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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Keywords
- Dynamic Spatial Reconstructor
- Routine Clinical Tool
- Ventricular Volume
- Regional Functional Analysis
- Left Ventricular
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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.
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).
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).
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).
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.
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).
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.
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).
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.
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).
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.
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).
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.
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Mahnken, A.H. (2018). Myocardial Function and Viability. In: Budoff, M., Achenbach, S., Hecht, H., Narula, J. (eds) Atlas of Cardiovascular Computed Tomography. Springer, London. https://doi.org/10.1007/978-1-4471-7357-1_15
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