MRI [4]. To assess myocardial mass, endocardial borders (black line) and epicardial borders (red line) must be drawn. Alternatively, end-systolic and end-diastolic two-chamber view images may be used, applying the area-length method (bottom). To calculate ventricular volumes from two-chamber–view images, the length from the apex to the mitral valve plane (L, arrows) and the cross-sectional area (A) are needed. Ventricular volumes are used to calculate ejection fraction (EF). A key disadvantage of the area-length method is the presence of geometric assumptions that may lead to wrong results, especially in patients with left ventricular pathologies such as ventricular aneurysms. Moreover, this approach is suited only for the left ventricle, as the right ventricle follows a more complex geometry

Fig. 15.7

The previously shown two-dimensional approaches (e.g., the Simpson method) are limited for the volumetric assessment of the left ventricular outflow tract, the apex, and the saddle-shaped mitral annulus. True three-dimensional measurements are better suited for assessing complex geometric objects such as the cardiac ventricles, and most tools now provide threshold-based, semi-automated assessment of left and right ventricular volumes. These tools are supported by complex geometric models that were designed to improve the robustness of these techniques. As a major advantage, these tools are much quicker than manual contour drawing. Although the ventricular volumes are comparable to those obtained using Simpson’s rule, there appears to be systematic underestimation of the left ventricular volumes, with a subsequent overestimation of the ejection fraction [5]. Moreover, comparability with other techniques such as echocardiography and MRI is limited, as most reference techniques are still limited to two-dimensional assessment of ventricular volumes

Fig. 15.8

For calculation of the right ventricular volumes, the modified Simpson method is used. Because of the complex geometry, reliable geometric models (which are suited for the left ventricle) are not available. Moreover, in most centers only the ventricular volumes are assessed, requiring the endocardial contours only. For reliable assessment of the right ventricular volumes, a sufficient enhancement of the right ventricle is needed, which can reliably be achieved by using the so-called split-bolus technique. A common approach is to add diluted contrast material (e.g., 20% contrast with 80% saline) after the main contrast bolus

The assessment of end-systolic and end-diastolic images is sufficient to determine ejection fraction and cardiac output, but heart rate and temporal resolution must be considered in interpreting these results (Figs. 15.9 and 15.10). An approach using indicator dilution allows a fast estimation of global ventricular function independent from the patient’s heart rate (Fig. 15.11).

Fig. 15.9

The assessment of end-systolic and end-diastolic images is sufficient to determine ejection fraction and cardiac output. Images from multiple cardiac phases must be analyzed (a) for calculation of volume-time-curves (b). The shape of these curves provides information on ventricular dynamics beyond ejection fraction. This information may be of particular interest in patients with ventricular asynergy, when a two-point method may be misleading. In clinical routine, the use of volume-time curves for the assessment of ventricular function is uncommon. This technique is mainly used for research purposes, as its added value for most clinical problems has not yet been unequivocally clarified

Fig. 15.10

Accuracy of measurements of left ventricular ejection fraction and volumes depends on heart rate and temporal resolution. Results of phantom measurements with a “true” ejection fraction of 60% show that a temporal resolution of 165 ms is still not enough to reliably determine the ejection fraction at every heart rate (a). At increased heart rates (>90 bpm), ejection fraction will be underestimated, as end-systole will be missed. The same is true for cardiac output (b). If the temporal resolution is down to 75 ms, both functional parameters can be reliably determined independent of the patient’s heart rate. Consequently, it is important to consider heart rate and temporal resolution when interpreting results of functional cardiac CT

Fig. 15.11

A different approach to assess ventricular function is to apply indicator dilution theory on test-bolus data. Interpreting contrast material as indicator and the CT scanner as a densitometer, cardiac output can be determined from a defined contrast material injection and a dynamic CT measurement (a). From fitted time-attenuation curves in the aorta (b) cardiac output (CO) can be calculated using a modification of the Stewart-Hamilton equation in which Q is the amount of indicator injected and c(t) is indicator concentrations as a function of time [6]. This technique allows a fast estimation of the global ventricular function. Moreover, this technique is independent from the patient’s heart rate and does not require ECG-gated data acquisition

Table 15.1 shows normal values for ventricular function that have been established using CT. Table 15.2 illustrates the excellent correlations between ventricular volumes determined by CT and MRI. Left ventricular function, however, is affected by the administration of beta-blockers, which are often given to slow down heart rate for coronary CT angiography. In these patients, stroke volume and ejection fraction are significantly decreased [9]. Another factor affecting global function values is automated 3D image analysis, which results in a systematic overestimation of the ejection fraction [12]. Corresponding results were reported from comparison of CT with echocardiography and ventriculography, proving CT a reliable technique to assess left ventricular volumes at rest.

Table 15.1 Normal values for global and regional ventricular function using CT^a

Table 15.2 Studies of correlation between ventricular volumes determined by CT and by MRI

Assessment of Ventricular Wall Motion

Regional ventricular wall motion is typically assessed using a semiquantitative four-point scale (Figs. 15.12 and 15.13). Various imaging modalities, including CT, can be used to assess global and regional left ventricular function (Fig. 15.14).

Fig. 15.12

Myocardial segments are identified for the assessment of regional myocardial function and viability, from the base (a) and the midsection (b) to the apex (c). The apex itself is considered a separate segment (d). Each myocardial segment can be assigned to the territories of the different coronary arteries: left anterior descending artery (white), left circumflex coronary artery (light green), and right coronary artery (dark green) [17]. So far, there is no uniform nomenclature for the right ventricle

Fig. 15.13

Regional ventricular wall motion is typically assessed using a semiquantitative four-point scale. Regular wall motion is classified as normokinetic (a). Reduced wall motion is classified as either hypokinetic (i.e., reduced systolic wall thickening) (b) or akinetic (i.e., absent regional wall thickening) (c). An outward movement of the ventricular wall segments during systolic contraction is called dyskinesis (d). Furthermore, asynchronous wall motion can be observed, such as in the presence of left or right bundle block. In combination with information on the coronary arteries, this wall motion analysis provides valuable insight into the functional state of the different coronary artery territories and therefore the coronary artery perfusion. Because CT wall motion analysis is performed only at rest, there is no information on contractile reserve, limiting the use of this technique

Fig. 15.14

Multiple imaging modalities can be used to assess global and regional left ventricular function. These include CT (a), MRI (b), ventriculography (c), and echocardiography (d). All of these techniques allow differentiation of systole (left) and diastole (right) and, therefore, calculation of ventricular volumes and function. As shown in this patient with severely reduced left ventricular function, all of these techniques are suited to visualize the impaired left ventricular function. CT, MRI, and echocardiography also have the potential to quantitatively assess wall thickening. These images show severely reduced wall thickness of the septal and inferior myocardial segments. Only the mid-anterolateral segment of the left ventricle presents with nearly normal wall thickness, but limited wall thickening

Studies have shown that the assessment of regional wall motion is feasible with cine CT. An excellent agreement has already been found with 16-slice CT, and these outstanding results have been confirmed with recent CT scanners, proving the assessment of regional wall motion from CT data to be a robust and reliable approach (Table 15.3). The combination of regional functional analysis with CT coronary angiography has also been shown to improve the conspicuity of coronary artery lesions in patients with acute chest pain [18]. Despite the trend towards prospective triggering as an image acquisition technique to reduce the radiation dose, a comprehensive data analysis should be performed whenever functional data are available.

Table 15.3 Studies of regional wall motion : agreement of MDCT with MRI or echocardiography

Left-ventricular remodeling is another important indicator of cardiac pathology, with different types of pathologic remodeling occurring in various diseases. Eccentric remodeling is a unique form that eventually follows myocardial infarction. It can be identified by adapting the principles of echocardiography to CT (Fig. 15.15).

Fig. 15.15

Left-ventricular (LV) remodeling is another important indicator of cardiac pathology. It may be physiological when the heart increases in size but maintains normal function, such as during growth or physical training. Different types of pathologic remodeling occur in a variety of diseases. The transition to pathologic remodeling goes along with progressive ventricular dilatation and distortion of the LV cavity shape, resulting in a disruption of its normal geometry. Eccentric remodeling is a unique form of LV remodeling that eventually follows myocardial infarction. Applying the principles of echocardiography to CT, it may be identified from the ratio of the LV myocardial mass index (LVMMI; g/m²) to relative wall thickness (RWT), with RWT being defined as: $RWT=\frac{2\bullet EDW{T}_{dia}}{EDID}$ where EDWT _dia is the end-diastolic wall thickness and EDID is the end-diastolic inner diameter of the left ventricle as measured from short-axis views (see Fig. 15.6) [23]

CT Myocardial Perfusion Imaging

Ischemic injury of the myocardium can be differentiated as reversible or irreversible, in conditions that are acute or chronic. Different contrast enhancement patterns may be seen on arterial and late-phase CT imaging (Fig. 15.16). During the arterial phase, ischemic damage to the myocardium appears as an area of reduced attenuation (Fig. 15.17). Various studies have shown that it is feasible to detect myocardial infarction using arterial-phase CT because infarcted myocardium shows reduced attenuation values (Table 15.4). Attenuation values of normal myocardium are typically more than twice as high as the values in infarcted myocardium. Applying a threshold of 20 HU to differentiate healthy from infarcted myocardium provides results with acceptable specificity. Nevertheless, the size of myocardial infarction is typically underestimated on arterial phase CT when compared with MRI, and unequivocal differentiation of reversible and irreversible myocardial injury is not feasible.

Fig. 15.16

Ischemic injury of the myocardium can be differentiated in reversible and irreversible conditions and in acute and chronic conditions. Single or repeated short periods of myocardial ischemia may result in myocardial stunning, a postischemic dysfunctional state of the myocardium that persists even if the coronary flow is restored. Hibernating myocardium is characterized as viable but nonfunctional myocardium with chronically impaired regional blood flow. The loss of cell membrane integrity marks the point of cell necrosis and irreversible myocardial infarction. Myocardial infarction may be occlusive or reperfused (e.g., after medical or mechanical revascularization therapy). Depending on the type of myocardial injury, different contrast enhancement patterns may be seen on arterial and late-phase CT imaging

Fig. 15.17

During the arterial phase, ischemic damage to the myocardium appears as an area of reduced attenuation when compared with healthy, remote myocardium. A focal decrease in myocardial enhancement of 20 HU or more is considered relevant. This finding is unspecific, however; it can be found in myocardial infarction but less commonly is seen in myocardial stunning or hibernation. Wall thickness may help to distinguish acute from chronic stages of disease. This example shows an arterial-phase, four-chamber view in a 54-year-old man with acute myocardial infarction, 2 days after an emergency coronary artery bypass graft procedure. Left panel, CT shows a large, hypodense area in the basal and mid-inferoseptal segments of the left ventricle (black arrows). The patent coronary artery bypass graft also is visible (white arrows). Middle panel, Information on myocardial perfusion can be visualized as a color-coded polar map, projecting information from the different myocardial segments onto a single image. In this example, areas of reduced perfusion are encoded blue, whereas normal myocardium appears green. Right panel, Single-photon emission CT (SPECT) images obtained the same day confirm the location and the extent of the area of reduced perfusion

Table 15.4 Detection of myocardial infarction with arterial-phase CT

Acute and chronic myocardial infarction cannot be differentiated based on the pattern of contrast enhancement, but they can be differentiated on the basis of regional wall thickness and the presence of myocardial calcifications. Chronic myocardial infarction presents with a combination of reduced contrast enhancement and regional wall thinning (Fig. 15.18).

Fig. 15.18

Whereas acute myocardial infarction is characterized by reduced contrast enhancement with normal wall thickness, chronic myocardial infarction presents with a combination of reduced contrast enhancement and regional wall thinning. Cine CT also shows a reduced regional wall thickening. This figure shows a typical example of a pig suffering from an acute myocardial infarction in the mid-anteroseptal myocardium, caused by balloon occlusion of the left anterior descending branch of the left coronary artery. Top left, During the acute phase of infarction , arterial-phase CT depicts myocardial infarction as a hypodense area (arrows). Bottom left, At 3 months follow-up, typical thinning of the mid-anteroseptal myocardium (arrows) can be observed. Right, Corresponding MRI findings

First-pass myocardial perfusion imaging has the potential to detect impaired microvascular function before the occurrence of clinical symptoms, but only preliminary data on first-pass perfusion CT imaging are available (Figs. 15.19 and 15.20). CT perfusion imaging offers several theoretical advantages over MRI, such as the linear relation between contrast enhancement and iodine concentration, which potentially allows for the direct quantification of myocardial blood flow and avoids the need for potentially error-bearing correction methods. Exposure to radiation and contrast material limits stress testing with CT, however, and MRI, a radiation-free reference technique, has become increasingly available.

Fig. 15.19

Because first-pass myocardial perfusion imaging may detect impaired microvascular function before the occurrence of clinical symptoms, this technique is suited to assess the physiologic relevance of a coronary artery lesion, as decreased myocardial perfusion represents the first consequence of obstructive coronary artery disease. The feasibility of CT perfusion imaging has been shown, but only preliminary data on first-pass perfusion CT imaging are available. Technically, sequential ECG-gated images are obtained at least during every second heartbeat. The presented sequence starts from the left to the right with a nonenhanced baseline image and shows the contrast passage from the right heart to the left ventricle, where an area of reduced myocardial perfusion becomes visible in the anterior wall (arrows). The nonenhanced baseline image allows calculation of the absolute contrast enhancement and is needed for quantitative analysis

Fig. 15.20

(a) For first-pass perfusion analysis , a region of interest is placed in the area of infarction (yellow) and the healthy remote myocardium (black). To allow interindividual and intraindividual comparison, data must be corrected for variations in the hemodynamic state. Therefore, attenuation values measured in the myocardium must be normalized to the attenuation values in the left ventricular cavum (red). (b) From these data, time-attenuation curves can be calculated, which are used for quantitative and semiquantitative analysis. For multidetector CT, few data on first-pass perfusion imaging are available. So far, mainly visual and sem iquantitative analyses have been performed. Typical parameters established for semiquantitative analysis include maximum signal intensity, wash-in time, and slope. Quantitative parameters include mean transit time, myocardial blood volume, and myocardial perfusion [30]

Assessment of Myocardial Viability

In patients with ischemic heart disease, dysfunctional but viable myocardium may experience functional improvement after revascularization therapy, but nonviable (necrotic) myocardium will not recover function. Therefore, it is important to be able to assess myocardial viability, which can be achieved with contrast-enhanced late-phase CT (Fig. 15.21 and Figs. 15.22). CT also allows differentiation of occlusive from reperfused myocardial infarction (Figs. 15.23 and 15.24). Differences in contrast enhancement patterns can be used to predict functional recovery after myocardial infarction (Fig. 15.25).

Fig. 15.21

On contrast-enhanced late-phase CT, hyperdense areas of the myocardium correspond to necrotic tissue, so this technique can be used to assess the individual patient’s prognosis. Top left, If delayed contrast enhancement affects less than 25% of left ventricular wall thickness, global improvement of left ventricular function can be expected. Top right, Regional wall motion improvement can be expected in segments with an extent of the hyperenhancing myocardium of less than 50% of the wall thickness [31]. Bottom left, Segments with more than 75% delayed contrast enhancement have very little chance for functional improvement [32]. Bottom right, Transmural infarcts will not improve function after revascularization

Fig. 15.22

Late phase contrast enhanced CT in a 76-year-old male patient with subacute myocardial infarction shows a marked late enhancement of the left ventricular myocardium. The hyperdense part of the lateral wall of the left ventricle (arrows) corresponds to necrotic myocardium. With about 75% transmural extent of the hyperenhancing myocardium the chance for functional recovery is low

Fig. 15.23

CT allows differentiation of occlusive (a) from reperfused myocardial infarction (b). On arterial-phase CT (left), both types of infarction present with reduced attenuation values corresponding to a lack of contrast enhancement. On contrast-enhanced late-phase CT, occlusive myocardial infarction remains hypodense (a, middle), whereas reperfused myocardial infarction shows a typical delayed contrast enhancement (b, middle). The size of infarction correlates well with MRI (right), indicating the reliability of CT for imaging myocardial viability

Fig. 15.24

Contrast dynamics for occlusive (a) and reperfused (b) myocardial infarction differ on CT, allowing differentiation of these types of infarction. Reperfused myocardial infarction shows a characteristic hyperenhancement on late-phase CT (b), whereas areas of occlusive infarction remain hypodense when compared with healthy remote myocardium (a). The best contrast between normal and infarcted myocardium appears about 5 min after contrast material injection. Especially in reperfused myocardial infarction, however, the contrast between infarcted myocardium and the blood pool is poor (b). LV left ventricular

Fig. 15.25

In contrast-enhanced CT of myocardial infarction, different contrast enhancement patterns can be observed when comparing arterial-phase CT (left) and late-phase CT (right). These patterns allow the prediction of functional recovery after myocardial infarction. Top row, The best results with respect to wall thickening and left ventricular ejection fraction were experienced when late enhancement (white regions) was seen, but no early perfusion deficit. Bottom row, The poorest results are expected in patients with early and late perfusion defects (dark regions) [33], which correlate well with so-called microvascular obstruction. This phenomenon is well known to limit the individual patient’s prognosis

CT Scan Protocols for Myocardial Assessment

So far, there are no recommendations for a uniform scan protocol for the CT assessment of myocardial function and viability. For a comprehensive analysis of cardiac function and viability, a contrast-enhanced dual-phase scan protocol is needed. Ventricular volumes and regional wall motion are assessed from multiphase, arterial-phase images, whereas myocardial viability is assessed from contrast-enhanced late-phase CT images. Both phases are needed to predict an individual patient’s outcome. Figure 15.26 illustrates different contrast injection protocols for coronary CT angiography with subsequent late-phase imaging. A different approach is the use of dual-energy CT for assessing delayed myocardial contrast enhancement. This approach requires a higher radiation exposure than single-energy CT, however, and its use is limited to very few centers.

Fig. 15.26

Contrast injection protocols for coronary CT angiography with subsequent late-phase imaging to assess myocardial viability. (a) Traditionally the total amount of contrast material is injected in a single bolus injection, and late-phase images are obtained 5–15 min after contrast injection. (b), A different approach is to administer a smaller contrast bolus for coronary imaging, followed by slow-flow injection (0.1–0.3 mL per s) for several minutes. (c) Adding an additional wash-out phase improves contrast between the blood pool and areas of delayed myocardial contrast enhancement For determination of the start delay, a test bolus or the bolus tracking technique may be used. Test-bolus data may be used to compute cardiac output. The total amount of contrast material should be between 0.75 and 1 g of iodine per kilogram to ensure sufficient contrast on late-enhanced CT images. Contrast will be further enhanced by using 80 kV or 100 kV scan protocols with iterative reconstruction techniques. Lowering tube voltage and using prospective triggering techniques helps to reduce the radiation dose of the additional viability scan