Cardiovascular Computed Tomography Including Calcium Scoring
I.Introduction. Cardiovascular computed tomography (CT) has continued to rapidly evolve over the past decade, gaining new and expanded indications for noninvasive assessment of the heart, great vessels, and peripheral vasculature. Technologic improvements, including increasing numbers of detectors, improved temporal and spatial resolution, and advanced postprocessing, have broadened the clinical utility of this imaging modality. Advanced multidetector computed tomography (MDCT) scanners and new scanning protocols have significantly reduced the required radiation and contrast dosages. Numerous considerations are involved in the proper selection of cardiovascular CT protocols, and skilled operators are required to plan and interpret these examinations.
II.Basics of Cardiac CT
A.CT physics. In CT, images are created by rotating an x-ray source emitting a fan-shaped beam of x-rays, which then pass through the body. Some x-rays are absorbed or scattered, but others are transmitted and subsequently sensed by detectors located directly across the x-ray source. In MDCT, the x-ray tube and detectors are mounted on a gantry that rotates rapidly around the patient as he or she passes through the scanner. As in traditional x-ray radiography, different structures attenuate the x-ray beam to differing extents depending on their atomic composition and density, as well as the energy of the incident photons. The data collected by the detectors then go through a complex set of mathematical reconstruction algorithms that create a set of axial images through the technique of back projection. Each voxel in the resulting axial image is ascribed a specific attenuation value, which is expressed in Hounsfield units (H.U.). Using a reference of 0 H.U. for water, −1000 H.U. for air, and +1000 H.U. for bony cortex, different points are assigned their respective attenuation values. This information is then converted into a grayscale image that can be manipulated by the interpreting physician.
B.Technical challenges for cardiac imaging
1.The fast cyclical motion of the heart requires high temporal resolution to avoid blurring or degradation of images because of cardiac motion artifact. In cardiac CT, image acquisition is referenced, or gated, to the cardiac cycle. Although data can be acquired throughout the cardiac cycle, most image data sets are reconstructed during periods of minimal cardiac motion, typically a brief 100- to 300-ms interval in late diastole (60% to 75% of the R–R interval).
2.High spatial resolution is required to image relatively small vessels such as the coronary arteries. Current MDCT scanners provide a spatial resolution of 0.3 to 0.5 mm, compared with a spatial resolution of 0.1 to 0.2 mm for invasive angiography and intravascular ultrasound.
3.Respiratory motion artifact can be minimized by having the patient hold their breath during image acquisition. Most of the prior generations of clinically available scanners can cover the entire heart in 10 to 12 seconds, whereas the current 320-detector, wide-volume MDCT scanner, as well as the current dual-source MDCT scanner (with high-pitch spiral acquisition—see subsequent text), can image the entire heart in just one heartbeat.
4.Rapid improvements in CT technology and protocoling have outpaced research in the field. Many studies investigating the diagnostic and prognostic yield of information gained from cardiac CT were not based on the latest generation MDCT scanners, but rather on single-beam or MDCT detector systems with fewer detectors (e.g., 16- to 64-detector MDCT scanners).
C.Current CT hardware
1.MDCT involves using an x-ray tube mounted opposite multiple detector rows on a gantry, which is then rotated around the patient at a rapid rate (220 to 400 ms/rotation). The patient is moved at either a fixed or variable speed, or pitch, through the scanner. An increasing number of detectors allows for an increased z-axis (cranial–caudal) coverage, permitting faster scans with improved image quality, because of less cardiac and respiratory motion artifact. Temporal resolution is improved by faster gantry rotation, the use of two x-ray tubes and detector arrays mounted at 90° angles to each other (dual-source MDCT), and special reconstruction techniques. Dual-source/dual-energy scanners provide substantial improvements by utilizing dual-source MDCT technology as well as dual-energy sources to improve temporal resolution and decrease scatter. The latest generation dual-source MDCT scanners provide a temporal resolution of 66 ms. Spatial resolution is largely determined by detector architecture (typically 0.4 mm isotropic resolution), although thicker slices (1 to 5 mm) can be acquired to reduce radiation dose according to the study indication. MDCT can be used for both cardiac and noncardiac studies, and it is now the most widely used type of CT hardware for cardiac imaging.
2.Electron beam computed tomography (EBCT), although rarely used today, was specifically developed for cardiac imaging. It involves the use of a rapidly oscillating electron beam reflected onto a stationary tungsten target. Because there is no mechanical motion within the gantry, EBCT is capable of very high temporal resolution (50 to 100 ms). EBCT was used primarily for the quantitative detection of coronary artery calcification (CAC).
D.Image acquisition techniques
1.Acquisition modes. Both prospectively triggered axial acquisition and spiral (helical) retrospectively gated acquisition are available for most MDCT scanners. High-pitch spiral acquisition (flash mode) is unique to current generation dual-source MDCT scanners.
2.Prospectively electrocardiogram (ECG)-triggered sequential (axial, “step-and-shoot”) mode. Single transaxial slices are sequentially acquired while the patient table is incrementally advanced between successive rotations of the gantry. For patients with adequate heart rate control, this mode should be the acquisition mode of choice for CT coronary angiography. With prospective ECG triggering, image acquisition occurs only during a prespecified part of the R–R interval. Significant reductions in radiation exposure (up to 90%) are obtained with this mode of image acquisition, compared with retrospective ECG gating.
3.Retrospectively ECG-gated spiral (helical) mode. Data are continuously acquired during constant rotation of the gantry with simultaneous, constant (z-axis) movement of the patient through the scanner. Because the tube does not perform a complete rotation in any plane, x-ray data are interpolated from a series of sequential frames to create a single tomographic image. This mode of image acquisition should be considered in situations where patients have arrhythmias and/or tachycardia.
4.High-pitch spiral acquisition (flash mode). This is a newer MDCT acquisition mode, available on current generation dual-source MDCT scanners. The latest generation dual-source scanners enable continuous sampling of the z-axis at a pitch value of 3.4, by interleaving data acquired from two detector systems. For a CT coronary angiography study, scan acquisition is most commonly triggered in early diastole (60% of the R–R interval) and completed in one cardiac cycle. The strength of this mode of acquisition is the low radiation dose (often <2 millisieverts [mSv]).
5.ECG gating
a.Prospective triggering. The trigger signal is derived from the patient’s ECG based on a prospective estimation of the R–R interval. The scan is usually triggered to begin at a defined point after the R-wave, usually allowing image acquisition to occur during diastole. Prospective ECG triggering is one of the most dose-efficient ways of cardiac scanning, because only the very minimum scan data needed for image reconstruction are acquired. Limitations of prospective triggering (or “gating”) include the fact that the acquired data set will be of a limited portion (or phase) of the cardiac cycle only, limiting the opportunity for evaluating image data sets from other cardiac phases. In addition, prospective triggering depends greatly on the regularity of the patient’s heart rate and can result in serious misregistration artifact in the setting of arrhythmia.
b.Retrospective gating. Unlike prospective triggering, retrospective ECG gating collects data during the entire cardiac cycle. Once the scan is complete, data from specific periods of the cardiac cycle are used for image reconstruction by retrospective referencing to the ECG signal. This approach allows reconstructions to be made from multiple segments of the cardiac cycle and allows assessment of ventricular function via dynamic four-dimensional imaging. However, retrospective gating results in significantly higher radiation dose exposure, although this can be somewhat mitigated by dose modulation (see subsequent text).
E.Other imaging considerations
1.Segmented reconstruction refers to image acquisition algorithms that use scan data from more than one cardiac cycle for image reconstruction. This can improve the effective temporal resolution of the scan at the cost of a slight increase in radiation dose.
2.Dose (or tube current) modulation. MDCT scanners may operate with fluctuating tube currents that increase radiation dose during portions of diastole (when diagnostic images are most likely to be obtained) and decrease during systole. Dose modulation typically reduces effective radiation dose by approximately 33%, and it is most effective at lower heart rates.
a.Image reconstruction and interpretation. Images are most frequently viewed from axial and double oblique planes, in which the three-dimensional data set is manipulated by the interpreting physician so that multiple planes can be viewed to assess cardiac morphology and coronary anatomy. Additional postprocessing techniques can be performed to provide further diagnostic information or, more frequently, to present to the referring physician.
b.Multiplanar reformation involves creating straight or curved image planes by cutting orthogonally or obliquely through the three-dimensional acquisition. This aids in evaluating complex three-dimensional structures, such as the coronary arteries.
c.Maximal-intensity projections are created by compressing a predetermined volume of image data into a two-dimensional projection of the brightest voxels. This is similar in principle to the two-dimensional images created by typical invasive angiography.
d.Three-dimensional or volume rendering is an advanced image processing approach that uses semitransparent visualization of the outer contours of volumetric data, giving the appearance of a three-dimensional structure. Although often not as useful for assessing smaller structures, these reconstructions can be very helpful for understanding complex spatial relationships between major intrathoracic structures.
e.Four-dimensional or cine imaging from retrospectively ECG-gated spiral data acquisition generates cine images of the CT data for evaluating ventricular and valvular function.
3.Contrast-enhanced imaging. Administration of iodinated contrast media increases the attenuation of the blood pool, improving vessel delineation and tissue characterization. When using contrast, image acquisition must be timed such that images are acquired when the blood pool saturation in the target structure is maximal. Various techniques exist to time the arrival of the contrast bolus in the arterial tree and initiate imaging. The specific risks of contrast media are discussed in Section IV.
III.Indications. The roles of cardiac CT in evaluating patients with cardiovascular disease are diverse, and continue to evolve. Generally accepted indications for cardiac CT are listed in Table 49.1 and are discussed in the context of specific clinical situations in Section VI. Appropriate-use criteria (AUC) for the use of cardiac CT have also been published and are discussed at the end of the chapter. The following is a brief listing of the more common indications for MDCT.
TABLE 49.1 Selected Appropriate Indications for Cardiac Computed Tomography |
|
Category |
Specific Appropriate Indications |
|
Suspected CAD with symptoms |
Intermediate pretest probability of CAD with uninterpretable ECG or unable to exercise |
Acute chest pain with intermediate pretest probability of CAD and no ECG changes and negative serial enzymes |
|
Evaluation of suspected anomalous coronary artery anatomy |
|
Coronary artery calcium scoring |
Asymptomatic patients at intermediate CAD risk Asymptomatic patients with low CAD risk, with a family history of premature CAD |
Evaluation of intra- and extracardiac structures |
Evaluation of a cardiac mass (suspected tumor or thrombus) in patients with suboptimal/limited images from other noninvasive imaging modalities (e.g., TTE, TEE) |
Pericardial disease |
Evaluation of pericardial anatomy |
Congenital heart disease |
Assessment of complex congenital heart disease including anomalies of great vessels |
Pulmonary vein anatomy |
Evaluation of pulmonary vein anatomy prior to invasive radiofrequency ablation for atrial fibrillation |
Biventricular pacing |
Mapping of the coronary vein anatomy prior to placement of biventricular pacemaker |
Assessment of cardiac structure |
Assessment of right ventricular morphology and suspected arrhythmogenic right ventricular dysplasia |
CAD, coronary artery disease; ECG, electrocardiogram; TEE, transesophageal echocardiography; TTE, transthoracic echocardiography.
Adapted from the Taylor AJ, Cerqueira M, Hodgson J, et al. ACCF/SCCT/ACR/AHA/ASE/ASNC/SCMR 2010 appropriate use criteria for cardiac computed tomography. J Am Coll Cardiol. 2010;56:1864.
A.Evaluation of chest pain in patients with low to intermediate pretest probability of obstructive disease and ongoing symptoms (e.g., chest pain, dyspnea) with an equivocal stress test
B.Suspicion of coronary artery anomalies: Because of the high spatial resolution and the ability to create three-dimensional reconstructions of the vasculature, MDCT has very high sensitivity and specificity for coronary artery anomalies.
C.Pulmonary vein evaluation: This can be performed often before or after pulmonary vein isolation (PVI) for atrial fibrillation. This is helpful for the mapping of pulmonary venous anatomy preprocedurally and to exclude pulmonary vein stenosis postprocedurally.
D.Evaluation of cardiac masses in conjunction with or when other noninvasive imaging modalities, such as transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE), are limited/unrevealing
E.Evaluation of pericardial disease in conjunction with or when other noninvasive imaging modalities, such as TTE and TEE, are not able to provide the complete set of diagnostic data (e.g., suboptimal/limited images)
F.Assessment of anatomy in complex congenital heart disease
G.Presurgical evaluation, particularly before redo open heart surgery: Noncontrast MDCT can be used to assess for the presence of significant calcification in the ascending aorta, as well as assess proximity of cardiovascular structures to the sternum.
H.Assessment of graft patency after prior bypass surgery may be feasible in select cases, although the study can be limited by artifacts related to calcification and surgical clips.
I.Evaluation of aortic disease: MDCT is the test of choice for evaluating thoracoabdominal aortic aneurysms. It is also useful in the long-term follow-up of patients who have undergone prior aortic surgery or endovascular stenting.
IV.Contraindications. In comparison to cardiac magnetic resonance imaging (MRI), few absolute contraindications exist for cardiac CT. However, there are important risks associated with radiation and/or contrast exposure that must be weighed against the benefits of the study. Relative contraindications to CT scanning are listed below.
A.Renal insufficiency. Given the potential for contrast-induced nephropathy, patients with significant renal insufficiency (i.e., estimated glomerular filtration rate < 30 mL/min/1.73 m2) should not undergo contrast-enhanced CT, unless the information from the scan is critical and the risks/benefits are thoroughly discussed with the patient.
B.Contrast (iodine) allergy. Patients with allergic reactions to contrast should be pretreated with diphenhydramine and steroids before contrast administration. A prior anaphylactic response to contrast is generally felt to be an absolute contraindication for intravenous iodinated contrast administration by many institutions.
C.Recent intravenous iodinated contrast administration. Patients who have received an intravenous dose of iodinated contrast should avoid contrast-enhanced CT scanning for 24 hours to reduce the risk of contrast-induced nephropathy. For younger patients with normal renal function without risk factors for contrast-induced nephropathy, contrast doses of up to 150 to 200 mL per 24 hours are generally well tolerated.
D.Hyperthyroidism. Iodinated contrast is contraindicated in the setting of uncontrolled hyperthyroidism because of possible precipitation of thyrotoxicosis.
E.Atrial fibrillation, or any significant arrhythmia, is a relative contraindication to CT coronary angiography because of image degradation from suboptimal ECG gating.
F.Inability to breath-hold for at least 10 seconds. Image quality will be significantly reduced because of respiratory motion artifact, if the patient cannot comply with breath-hold instructions.
V.Safety
A.Radiation exposure is an important consideration in various cardiac imaging modalities, including CT. Radiation doses of cardiac CT scans vary greatly, depending on the scan parameter settings, scan range (cranial–caudal length of the scan), gender (women receive more radiation because of breast tissue), and patient’s body habitus (obesity increases exposure).
1.Estimates of radiation dose from MDCT have varied widely in the literature. Effective dose is an estimate of the dose to patients during an ionizing radiation procedure and is expressed in millisieverts. For reference, the estimated dose from a chest x-ray is 0.04 to 0.10 mSv, and the average annual background radiation in the United States is 3 to 3.6 mSv. Invasive diagnostic coronary angiography provides effective doses of 2.1 to 4 mSv. In comparison, CT coronary angiography studies have reported doses ranging from 3.6 mSv to as high as 18 mSv, depending on the scan parameters, with most estimates ranging from roughly 4 to 11 mSv.
2.Table 49.2 lists radiation dose ranges for the most commonly used cardiac imaging modalities.
TABLE 49.2 Estimated Radiation Exposure from Cardiac Imaging Procedures |
||
Diagnostic Procedure |
Typical Effective Dose (mSv) |
Equivalent Period of Natural Background Radiation |
Natural background radiation |
3–4 (range 1.5–7.5) |
1 y |
Chest x-ray (PA and lateral) |
0.04 |
6 d |
Transatlantic flight |
0.03 |
5 d |
Lung ventilation (Kr-81m) |
0.1 |
2–4 wk |
Lung perfusion study (Tc-99m) |
1 |
4–6 mo |
Calcium scoring |
0.8–2 |
3–6 mo |
CT head |
2 |
6 mo |
Cardiac catheterization (diagnostic) |
3–4 |
1 y |
64-Slice MDCT (with dose modulation) |
8–12 |
2–3 y |
Second-generation, wide-volume 320-detector row MDCT Latest generation, dual-source MDCT (high-pitch spiral/flash mode) |
1–2 |
3–6 mo |
Myocardial perfusion (Tl-201) |
15–18 |
4–5 y |
CT abdomen/pelvis |
10–20 |
3–6 y |
Cardiac PET |
14–20 |
4–6 y |
CT, computed tomography; MDCT, multidetector computed tomography; mSv, millisievert; PA, posterolateral; PET, positron emission tomography; R, Roentgen units.
3.Feasibility of low-dose CT coronary angiography. With the use of prospective ECG triggering, axial imaging modes, dose reduction, and software adaptations, recent studies have reported the feasibility of CT coronary angiography with comparable image quality and substantially reduced radiation doses (i.e., 1.1 to 3.0 mSv). This remains an area of active investigation. For second-generation, wide-volume 320-detector row MDCT scanners, submillisievert radiation dose has been reported for CT coronary angiography studies. With the current generation, dual-source MDCT scanners utilizing the high-pitch spiral acquisition (flash mode), many CT coronary angiographic studies can be performed with less than 2 mSv of radiation dose.
B.Contrast-induced nephropathy. Iodinated contrast media can cause renal ischemia by reducing renal blood flow or increasing oxygen demand and may also have a direct toxic effect on tubular epithelial cells. If a contrast-enhanced CT study is necessary in patients with significant renal insufficiency, prophylactic measures should be taken to reduce the risk of renal damage. Most cardiac CT studies require between 80 and 100 mL of contrast.
1.Risk factors
a.Preexisting renal insufficiency
b.Diabetes mellitus
c.Increased volume of contrast media
2.Prophylactic measures include saline hydration, use of low-osmolar agents, and sodium bicarbonate infusion, although the data for each of these measures remain somewhat controversial. The use of N-acetylcysteine has been shown to have no effect in slowing the progression of contrast-induced nephropathy.
VI.Clinical Applications
A.Coronary calcium scoring uses the observation that coronary calcium is a surrogate marker for coronary atherosclerotic plaque. Studies have shown that the complete absence of coronary artery calcium makes the presence of significant coronary luminal obstruction highly unlikely and indicates a very low risk of future coronary events. Men tend to have higher calcium scores, and individuals of either gender with renal insufficiency or diabetes mellitus tend to have higher coronary calcium scores. Coronary calcium scoring is considered an appropriate test for patients with intermediate coronary artery disease (CAD) risk and those low-risk patients with a family history of premature CAD.
1.Either noncontrast EBCT or MDCT can be used (typically with 3.0 mm slice thickness). Contrast is not necessary because calcium is readily identified secondary to its very high x-ray attenuation coefficient (high H.U. score).
2.The Agatston CAC volume score is the most frequently used scoring system. It is derived by measuring the area of each calcified coronary lesion and multiplying it by a coefficient of 1 to 4, depending on the maximum CT attenuation within that lesion. It is important to note that interobserver variability exists in Agatston scores. However, with very low and very high scores, such interobserver variability has little clinical meaning. The interobserver variability can be as high as 3%.
a.The CAC score can be classified into five groups: (1) 0, no coronary calcification; (2) 1-100, mild coronary calcification; (3) >100 to 399, moderate calcification; (4) 400 to 999, severe calcification; and (5) ≥1,000, extensive calcification.
b.The CAC score is age specific and gender specific. Therefore, there has to be a comparison of the individual data with a “normal” cohort in order to produce meaningful data, usually presented as a percentile distribution (e.g., Multi-Ethic Study of Atherosclerosis Risk Score). In general, CAC develops 10 to 15 years later in life in women than in men. Similarly, CAC is generally five to seven times lower at any given age in women than in men.
c.In a typical cohort of patients with CAD, the median CAC score is 975 for men and 370 for women. In comparison with a CAC score of 0, the presence of any CAC is associated with a fourfold risk of coronary events over 3 to 5 years.
d.In patients at intermediate clinical risk for coronary events (e.g., by Framingham score), the CAC score can help reclassify patients to a higher or lower risk group. For instance, a CAC score of 0 confirms low risk of events. Conversely, a CAC score of >400 is observed with a significant cardiac event rate (>2% per year) in patients who appear to be of intermediate risk, according to the Framingham score.
e.Because statins have no documented effect on CAC progression, there is no value in repeating CAC in persons with a score of >100 or the 75th percentile.
3.However, not every atherosclerotic plaque is calcified, and even the detection of a large amount of calcium does not directly translate into the presence of significant obstructive coronary artery lesions. Therefore, CAC adds incrementally to traditional risk factor assessment and should not be used in isolation. The test is most useful in intermediate-risk populations, in which a high or low score may reclassify individuals to a higher or lower risk group, respectively. Unselected screening is not recommended.
B.CT coronary angiography has been shown to be an accurate noninvasive modality for visualizing the coronary arteries, with high sensitivity (85% to 95%) and specificity (95% to 98%), compared with invasive coronary angiography.
1.CT coronary angiography for evaluating CAD is most useful in low- to intermediate-risk patients with angina or anginal equivalent symptoms. The negative predictive value of CT coronary angiography is uniformly high in clinical studies, approaching 95% to 100%; in other words, CT coronary angiography is an excellent modality for ruling out coronary disease.
2.Patients who are generally poor candidates for CT coronary angiography include those who are likely to have heavily calcified coronary arteries (older than 75 years, end-stage renal disease, and Paget disease), atrial fibrillation/flutter, frequent ventricular ectopic beats, or uncontrolled tachycardia. Quantification of stenosis severity is often impossible in densely calcified arteries, whereas image quality is significantly degraded in patients with arrhythmias or tachycardia. The negative predictive value dropped to 83% in one study, where patients with Agatston CAC score of <600 were included.
3.Known severe CAD is generally a contraindication to CT coronary angiography. However, cardiac CT has been shown to have high sensitivity and specificity for the assessment of bypass graft patency in patients with previous coronary artery bypass grafting (see subsequent text).
4.Stent patency. Patients with prior coronary artery stents are generally poor candidates for CAC and CT angiography, although selected patients with proximal left anterior descending or left main stents may be successfully imaged. Current CT technology does not allow for the accurate quantification of in-stent restenosis severity, because of blooming artifact from the metallic struts of the stent.
5.When assessing the coronary arteries, noncalcified plaque appears as a low to intermediate attenuation irregularity in the vessel wall. Calcified plaques are bright, high-attenuation lesions in the vessel wall and may be associated with positive remodeling of the vessel. Densely calcified plaques are often associated with calcium blooming artifact, which can lead to overestimation of luminal stenosis severity.
6.Certain characteristics of noncalcified plaque, such as positive remodeling, have been reported to predict atherosclerotic lesions at higher risk of developing subsequent acute coronary syndromes.
7.The accuracy of CT coronary angiography is highest in the larger proximal to medium vessels, which are more likely to benefit from an invasive management strategy. Coronary stenoses are generally categorized as mild (<50% diameter stenosis), moderate (50% to 70% diameter stenosis), or severe (>70% diameter stenosis).
C.CT coronary angiography in stable chest pain
1.Recent randomized data (PROMISE study ) from 10,003 patients, comparing anatomical (with CT coronary angiography) versus functional testing (exercise electrocardiography, nuclear stress testing, or stress echocardiography) for the assessment of symptoms suggestive of CAD, over a median follow-up of 25 months, found that long-term patient outcomes were equivalent with both strategies. CT coronary angiography was associated with fewer catheterizations showing no obstructive CAD than functional testing, although more patients in the anatomical testing group underwent subsequent catheterization.
2.In a prospective, open-label, multicenter trial, 4,146 patients with stable chest pain were randomly assigned to standard care or standard care plus CT coronary angiography (SCOT-HEART). CT coronary angiography helped clarify the diagnosis of angina: At 6 weeks, CT coronary angiography reclassified the diagnosis of CAD in 558 (27%) patients and the diagnosis of angina because of CAD in 481 (23%) patients. This led to decreased functional testing, increased invasive angiography, and more focused treatment regimens for patients. After 1.7 years, CT coronary angiography was associated with an apparent, but statistically nonsignificant reduction in fatal and nonfatal myocardial infarction.
D.CT coronary angiography in acute chest pain
1.In a multicenter study of 1,000 patients presenting to the emergency department with suspected acute coronary syndrome (ROMICAT-II), early CT coronary angiography, compared with standard evaluation, reduced the mean length of stay in hospital by 7.6 hours (p < 0.001), and more patients were discharged directly from the emergency department (47% vs. 12%, p < 0.001). CT coronary angiography resulted in more downstream testing and higher radiation exposure, without a significant increase in overall costs of care.
2.Another multicenter study enrolled 1,370 patients presenting to the emergency department with a suspected acute coronary syndrome: 908 patients in the CT coronary angiography group and 462 patients in the conventional care group. Compared with conventional care, CT coronary angiography was found to be safe and enabled early discharge from the emergency department (49.6% vs. 22.7%). Of 640 patients with a negative CT coronary angiography, none died or had a myocardial infarction within 30 days.
E.Bypass graft imaging
1.Graft location. MDCT can accurately characterize the origin, course, and touchdown of prior bypass grafts using intermediate slice thickness (e.g., 1.5 mm). This can be important for surgical planning (see subsequent text).
2.Graft patency. Using a protocol similar to that used for coronary artery assessment (>1 mm slice thickness), the patency of both arterial and venous bypass grafts can be assessed. Studies have suggested that the sensitivity and specificity of MDCT for detecting stenosis or occlusion of bypass grafts, when compared with invasive angiography, are excellent (97%). Occasionally, artifacts related to metallic clips can interfere with the assessment of distal anastomosis of an arterial graft (internal mammary or radial artery graft).
F.Coronary artery anomalies. Because of the three-dimensional data acquisition, MDCT is an excellent modality for assessing patients with known or suspected coronary artery anomalies. MDCT can accurately assess the origin and course of anomalous coronaries and can delineate the relationship of the coronary artery to neighboring structures. Although MRI can also be used to assess anomalous coronaries without the need for radiation exposure, the spatial resolution, ease of data acquisition, and reliable image quality of MDCT make it a reasonable first choice (Fig. 49.1). Intramyocardial bridging can also be detected with high sensitivity, although the clinical significance of this relatively common finding is uncertain.
FIGURE 49.1 Axial multidetector computed tomography (MDCT) reconstruction demonstrating anomalous origin of the right coronary artery (RCA; arrow) from the left coronary cusp, with an intramural course, and compression of the ostium of the vessel between the ascending aorta and pulmonary artery. Note that the ostium of the RCA has a slit-like appearance. This image is from a 68-year-old male, with recurrent chest pain, who is being considered for surgical intervention.
G.Cardiac morphology/function. Contrast-enhanced MDCT can provide high-resolution morphologic images of the cardiac chambers as well as accurate assessment of right and left ventricular systolic function. However, other imaging modalities such as echocardiography or MRI, which do not require radiation exposure, are generally preferred for the initial assessment of ventricular function.
1.Patients with prior myocardial infarction can have fibrous replacement of myocardium with or without calcification, ventricular wall thinning, aneurysm formation, and intracavitary thrombus. Cardiac CT is rarely used as the primary imaging investigation to assess for prior ischemic damage. Delayed-enhancement imaging with cardiac MRI remains the most widely accepted imaging modality of choice for this indication.
2.Ventricular dysplasia is characterized by fibrous and/or fatty replacement of myocardium, ventricular wall thinning and/or focal aneurysm formation, and ventricular cavity dilation with regional or global wall motion abnormalities.
3.Mass. Compared with cardiac MRI, CT provides less tissue characterization, although the attenuation of a mass (in H.U.) can be helpful. For instance, lipomas have low H.U. numbers, cysts have water density (i.e., 0 to 10 H.U.), and thrombi have low to intermediate H.U. numbers. Atrial myxoma can be visualized easily in the left atrium, although right atrial masses may be more difficult to assess, because of contrast mixing at the junction of the right atrium and inferior vena cava (IVC).
H.Pericardial diseases. The pericardium appears as a thin structure (1 to 2 mm) surrounding the heart, usually visible with a small amount of adjacent epicardial/pericardial fat.
1.Findings of pericardial constriction on CT include irregular pericardial thickening and calcification, conical or tubular deformities of one or both ventricles, enlargement of one or both atria, dilation of the IVC, and a characteristic diastolic bounce of the interventricular septum.
2.Pericardial effusions can be reliably detected by CT, with the presence of a trivial to small amount of pericardial fluid considered physiologic. Cardiac tamponade is better evaluated by echocardiography, because of its ability to provide hemodynamic assessment.
3.A pericardial cyst will appear as a well-circumscribed paracardiac mass with characteristic water attenuation (H.U. = 0), usually in the right costophrenic angle.
4.Both primary neoplasms and, more commonly, metastatic neoplasms can be visualized in the pericardium.
5.Rare pericardial disorders such as partial or complete congenital absence of the pericardium can be diagnosed by CT, with the demonstration of posterolateral displacement of the left ventricular apex and interposition of lung tissue between the aorta and pulmonary artery.
I.Congenital heart disease. MDCT may be useful in select patients in whom echocardiography is nondiagnostic or inadequate and MRI is not available or contraindicated. The ability to evaluate cardiovascular anatomy in multiple planes is often helpful for delineating cardiac morphology in congenital heart disease, particularly with regard to the relationship of the great vessels, pulmonary veins, and coronary arteries. Specific situations in which MDCT can be helpful include the detection of shunts (e.g., sinus venosus atrial septal defect, unroofed coronary sinus, patent ductus arteriosus), visualization of pulmonary arteries in cyanotic congenital heart disease, precise definition of aortic anatomy in Marfan syndrome or coarctation, and delineation of partial or total anomalous pulmonary venous drainage. Additionally, CT can be useful as the follow-up imaging modality in patients with congenital heart disease, such as l-transposition of the great arteries, who have had prior pacemaker or ICD implantation, contraindicating cardiac MRI.
J.Diseases of the aorta constitute a common and important indication for CT examinations. Contrast-enhanced MDCT is nearly 100% sensitive and specific for evaluating acute aortic syndromes. ECG gating is critically important for studies of the aortic root and ascending aorta, given the propensity for motion artifacts to mimic dissection flaps on nongated studies.
1.Acute aortic dissection (see Chapter 26) is characterized on CT by visualization of a dissection flap (i.e., separation of the intima from the media) that forms true and false lumens. The CT study can characterize the origin and extent of the dissection, classify it as type A or B, assess for concomitant aneurysmal aortic dilation, and identify branch vessel involvement.
2.Aortic intramural hematomas are believed to be caused by spontaneous hemorrhage of the vasa vasorum of the medial layer. They appear as crescent-shaped areas of increased attenuation with eccentric aortic wall thickening. Unlike dissections, hematomas do not spiral around the aorta.
3.Aortic aneurysm occurs when there is enlargement (≥150%) of the normal aortic caliber (usually >5 cm in the thoracic aorta and >3 cm in the abdominal aorta). Given the often tortuous course of a dilated aorta, it is important that these measurements be made in the true short axis of the aorta, because oblique cuts can result in erroneous overestimation. Quantitative measurements of an aortic aneurysm can be made for planning endovascular repair with a stent graft.
4.Penetrating atherosclerotic ulcer. These tend to be focal lesions of the descending thoracic aorta that appear as contrast-filled irregular outpouchings of the aortic wall.
K.Evaluation of pulmonary veins. In the context of electrophysiology interventions such as PVI, preprocedural MDCT can be used to define pulmonary venous anatomy and identify supernumerary veins. Postprocedural MDCT can be used to evaluate for pulmonary vein stenosis. Additionally, in the setting of congenital heart disease, CT can be used to identify anomalous pulmonary venous return.
L.Evaluation of pulmonary embolism (PE). MDCT is highly accurate in detecting PE, which appears as a filling defect in the pulmonary arteries. This modality is most sensitive for proximal (main through segmental branches) thrombi. Small, more distal emboli may be missed.
M.Valvular heart disease. Visualization of the valve leaflets, particularly the aortic valve, is feasible with newer generation scanners because of their improved temporal resolution. Dynamic four-dimensional MDCT imaging is particularly useful for the assessment of prosthetic valves for suspected thrombus, as well as infective endocarditis.
N.Surgical planning. The utility of MDCT in surgical planning before cardiothoracic surgery, particularly for reoperations, is increasingly recognized. Preoperative scans can evaluate the proximity of mediastinal structures to the sternum (i.e., aorta, right ventricle, and bypass grafts) and the degree of aortic calcification (i.e., to guide cannulation sites) and concomitantly provide information about cardiac morphology (e.g., presence of a ventricular aneurysm).
O.Peripheral arteries. MDCT can be used to evaluate peripheral arteries, including the carotid, renal, visceral, and lower extremity vessels. Indeed, imaging these vessels is generally more straightforward than coronary imaging, because of their large caliber and minimal motion. CT can be used for planning and follow-up of vascular disease in these peripheral vascular beds. Given the larger caliber of these vessels, assessment of stent patency is often quite feasible.
P.Structural cardiac interventions. MDCT is now the accepted gold-standard imaging investigation for the assessment of the aortic annulus, as well as iliac–femoral anatomy for the purpose of planning for transcatheter aortic valve replacement. MDCT has also been increasingly used for the assessment and sizing of the mitral valve annulus, to guide transcatheter mitral valve replacement. Recent data have also emerged that MDCT assessment of the left atrial appendage ostium is superior to TEE for sizing the Watchman device for left atrial appendage occlusion.
VII.Appropriateness Criteria. Appropriateness criteria (AUC) for the appropriate use of cardiac CT, endorsed by multiple societies and led by the American College of Cardiology, have been published. The current iteration of AUC for cardiac CT was published in 2010. They serve as guidelines, helping clinicians with the appropriate use of cardiac CT in the context of different clinical situations. Each clinical situation is designated appropriate (A), uncertain (U), or inappropriate (I) for the use of cardiac CT. For instance, cardiac MDCT is considered an appropriate (A) investigation for intermediate-risk patients with suspected CAD and chest pain, but negative ECG and biomarkers; it is considered an uncertain (U) indication to use cardiac MDCT for routine evaluation of coronary arteries following heart transplantation; it is considered an inappropriate (I) indication to use cardiac CT to investigate patients with documented moderate or severe ischemia on functional testing.
ACKNOWLEDGMENTS: The authors would like to thank Drs. Parag R. Patel, Milind Desai, Deepu Nair, Paul Schoenhagen, Richard D. White, Sandra S. Halliburton, and Stacie A. Kuzmiak for their contributions to earlier editions of this chapter.
Achenbach S, Moselewski F, Ropers D, et al. Detection of calcified and noncalcified coronary atherosclerotic plaque by contrast-enhanced, submillimeter multidetector spiral computed tomography: a segment-based comparison with intravascular ultrasound. Circulation. 2004;109:14–17.
Chen MY, Shanbhag Sm, Arai AE. Submillisievert median radiation dose for coronary angiography with a second-generation 320-detector row CT scanner in 107 consecutive patients. Radiology. 2013;267(1):76–85.
Douglas PS, Hoffmann U, Patel MR, et al. Outcomes of anatomical versus functional testing for coronary artery disease. N Engl J Med. 2015;372(14):1291–1300.
Hoffman U, Ferencik M, Udelson JE, et al. Prognostic value of noninvasive cardiovascular testing in patients with stable chest pain: insights from the PROMISE trial. Circulation. 2017;135(24):2320–2332.
Hoffmann U, Truong QA, Schoenfeld DA, et al. Coronary CT angiography versus standard evaluation in acute chest pain. N Engl J Med. 2012;367(4):299–308.
Hunold P, Vogt FM, Schmermund A, et al. Radiation exposure during cardiac CT: effective doses at multi-detector row CT and electron-beam CT. Radiology. 2003;226:145–152.
Leber AW, Knez A, Becker A, et al. Accuracy of multidetector spiral computed tomography in identifying and differentiating the composition of coronary atherosclerotic plaques: a comparative study with intracoronary ultrasound. J Am Coll Cardiol. 2004;43:1241–1247.
Litt HI, Gatsonis C, Snyder B, et al. CT angiography for safe discharge of patients with possible acute coronary syndromes. N Engl J Med. 2012;366(15):1393–1403.
McClelland RL, Chung H, Detrano R, et al. Distribution of coronary artery calcium by race, gender, and age: results from the Multi-Ethnic Study of Atherosclerosis (MESA). Circulation. 2006;113(1):30–37.
Meyer TS, Martinoff S, Hadamitzky M, et al. Improved noninvasive assessment of coronary artery bypass grafts with 64-slice computed tomographic angiography in an unselected patient population. J Am Coll Cardiol. 2007;49:946–950.
Miller JM, Rochitte CE, Dewey M, et al. Diagnostic performance of coronary angiography by 64-row CT. N Engl J Med. 2008;359(22):2324–2336.
Motoyama S, Sarai M, Harigaya H, et al. Computed tomographic angiography characteristics of atherosclerotic plaques subsequently resulting in acute coronary syndrome. J Am Coll Cardiol. 2009;54(1):49–57.
Pundziute G, Schuijf JD, Jukema JW, et al. Prognostic value of multislice computed tomography coronary angiography in patients with known or suspected coronary artery disease. J Am Coll Cardiol. 2007;49:62–70.
SCOT-HEART investigators. CT coronary angiography in patients with suspected angina due to coronary heart disease (SCOT-HEART): an open-label, parallel-group, multicentre trial. Lancet. 2015;385(9985):2383–2391.
Xu B, Betancor J, Sato K, et al. Computed tomography measurement of the left atrial appendage for optimal sizing of the Watchman device. J Cardiovasc Comput Tomogr. 2018;12(1):50–55.
Relevant Book Chapters
Murphy RT, Garcia MJ. Computed tomography of the heart. In: Topol EJ, ed. Textbook of Cardiovascular Medicine. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2007:941–960.
Taylor AJ. Cardiac computed tomography. In: Braunwald E, ed. Braunwald’s Heart disease: A Textbook of Cardiovascular Medicine. 10th ed. Philadelphia, PA: WB Saunders; 2015:341–363.
To A, Desai MY. Cardiac MRI and CT. In: Griffin BP, Kapadia SR, Rimmerman CM, eds. The Cleveland Clinic Cardiology Board Review. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2013:137–154.
Relevant Guidelines and Appropriateness Criteria
Abbara S, Blanke P, Maroules CD, et al. SCCT guidelines for the performance and acquisition of coronary computed tomographic angiography: a report of the Society of Cardiovascular Computed Tomography Guidelines Committee endorsed by the North American Society of Cardiovascular Imaging (NASCI). J Cardiovasc Comput Tomogr. 2016;10(6):435–449.
Hecht HS, Cronin P, Blaha MJ, et al. 2016 SCCT/STR guidelines for coronary artery calcium scoring of noncontrast noncardiac chest CT scans: a report of the Society of Cardiovascular Computed Tomography and Society of Thoracic Radiology. J Cardiovasc Comput Tomogr. 2017;11(1):74–84.
Rybicki FJ, Udelson JE, Peacock, WF, et al. 2015 ACR/ACC/AHA/AATS/ACEP/ASNC/NASCI/SAEM/SCCT/SCMR/SCPC/SNMMI/STR/STS Appropriate utilization of cardiovascular imaging in emergency department patients with chest pain. J Am Coll Cardiol. 2016;13(2):e1–e29.
Taylor AJ, Cerqueira M, Hodgson J, et al. ACCF/SCCT/ACR/AHA/ASE/ASNC/SCMR 2010 appropriate use criteria for cardiac computed tomography. J Am Coll Cardiol. 2010;56:1864.