Cardiac Computed Tomography
Ron Blankstein
Vasvi Singh
INTRODUCTION
Cardiac computed tomography (CCT) has evolved considerably over the past 15 years and now serves an essential role in the practice of cardiology and cardiac surgery. The growth in CCT has been due to both technical advances in the field and clinical effectiveness studies showing how CCT can be useful in diagnosis and patient management. As a result, CCT now provides an opportunity to image many forms of cardiovascular diseases ranging from coronary artery disease (CAD) and myocardial disease to valvular and pericardial heart disease (Table 36.1).
CCT—whether with or without contrast—utilizes x-rays to obtain high-resolution three-dimensional (3D) data sets that then allow users to view various parts of the heart. A fundamental advancement that enables computed tomography (CT) imaging of the heart is the use of electrocardiographic (ECG) gating, whereby image acquisition is performed during predetermined phases of the cardiac cycle, thus “freezing” the motion of the heart. There are various protocols and applications of CCT, among them:
TABLE 36.1 Examples of Cardiac Pathology That Can Be Evaluated Using Different Cardiac Computed Tomography Applications | ||||||||||||||||||||||||||||||||||||
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Coronary artery calcium (CAC) scan—non-contrast-enhanced ECG-gated images that are performed to identify the presence and amount of coronary calcium, which is quantified as an Agatston score.
Coronary computed tomography angiography (CCTA)—contrast-enhanced ECG-gated images that are performed to identify the presence of both calcified and noncalcified plaque, as well as estimate the severity of stenoses.
Cine-CCT—contrast-enhanced ECG-gated images that are obtained throughout multiple phases of the cardiac cycle, thus allowing for the reconstruction of cine images. The ability to view the heart throughout the cardiac cycle can be used to determine left ventricular (LV) or right ventricular systolic function and to assess valvular heart disease.
This chapter provides readers with an overview of the various uses of CCT and how to utilize this test in clinical practice.
PHYSICAL PRINCIPLES AND INSTRUMENTATION
Cardiac imaging with CT requires high temporal resolution to limit cardiac motion artifacts, high spatial resolution to visualize small cardiac anatomy, fast anatomic coverage allowing scanning of the heart during a breath-hold to reduce respiratory motion artifacts, and synchronization of data acquisition or reconstruction to the cardiac cycle to ensure imaging during a desired cardiac phase.
In a CT system, an x-ray source rotates continuously in a gantry, emitting a beam that is attenuated by the target (patient’s chest) and detected by an array of sensors opposite the x-ray source. Modern scanners permit the simultaneous acquisition of multiple (64, 128, and 320 submillimeter thin) slices at rotation times as short as 270 ms. In the source, x-rays are produced when highly energetic electrons interact with matter. Electrons are accelerated toward a target to gain kinetic energy. For most diagnostic imaging applications, the electrons gain a maximum kinetic energy between 80 and 140 keV. The attenuation, or removal, of photons from x-rays passing through tissue is dependent on both the energy of the x-rays and density of the tissue within the scanned region. Attenuated x-rays are detected opposite the image source by multiple rows of detectors. An 180-degree rotation of the x-ray tube/detector system is required to generate an image.
Data covering the entire heart are acquired using either axial or helical modes within a single breath-hold. Images are reconstructed with thicknesses ranging from 0.5 to 3 mm depending on the specific cardiac application. An ECG signal is used to reference data to the cardiac cycle. The ECG signal may be used to either prospectively trigger data acquisition or retrospectively gate data reconstruction. For static morphologic evaluation of most cardiac structures, data is usually selected from the diastolic phase of the cardiac cycle where heart motion is minimized. During axial mode acquisition (step-and-shoot), data acquisition is prospectively triggered by the ECG signal during the desired cardiac phase. The patient table moves at incremental steps between periods of data acquisition. The number of steps depends on the coverage width of specific scanner models (typically 4 cm/64 slices, 8 cm/128 slices, and 16 cm/320 slices). In the helical mode of operation, data are acquired continuously with simultaneous recording of the ECG signal. Data are then retrospectively gated to the ECG signal after acquisition and reconstructed during one or more cardiac phases.
ANATOMIC CONSIDERATIONS
CCT enables detailed visualization of all cardiac structures, including pericardium, pericardial and epicardial fat, myocardium, coronary arteries and veins, and valves. The strength of CCT is the ability to obtain high spatial resolution images. Accordingly, CCT can provide detailed visualization of coronary artery plaque and stenoses. On the other hand, CCT has lower contrast resolution than do other techniques such as cardiac magnetic resonance (CMR) imaging. Thus CMR—particularly using late gadolinium enhancement imaging—is better suited for evaluating infiltrative disease of the myocardium, or certain types of cardiac masses.
FUNDAMENTALS OF CARDIAC COMPUTED TOMOGRAPHY IMAGING
CAC scan—CAC assessment does not require administration of radiographic contrast agent or premedication. The scan consists of an ECG-gated, limited field-of-view 3D image of the heart during a single phase of the cardiac cycle. The radiation dose of a CAC study is usually approximately 1 mSV, which is similar to that for a mammogram.
Coronary computed tomography angiography—CCTA requires a large-bore (18-gauge) intravenous (IV) catheter for the delivery of contrast material at a rate of 5 to 6 cc/second. To achieve good image quality, patients are often administered beta-blockers (to lower the heart rate) and nitroglycerin (to dilate the coronary arteries). The radiation dose of CCTA using contemporary techniques is usually approximately 2 to 5 mSv,1 but can be higher when older techniques/scanners are employed, especially if data are acquired throughout multiple phases of the cardiac cycle using a helical acquisition mode. The presence of arrhythmias can also increase the radiation dose.2
Cine-CCT—Cine-CCT is acquired in a way similar to that for CCTA scans, except that data from multiple cardiac phases are acquired, and thus the radiation dose may be higher. When detailed anatomy of the coronary arteries is not needed, nitroglycerin is not required. Also, contrast agent administration may differ, particularly when there is a need to image right-sided heart structures.
CLINICAL APPLICATION AND INDICATIONS
Coronary Artery Calcium Scan
CAC testing can quantify the amount of calcified coronary plaque (Figure 36.1). There is considerable data showing that among patients who do not have established CAD, there is increased risk of atherosclerotic cardiovascular disease (ASCVD) events with increasing CAC scores. Conversely, patients who lack calcified plaque (CAC = 0) generally have a very low event rate.3 In addition, the addition of CAC to traditional risk factors results in improved risk assessment and risk reclassification.
 FIGURE 36.1 Example of coronary artery calcium (CAC) testing. A 54-year-old female with impaired glucose tolerance and hyperlipidemia was referred for CAC scan for cardiovascular risk assessment. A prospectively electrocardiography (ECG)-gated non-contrast computed tomography (CT) scan with field-of-view limited to the heart was obtained. Images were reconstructed at 3-mm intervals. CAC scan showing (A) left anterior descending artery calcifications (green arrow), (B) right coronary artery calcifications (red arrow), (C,D) overall, mild amount of coronary calcifications with an Agatston score of 49. |
Indications for Coronary Artery Calcium Testing
Statin averse—Improved risk assessment, in the context of shared decision-making, among individuals who meet criteria for statin therapy (ie, 10-year ASCVD risk of 5%-20%), but have a preference to defer therapy. In such patients, the presence of a CAC score of zero could be used to identify low-risk patients and defer therapy.3,4,5 Notably, patients who have a high risk of ASCVD events are less likely to benefit from CAC testing because their risk may remain elevated even in the absence of CAC.
Statin-intolerant patients may benefit from CAC testing to determine the likelihood that they will benefit from statin, or in some cases from additional lipid-lowering therapies that may be more costly.
Screening in the lower risk cohort who have either traditional or nontraditional risk factors, when decision-making regarding risk versus benefit of statin therapy is uncertain. For example, some individuals who have a low 10-year ASCVD risk but who have a family history of premature ASCVD events may benefit from CAC testing.4,6
In general, the yield of CAC testing in those who are younger than age 40 is low. However, some studies have suggested a limited role for selected use of CAC testing among men and women who are younger than age 40.7,8 In such cases, the finding of any atherosclerosis (ie, CAC > 0) implies a higher risk of future ASCVD events; however, the absence of CAC may be less reassuring, especially when considering a longer time perspective.
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