Chapter 5. Cardiac Computed Tomography

Introduction

Cardiac computed tomography (CT) has become one of the fast growing areas of cardiovascular imaging since the advent of multidetector scanners in 1998. Early cross-sectional CT imaging of the heart was performed using electron beam CT. Continued advancements in multidetector CT (MDCT) scanners as well as their widespread availability, however, have further increased the utility of cardiac MDCT for evaluation of a variety of cardiovascular diseases.

CT uses rapidly rotating x-ray sources and detectors to create an image. The x-ray sources and detectors are mounted opposite one another on a ring, and the rotating unit is called the gantry. The patient is placed inside the ring, and radiation that is not absorbed by the patient passes into multiple detectors, and, through a process known as filtered back projection, a cross-sectional image is generated (Fig. 5–1). Images are acquired in an axial (transverse) plane in thin sections to yield isotropic voxels, matching that of in-plane spatial resolution. CT angiography (CTA) of the cardiovascular system refers to CT scanning that is performed during the injection of an iodinated contrast agent. Although much information can be obtained from a noncontrast CT, optimal evaluation of cardiac structures, including vascular anatomy, usually requires CTA.

Important aspects of cardiac CT imaging include:

1. High temporal resolution with electrocardiogram (ECG) synchronization of data acquisition to minimize cardiac motion artifacts

2. High spatial resolution with isotropic voxels to allow for oblique reconstructions without loss of resolution

3. Contrast resolution

4. Radiation dose

Image quality optimization mainly consists of elimination of artifacts related to cardiac motion, respiratory motion, and partial volume effects. Different aspects of CT imaging, including collimation, pitch, breath-hold period, field of view, reconstruction interval, rate and volume of intravenous contrast administration, reconstruction algorithm, and radiation dose, affect overall image quality. Unfortunately, no single set of optimal scan parameters currently suffices for all cardiac CT applications on all CT systems. Instead, CT techniques often reflect a combination of personal or institutional preferences coupled with individual manufacturer capabilities and tailored, to a large extent, to the individual patient's specific clinical needs.

Specific Technical Aspects

Scanner Design

The gantry design of all MDCT scanners is similar with only minor variations. Single-source systems consist of a single set of x-ray source and detectors, and dual-source systems have two sets of x-ray sources and detectors mounted on the gantry. Detector geometry and postprocessing algorithms are system specific and somewhat dependent on the number of detectors used in the gantry design and number of slices generated, currently anywhere from 4 to 320 slices. Special three-dimensional (3D) back-projection algorithms are used to correct for artifacts that arise from the cone-beam geometry that results from the oblique x-ray projections of thin collimated slices generated from the gantry.1 Thin-slice collimations (0.5-0.625 mm) with reconstructed slice thickness of 0.4 to 0.7 mm allow for near isotropic voxels, which are important for oblique reconstructions. Gantry speed rotations of 280 to 420 ms/360° allow for high temporal resolution in the range of 140 to 210 ms. Further improvements in temporal resolution are obtained with the use of dual-source technology and multisegment postprocessing algorithms, which are important for functional assessment of the heart as well as for improving image quality in patients with fast heart rates.

ECG Referencing

Normal motion of the heart is a source of significant motion artifact on conventional CT imaging. Because cardiac motion varies throughout the cardiac cycle, often in a predictable manner, synchronization of data acquisition to the heart cycle allows for combination of data acquired from consecutive gantry rotations in volumetric data sets with minimal blurring artifact related to cardiac motion.

The ECG signal is commonly used for synchronization. The time between two consecutive heartbeats is defined by the R-R interval (the interval between consecutive R waves of the ECG; 1000 ms for a heart rate of 60 beats per minute). Depending on the acquisition mode, the R-R interval of the ECG signal is used to either prospectively trigger data acquisition or to retrospectively determine the timing of reconstruction of the image data sets.

With prospective triggering, data acquisition is timed to the ECG R peak after a preselected delay within the following cardiac cycle at a given table position. After table movement, data are again sampled based on the next available R trigger (Fig. 5–2). With retrospective ECG gating, a conventional spiral data set is acquired with simultaneous tracing and recording of the patient's ECG. Following data acquisition, image data and ECG data are merged to allow for ECG-related image reconstruction after scanning (Fig. 5–3). In contrast to other CT spiral acquisitions, cardiac algorithms usually require a substantially reduced pitch factor (0.2-0.3) in order to allow image reconstruction at any time point of the cardiac cycle. A comparison of these two acquisition techniques is shown in Table 5–1.

Temporal Resolution

Temporal resolution is the amount of time needed to acquire the necessary x-ray data in order to reconstruct an image.2 In cardiac MDCT, an image can be generated using half-scan reconstruction algorithms, and hence, temporal resolution for a particular scanner corresponds to the time needed for the gantry to rotate half of a full 360° rotation, or 175 ms with a 350 ms/360° rotation speed. Further improvements in temporal resolution can be achieved with multisector reconstruction algorithms, which are based on merging data of adjacent cardiac cycles (two to four heartbeats) in order to fill a data segment more rapidly (Fig. 5–4). However, with multisector reconstructions, image quality may be degraded due to blurring artifacts related to differences in the position of cardiac structures from cycle to cycle even within the same phase of the cycle. In this regard, multisegment reconstruction is most useful when coupled to gantry rotation and the patient's heart rate using a computerized algorithm.2,3 Dual-source CT scanners achieve improved temporal resolution (75-83 ms depending on scanner) with simultaneous scanning using two sets of sources and detectors, and similar to single-source scanners, multisector reconstruction algorithms can further improve the temporal resolution.

Spatial Resolution

Spatial resolution is mainly dependent on detector width, slice collimations, and data sampling. Coronary arteries are small, ranging from 1 to 5.5 mm in diameter.4 Nitroglycerin given just prior to scanning is often used to dilate the coronary arteries.5,6 With current generation MDCT scanners, there have been great improvements in spatial resolution, particularly in the z-axis resolution, leading to near isotropic voxels with spatial resolution up to 0.4 × 0.4 × 0.4 mm.7 These advances have greatly enhanced visualization of small structures such as the coronary arteries but still remain inferior to conventional coronary angiography, which has a spatial resolution of 0.2 × 0.2 mm.8

Contrast Administration

Several considerations are important to be aware of with contrast use in cardiac imaging. First, because image quality is greatly dependent on regularity of the heart rhythm, nonionic contrast is used to prevent perturbations in the heart rate during image acquisition.9 Second, patients with cardiac disease may have low cardiac outputs, and as such, the timing of contrast arrival into the regions of interest can be variable. Hence, it is important to accurately track contrast delivery times individually for each patient. Third, because imaging time is so short in cardiac imaging, contrast osmolarity also becomes an important factor. In particular, when using third-generation scanners with very short imaging times, maximal opacification with greater osmolarity is advantageous. Finally, the physiologic impact of receiving contrast media must be taken into consideration because this can also affect the timing of contrast arrival.

Two types of contrast timing methods have been employed in current MDCT scanning systems. The first involves determining the circulation time by giving a small-volume test bolus (10-20 cc) of contrast and observing the time of its arrival in the heart, also known as a timing run. The second involves imaging a specific region of interest (eg, ascending aorta), and once the contrast density attenuation reaches a prespecified point, image acquisition starts, also known as bolus tracking. Again, exact values are dependent on scanner type, contrast type, amount and delivery method, scan delay time, and individual preference.

Post Processing

Because cardiac MDCT data are acquired with near-isotropic resolution, oblique multiplanar and thin maximum intensity projection reconstructions, including curved reformations, are used, in addition to standard axial two-dimensional (2D) evaluation, for evaluation of the cardiac structures. These oblique reconstructions are particularly important for visualization of the coronary vessels throughout their course in both a longitudinal and axial plane to the vessel of interest for the assessment of atherosclerotic plaque and luminal stenosis (Fig. 5–5). In addition to 2D evaluations, volume- and shaded surface-rendered 3D reconstructions with advanced visualization algorithms allow for the extraction of specific overlying structures such as bones, lung tissue and vessels, and soft tissue structures to focus on cardiac anatomy. These volume-rendered 3D images (Fig. 5–6) are invaluable for understanding complex cardiac anatomy (eg, anomalous coronary arteries) and in surgical planning. Shaded surface displays are commonly used for visualization of internal structures from a specified point simulating an endoscopic or angioscopic view (Fig. 5–7) and can provide important information on the internal relationship of structures, which may be important for guiding procedures (eg, pulmonary vein isolation).

Retrospective ECG gating allows for the reconstruction of volumetric image data sets during multiple phases of the cardiac cycle. Although currently temporal resolution is still limited (75 ms at best), qualitative evaluation of wall motion and valve function can be made. Additionally, precise quantitative calculation of ventricular volumes and ejection fraction can be obtained from the data sets.10-13

Radiation Considerations

Radiation dose exposure is largely dependent on the acquisition protocol used. Typical effective radiation doses for coronary CTA are 5 to 12 mSv, although doses as high as 36 mSv have been reported with some protocols. In comparison, the typical radiation dose for chest CT is 5 to 7 mSv and that of a chest x-ray is <0.1 mSv. Several methods can be used to reduce overall radiation dose. These include minimizing scan length, reducing tube voltage (eg, 100 kVp instead of 120 kVp), using ECG dose modulation schemes in which tube current is lowered during acquisition of nondesired phases, reducing amperage (mA), limiting scan field of view, using step and shoot scanning (eg, prospective gating techniques), and increasing the table speed or pitch.14 The ability to use one or multiple dose-reduction schemes is dependent on multiple factors including patient body habitus, patient heart rate, and CT scanner capabilities.

Specific Clinical Considerations

MDCT represents an important diagnostic imaging modality in the evaluation of patients for cardiac disease. Although MDCT has the ability to provide spectacular 3D information for a variety of diseases, specific technical and clinical aspects must be considered for each individual patient.

Motion Artifacts

Cardiac motion is a significant source of motion artifact for conventional CT imaging. Fast gantry speed CT scanners with resultant high temporal resolution and ECG-gated acquisitions, either prospective or retrospective, are used in cardiac CT imaging to improve image quality by minimizing cardiac motion artifact. Given the limited temporal resolution offered by current MDCT technologies, image quality can also be improved through optimization of the patient's heart rate. Reconstruction is commonly performed during mid to late diastole, when there is minimal cardiac motion. Diastolic time is approximately two-thirds of the cardiac cycle; however, with increasing heart rate, the proportion of the cardiac cycle spent in diastole decreases.1,15 β-Blockers are often used to both lower the heart rate and promote regularity of the heart rhythm or decrease arrhythmias. Depending on the optimal diastolic time interval desired, oral or intravenous β-blockers have been shown to be safe when administered to patients prior to CT scanning.15-17 In patients unable to tolerate β-blockers (eg, severe asthmatics), calcium channel blockers may provide a similar benefit.

Respiratory motion is another potential source of motion artifact that can degrade image quality on CT imaging. The use of multidetector scanners with wide coverage and minimizing scan length are methods that can decrease overall scan acquisition time and hence the amount of time required for the patient to breath-hold. Thus, for coronary imaging, scan acquisition coverage is limited to the heart. By contrast, for evaluation of a patient with bypass grafts, coverage needs to be increased to include any aortocoronary bypass grafts. This necessitates increased scan acquisition time and hence increased breath-hold time. It is important to instruct the patient prior to image acquisition on the importance of breath-holding during scan acquisition. For patients unable to breath-hold the requisite amount of time needed for scan acquisition, methods to reduce scan acquisition time or use of alternative imaging techniques should be considered.

Contrast Administration

Many cardiac CT imaging applications require the concomitant administration of intravenous iodinated contrast to improve visualization and distinction of the cardiac structures. For cardiac CT imaging, regularity of the heart rate and rhythm throughout image acquisition is important in order to optimize image quality, and potential changes in heart rate and rhythm may occur with administration of intravenous contrast. Care should be taken to screen patients with potential contraindications to contrast use, including a history of prior reaction to iodinated contrast and/or known renal insufficiency.

Image Quality

Image quality is important for diagnosis. Spatial resolution, temporal resolution, and contrast resolution are all important aspects for image quality related to CT scanners and protocol design. However, several patient-specific parameters, including body habitus, heart rate and rhythm, respiratory status, and cardiac structure being imaged, are also important aspects for diagnostic image quality. Patients with high body mass index often require a high tube current in order to produce diagnostic quality images with low image noise. Depending on the CT scanner, different methods may be used to achieve diagnostic image quality while minimizing overall radiation dose. As mentioned earlier, heart rate can be optimized for cardiac CT imaging by the administration of β-blockers to increase diastolic time. Patients with irregular heart rhythms may also benefit from β-blocker administration to regularize the heart rate (eg, suppress premature contractions). However, these drugs may be of no benefit in a patient with irregularly irregular rhythm, as in atrial fibrillation. Finally, it is important to note that although higher specifications and restrictions on these patient-specific parameters may be necessary for imaging small mobile structures such as the coronary arteries, these restrictions may not be so stringent when imaging other larger cardiac structures such as the pulmonary veins or pericardium.

Coronary Artery Evaluation

Goals in the assessment of coronary artery disease include identification of coronary plaques and flow-limiting coronary stenoses, measurement of atherosclerotic burden, and characterization of plaque components.18 Identification and quantification of calcified plaques can be performed using a noncontrast CT study. Further information on atherosclerotic burden, presence of noncalcified or mixed plaques, and assessment for stenoses requires a contrast CT study.

Noncontrast CT for Calcium Scoring

Atherosclerotic coronary calcifications are often found in patients with long-standing coronary atherosclerosis. Elevated levels of coronary calcification have been shown to correspond to increased risk of myocardial events.19-22 Current clinical indications for quantification of coronary calcium are in patients with atypical chest pain, as well as asymptomatic patients with traditional cardiovascular risk factors.23 Noncontrast CT is currently the only method available to accurately quantify the coronary calcium plaque burden. Coronary artery calcium is assessed through the measurement of the number of pixels in the CT image with a density ≥130 Hounsfield units (HU) to calculate a total calcium score,24 a calcium volumetric score,25 or an absolute calcium mass26 (Fig. 5–8). Each method has shown similar reproducibility,27-29 and most software processing programs can easily provide all three scores simultaneously. A high coronary calcium score is a sensitive but nonspecific marker for obstructive coronary artery disease (CAD), and changes in the calcium score have not been shown to correspond to changes in cardiovascular event risk.23

Contrast Coronary Cta

Coronary Artery Disease

Noncalcified atherosclerotic plaques in the coronary arteries can be visualized using contrast-enhanced CT. The high spatial resolution and soft tissue delineation can provide information about the content of atherosclerotic plaques and coronary artery wall,30 including distinction between low-density lipid-rich (47 ± 9 HU) versus higher density fibrous (104 ± 28 HU) plaques.31,32 Several studies have demonstrated the high sensitivity (72%-99%) and specificity (86%-97%) and particularly the high negative predictive value (97%-100%) of MDCT when compared against the gold standards of x-ray coronary angiography and intravascular ultrasound for the detection of significant CAD.33-39 More importantly, with increasing number of detectors and faster gantry speeds leading to an improvement in image resolution, there has been a significant increase in the number of evaluable segments compared with early-generation scanners. The presence of coarse calcifications with related blooming and streak artifact, however, still remains problematic in regard to accurate interpretation of degree of luminal narrowing.

Coronary Anomalies

Anomalies of the coronary arteries affect approximately 1% of the population, with 87% of these individuals having anomalies of the origin and distribution and 13% having coronary artery fistulae.40,41 The true incidence of ectopic origin of the coronary arteries from the aorta in the population is unknown but estimated to be between 0.17% and 0.6%. Cardiac catheteri-zation has traditionally been the preferred imaging modality for characterization of coronary artery anomalies. However, given the complex 3D nature of these anomalies, not infrequently, conventional angiography incompletely delineates the anatomic origin and course of the coronary artery. CTA of the coronary arteries has therefore become one of the accepted standards for complete evaluation of coronary artery anomalies. With the high spatial resolution, volumetric acquisition, and orientation in any plane, CT can accurately depict the origin and course of coronary artery anomalies with high sensitivity, although specificity is reduced in patients with high heart rates due to the limited temporal resolution even with cardiac gating.42,43

Assessment of Stent Patency

With the growth of nonsurgical revascularization techniques, there has been an enormous increase in the number of patients with CAD who receive coronary artery stents.44,45 Hence, early identification of in-stent restenosis is important in the management of these patients. MDCT has shown variable success for evaluation of in-stent restenosis with steady improvements noted when using newer 64-slice systems offering improvement in spatial resolution, particularly when using dedicated reconstruction kernels.45-53 However, there is still high variability of stent lumen visibility depending on the stent type, size, orientation, and surrounding tissue (Fig. 5–9). Sharp reconstruction kernels, in addition to the routine medium kernels, may provide improvement in visible lumen diameter as well as more realistic intraluminal attenuation values.44,45

Assessment of Bypass Grafts

Surgical revascularization of CAD is accomplished by coronary artery bypass grafting in which a graft (arterial or venous) is used to bypass an occluded or stenosed coronary artery. Compared with the native coronary arteries, reversed saphenous vein and internal mammary artery grafts are easier to visualize due to the reduced overall motion, larger lumen, and less convoluted course. Familiarity with the common types of grafts used and placement can aid in both planning and interpretation of images (Fig. 5–10). Venous conduits are generally wider and longer than their arterial counterparts but have reduced long-term patency.

With the MDCT technique, special considerations must be taken into account during data acquisition. Because most patients have an in situ mammary artery graft, image acquisition must cover the entire thorax in order to visualize the proximal anastomosis and origin of the internal mammary arteries. Given the larger volume of coverage, adjustments in contrast dose and scan delay timing after contrast administration should be made to ensure adequate contrast enhancement of the bypass grafts and native coronary artery vessels during image acquisition. MDCT has been shown to have high sensitivity (97%-100%) and specificity (98%) for diagnosing graft occlusion, although somewhat lower sensitivity (75%-82%) and specificity (88%-92%) for detecting significant stenoses.54,55

Pulmonary Vein Evaluation

There has been an increase in the number of interventional procedures performed for the treatment of atrial arrhythmias. In particular, atrial fibrillation, which affects approximately 2 million people in the United States,56 can now be treated using an ablation technique, often pulmonary vein isolation. Imaging evaluation of the left atrial anatomy and pulmonary venous anatomy can aid the eletrophysiologist in preprocedural planning and can be used to asses for postprocedural complications. Three-dimensional visualization of the left atrium and pulmonary venous anatomy is often used to guide the electrophysiologist in navigating the left atrial cavity during catheter ablation.

The location, size, and number of pulmonary veins need to be defined. Ostial branches, which are venous branches noted within 5 mm of the atriopulmonary venous junction, are also important to note. Commonly, there are four pulmonary veins with separate ostia into the left atrium. Knowledge of aberrant pulmonary veins (Fig. 5–11) such as accessory or conjoined veins and incidental left atrial diverticula can help guide procedures to ensure isolation of the electrical potentials arising from all the pulmonary veins and avoid potential complications from attempts to enter atrial diverticula. Diagnosis of the presence of left atrial thrombus, especially prevalent in the left atrial appendage, is important to prevent iatrogenic systemic embolism (Fig. 5–12).

Postprocedural complications of radiofrequency ablation include endocardial scarring (Fig. 5–13), pulmonary vein dissection, and perforation.57 Small pleural or pericardial effusions, small atrial septal defects, and pulmonary venous stenosis are also potential complications. Serious complications include stroke, hemopericardium, hemothorax, pulmonary vein thrombosis, and hemodynamically significant pulmonary vein stenosis, which can result in pulmonary veno-occlusive disease including focal pulmonary edema.58 Pulmonary vein stenosis may develop up to 8 months after the procedure. Current ablation techniques, however, have greatly diminished the complication rate of pulmonary venous stenosis.

Imaging of the left atrial and pulmonary venous anatomy can be performed with CT or magnetic resonance imaging (MRI). CTA imaging can be performed as a gated or nongated study and should encompass the area from the aortic arch through the apex of the heart during a single breath-hold.59 Postprocessing of the image data sets, including 2D multiplanar reconstructions of the pulmonary venous ostia and atrial appendage, and 3D volume-rendered and shaded surface displays demonstrate the complex anatomy. Depending on the software vendor, volumetric CT data sets acquired through the left atrium can be synchronized and fused with electrical mapping systems in the electrophysiology laboratory (Fig. 5–14).

Congenital Heart Disease

Over the past five decades, the number of adults with congenital heart disease has grown due to the advances in cardiac surgery, intensive care, and noninvasive diagnosis.60 Approximately 85% of infants with cardiovascular anomalies can be expected to survive into adulthood, and with further advancements in surgical techniques, this number may continue to grow.61 Although surgical correction of an anomaly may reestablish a relatively normal pattern of blood flow, these adult survivors still remain at significant risk for developing complications from their operative procedures or from lingering effects of the original anomaly. Hence, these patients need to be followed closely regardless of their stage of treatment. As such, comprehensive imaging evaluation of the cardiovascular anatomy, flow, and function is important in the management of these patients. Cardiac MDCT with its fast acquisition times and capacity to obtain volumetric 3D information with high spatial resolution has often been used in the anatomic evaluation of congenital heart disease (Fig. 5–15). Advantages of CT compared with other imaging modalities such as cardiac MRI include short examination, fewer requirements for sedation, simultaneous evaluation of airways and lung parenchyma, and high spatial resolution. Disadvantages include radiation exposure, use of iodinated contrast, and lack of hemodynamic information.

CT imaging protocol is dependent on the structure of interest and information needed. For patients with abnormalities in the vascular system, image acquisition is usually started a few centimeters above the aortic arch and continued to the diaphragm. Because congenital heart disease often involves abnormalities in both the right and left heart chambers, the timing of data acquisition is set such that there is optimal contrast enhancement seen in both sides of the heart. In cases with intracardiac shunting, imaging can be performed early (within 5 seconds of contrast administration) and late (approximately 30 seconds after contrast administration) in order to determine the degree and direction of shunting.62 Images can be evaluated using multiplanar reconstructions, maximum intensity projections and 3D volume rendered image sets. Data can also be reconstructed in multiple phases through the cardiac cycle for calculation of cardiac volumes and function.

Pericardial Disease

Diseases of the pericardium encompass a spectrum of disorders including congenital malformations and infection-related, infarction-related, metabolic, autoimmune, traumatic, neoplastic, and idiopathic disorders. Depending on the disease process, clinical manifestations of pericardial involvement can vary. Suspected pericardial disease is usually initially evaluated with echocardiography. However, CT imaging can also provide additional valuable information.

CT imaging has been widely accepted for the evaluation of structural changes in the pericardium. The best quality images in the evaluation of pericardial disease are obtained with the use of cardiac gating and fast imaging to minimize motion blurring, although more prominent pericardial disease findings may be evident even on conventional studies performed for other indications. On CT, the pericardium is usually well delineated from the adjacent low-attenuation fat. Anatomic features of pericardial disease such as pericardial thickening, pericardial calcification, and pericardial effusion or masses are easily detected and evaluated with CT. Limited tissue characterization of pericardial fluid/masses can make it difficult to differentiate thickened pericardium from an exudative pericardial effusion with high protein content. Functional and hemodynamic information is also limited with CT. However, CT is the most sensitive technique for detection of pericardial calcification.

Pericardial Effusion

The CT appearance of pericardial effusion is often not very specific for a particular etiology, and further evaluation is usually necessary to help narrow the range of differential possibilities. CT attenuation measurements can provide initial characterization of pericardial fluid. On CT, simple effusions have attenuation similar to that of water. In contrast, more proteinaceous fluid (exudates or inspissated fluid collections) will have attenuation greater than water on CT.63 CT characteristics of malignant effusions may be similar to hemorrhagic effusions, depending on blood content, and are often associated with an irregularly thickened pericardium or pericardial nodularity. In addition, because of the wide field of view, associated abnormalities of the mediastinum and lungs may also be detected during the examination.64

Acute Pericarditis

Acute pericarditis may or may not be accompanied by pericardial effusion and some degree of myocarditis depending on etiology. Clinical symptoms and serologic markers of inflammation can often support the diagnosis. CT imaging can be used to aid in the diagnosis, especially when other tests prove inconclusive. Structural changes of the pericardium such as thickening, inflammation, pericardial effusion, and associated myocarditis or other concomitant heart disease and mediastinal pathology can be demonstrated.

Constrictive Pericarditis

Chronic inflammation of the pericardium can lead to constrictive pericarditis, which is characterized by impaired ventricular filling. The process may extend to involve the underlying myocardium, resulting in reduced ventricular function. The etiology of constrictive pericardial disease has often been attributed to antecedent acute pericarditis due to infectious, inflammatory, or idiopathic causes. However, other etiologies such as neoplastic diseases, postradiation therapy, postcardiac surgery, and remote nonpenetrating or penetrating cardiac trauma have been frequently seen. Accurate diagnosis and differentiation from restrictive cardiomyopathy are important because curative treatment for constrictive pericarditis is possible with surgical pericardiectomy. Initial diagnostic workup usually includes 2D, Doppler, and tissue Doppler echocardiography with analysis of respiratory changes associated or not with changes of preload. However, equivocal findings may be present in up to one-third of patients with possible pericardial constriction, and further testing is usually required.

Direct visualization of the pericardium can be performed with CT for measurement of pericardial thickness. However, the finding of a thickened pericardium is not necessarily confirmatory of constrictive pericarditis. In addition, up to 20% of patients with constrictive pericarditis may present with normal-thickness pericardium on current imaging methods.65 CT can also demonstrate characteristic anatomic features for constrictive pericarditis including a conical or tubular diastolic ventricular shape, an associated pericardial effusion, pericardial calcification (Fig. 5–16), and secondary sequelae including dilated atria, hepatic veins, and pulmonary veins, which can further support the diagnosis of constrictive pericarditis.

Pericardial Masses

The differential diagnosis of pericardial masses includes pericardial cyst, hematoma, neoplasm, loculated effusions, or pseudoaneurysms. Although often detected initially with echocardiography, evaluation with CT can be helpful in diagnosis and treatment. CT characteristics such as tissue attenuation, degree of contrast enhancement, and presence or absence of blood flow into the mass can help differentiate among pericardial masses. In addition, the size, anatomic extent, associated lesions, vascularity, and effects on cardiac function of the masses can be clarified. Furthermore, CT can guide other diagnostic testing, such as biopsy, and facilitate treatment and follow-up.

Congenital Pericardial Lesions

Congenital defects of the pericardium comprise partial left, right, or diaphragmatic or total absence of the pericardium. Most patients are asymptomatic, although approximately 30% may have additional congenital abnormalities. Complete absence of the pericardium poses a risk of homolateral cardiac displacement and amplified heart mobility and an increased risk of traumatic aortic type A dissection. Partial left-sided defects can be complicated by cardiac strangulation caused by herniation of the left atrial appendage, atrium, or left ventricle through the defect. Although the diagnosis can be suggested by chest x-ray and echocardiography, definitive diagnosis and complete evaluation of the pericardium can be performed with CT.

Cardiac Masses

Cardiac masses may be divided into benign or malignant lesions. Benign cardiac tumors include cardiac neoplasms, pseudotumors such as thrombi, and normal structures that resemble masses. Malignant cardiac tumors include both primary and secondary malignancies that can affect the heart. Echocardiography is often the initial diagnostic test to evaluate cardiac masses but often provides limited information on tissue characterization and extent of extracardiac involvement or presence of metastatic disease. Therefore, cross-sectional imaging with CT can provide additional information including the precise location of cardiac tumors (eg, paracardiac, mural, or intracavitary), the extent of disease, the presence of associated effusions, and the presence of metastases66 (Fig. 5–17). Additionally, CT imaging findings including tissue characterization may suggest the tumor type, which can be important in treatment planning (Table 5–2).

Depending on the size and location of the cardiac mass, CT image acquisition is tailored to obtain the necessary information. Noncontrast CT imaging can provide limited information regarding the nature of cardiac masses, but contrast CT imaging with cardiac gating can better assess the precise location, size, and extent of cardiac masses.

Benign Cardiac Neoplasms

Benign cardiac masses include myxoma, papillary fibroelastomas, lipoma, and rhabdomyoma. Benign tumors tend to affect the left-sided chambers of the heart. Although these lesions are histologically benign, they can act malignantly via secondary effects on the heart and vasculature. Therefore, successful treatment depends on the early detection and characterization of these masses.

Myxoma

Myxomas on CT often demonstrate a distinct, intracavitary sphere, typically with calcification, heterogeneous hypoattenuation consistent with its gelatinous nature,67-69 and neovascularization.70 They commonly demonstrate a narrow base of attachment (somewhat pedunculated), and attachment to the interatrial septum virtually cements the diagnosis.76,71 Functional consequences of large myxomas can be seen in cine cardiac CT in which the tumor mass may prolapse into the ventricles or cause obstruction of the atrioventricular valves.

Papillary Fibroelastoma

Papillary fibroelastomas are avascular papillomas lined with endothelium. Depending on the size, location, and mobility, these masses may not be well seen on CT, especially if cardiac gating is not used. However, on CT they often appear as small (<2 cm in diameter), solitary, mobile, pedunculated, hypodense,72 homogeneous valvular, or endocardial masses that flutter or prolapse with cardiac motion.69

Lipoma

Lipomas typically occur in adults, although they can also be seen in children. They generally occur as solitary masses that can arise from the epicardial surface spreading into the pericardial space73 or from the interatrial septum or endocardial surface as a broad base from which they can grow into any of the cardiac chambers.74 Lipomas are seen as encapsulated masses demonstrating the hypodense75 attenuation of fat on CT. There is often lack of contrast enhancement, but use of contrast can increase their conspicuity. In addition, a smooth contour and capsule can further distinguish benign lipomas from the irregular, multilobar liposarcomas.69

Although not a tumor, lipomatous hypertrophy of the interatrial septum can be confused with the truly neoplastic lipoma. Lipomatous hypertrophy results from an increase in cell number, or hyperplasia, seen in obese, elderly individuals as an unencapsulated fatty infiltration, defined formally as any deposit of fat in the atrial septum at the level of the fossa ovalis that exceeds 2 cm in transverse diameter.76 In addition to characteristic sparing of the fossa ovalis seen on echocardiography, this tissue may appear wedge shaped or as diffuse septal thickening on CT.

Fibroma

On CT, fibromas commonly appear as a large solitary, calcified mass within the ventricular myocardium. This is in contrast to other intramyocardial tumors including rhabdomyomas, which are often multicentric, and rhabdomyosarcomas, which are frequently cystic or necrotic.77 On CT, fibromas are commonly hypodense due to the dense, fibrous tissue with bright areas of calcification.78,79

Rhabdomyoma

Cardiac rhabdomyomas account for the majority of cardiac tumors seen in infants.71,77 Unlike other cardiac tumors, rhabdomyomas frequently regress over time and thus are treated conservatively. However, complications can result from an obstructive syndrome or from severe arrhythmias.80 On imaging studies, rhabdomyomas can be seen in both right and left ventricular myocardium and interventricular septum. They may be intracavitary or intramural and are often multiple rather than single. On CT, rhabdomyomas appear hypodense to myocardium after contrast administration.

Teratomas

Cardiac teratomas generally occur in infants and children as a predominantly right-sided pericardial mass.81 Although considered benign, these tumors are often accompanied by pericardial effusions, which may progress to cardiac tamponade with subsequent respiratory distress and cyanosis.81,82 However, surgical resection is often curative.83 CT imaging of teratomas demonstrates a heterogenous, multicystic appearance with calcification, often in the form of teeth, and can confirm the diagnosis of teratoma over other tumors.71,77

Hemangiomas

Cardiac hemangiomas are rare, benign, vascular tumors of the heart. They usually occur in the epicardial layer but can involve all the cardiac chambers. Symptoms are usually the result of tumor compression of surrounding structures or embolization.84 On noncontrast CT, cardiac hemangiomas have a heterogeneous density with occasional interspersed calcification.69 In addition, they enhance intensely with contrast, which may be inhomogeneous due to interspersed calcification and fibrous septa within the mass.85

Malignant Cardiac Neoplasms

Malignant cardiac masses include metastatic neoplasms, primary cardiac sarcomas, and primary cardiac lymphoma. Although no single finding is specific, malignant cardiac tumors commonly affect the right-sided chambers of the heart, demonstrate inhomogeneity of tumor tissue, appear infiltrative, and are associated with pericardial or pleural effusion.86

Metastatic Disease

Most metastases to the heart occur from lung or breast cancer, but they can also occur from melanoma, lymphoma, and leukemia.87-89 Involvement of cardiac structures primarily occurs through the lymphatic pathway, although hematogenous, contiguous, or transvenous extension does occur.90 Cardiac involvement is commonly seen as a late manifestation of primary tumor extension,91 but characteristic vascular involvement signs may help facilitate diagnosis. Pulmonary vein extension may signal bronchogenic carcinoma, inferior vena caval extension can be seen with renal cell and hepatocellular carcinoma, and superior vena caval extension may indicate supracardiac tumors such as thymic carcinomas.92 CT imaging of metastases can demonstrate nodular masses or pericardial thickening often with contrast enhancement due to vascularity of the lesions.93,94 Other common features of malignancy include right-sided involvement, ventricular infiltration, and hemorrhagic pericardial effusion.95

Sarcoma

Cardiac sarcomas are extremely rare tumors occurring in less than 0.2% of the population but represent the most common primary malignant cardiac tumor seen in adults.96 These tumors may originate in the epicardium or pericardium, involve the myocardium and cardiac valves, and cause nonspecific signs and symptoms that make clinical diagnosis difficult. There are several subtypes of sarcomas including angiosarcoma, osteosarcoma, fibrosarcoma, malignant fibrous histiocytoma, leiomyosarcoma, myxosarcoma, synovial sarcoma, neurofibrosarcoma, lymphosarcoma, reticulum cell sarcoma, and undifferentiated sarcoma.

Angiosarcoma is a tumor of endothelial cells and represents the most prevalent subtype. These tumors most commonly affect the right atrium; are highly vascular lesions with hemorrhagic, necrotic foci; and frequently invade the pericardium with associated hemorrhagic effusion that can lead to cardiac tamponade.97 On CT, angiosarcomas appear as hypodense irregular or nodular lesions, often arising from the right atrial free wall with heterogeneous enhancement, or in the case of pericardial infiltration, they may appear as pericardial effusion or thickening.98

Rhabdomyosarcomas are striated muscle tumors that represent the most common cardiac malignancy in infants and children.99 These tumors may arise anywhere within the heart, involving any cardiac chamber or valve,100 although pericardial infiltration is rare.71 On CT, these tumors demonstrate low attenuation with a smooth or irregular contour that enhances with contrast.101 Extracardiac extension into the pulmonary arteries, aorta, or valvular structures can be demonstrated with CT.100

Fibrosarcomas consist primarily of malignant fibroblasts and comprise 5% of all primary cardiac tumors.102,103 They commonly affect the left atrium, and valvular involvement is found in as many as 50%. On CT, fibrosarcomas appear as a low-attenuation, often obliterative mass. Fibrosarcomas may infiltrate the pericardium by direct invasion104 or tumor deposition nodules,105 or they may primarily involve the pericardium, appearing similar to malignant mesothelioma.96

Osteosarcomas in the heart consist of malignant bone-producing tumor cells.96 Primary osteosarcomas commonly arise in the left atrium usually accompanied by signs and symptoms of congestive heart failure.106 CT may show dense calcifications within a low-attenuation, left-sided mass.107 However, early lesions may have minimal calcification and can be mistaken for dystrophic calcifications. Other distinguishing features include a broad base of attachment and an aggressive growth pattern with extension into the pulmonary veins or atrial septum or infiltrative growth along the epicardium.108-110

Leiomyosarcoma is a malignant tumor with smooth muscle differentiation. It may arise from the subendocardium of the cardiac chambers but more commonly arises from smooth muscle of the pulmonary veins and arteries and then spreads into the heart.96 CT imaging demonstrates lobulated, irregular, low-attenuation masses.111

Liposarcoma is a malignant mesenchymal tumor that consists of lipoblasts. These tumors are extremely rare.96 They tend to arise in the atria but have been reported to occur in any cardiac chamber, the pericardium, and cardiac valves.112,113 Unlike benign lipomas, liposarcomas have little or no macroscopic fat.69 CT imaging demonstrates a large, multilobular, heterogeneous mass with areas of necrosis and hemorrhage.100

Lymphomas

Primary cardiac lymphoma includes lymphoma that is mostly confined to the heart or pericardium,96 as opposed to the more common cardiac spread of non-Hodgkin lymphoma.114 These lymphomas are commonly aggressive B-cell lymphomas.71,115 Although there is increased prevalence in immunocompromised patients, these lymphomas are also seen in immunocompetent patients.115 Cardiac lymphoma may appear as either multiple circumscribed polypoid masses116 or an ill-defined infiltrative lesion.117,118 Cardiac lymphomas are less likely than sarcomas to have necrosis.96 They commonly arise in the right side of the heart but may involve any chamber. Pericardial effusions are common and may be the only finding at the time of imaging. On CT, lymphomas appear hypodense or isodense relative to myocardium and demonstrate heterogeneous contrast enhancement.115,118

Pseudotumors

Normal Cardiac Structures

Due to the complex nature of the cardiac anatomy and the unique individual variations seen, normal cardiac structures can often be mistaken for cardiac tumors. In particular, fetal remnants (eg, Eustachian valve, Chiari network) or normal variants (eg, prominent trabeculations, false tendons) may give the impression of a cardiac tumor. Given the 3D nature of cross-sectional imaging with CT, differentiation of normal cardiac structures from cardiac tumors can be made. In addition, the distinction between cardiac neoplasms versus other cardiac masses can also be made.

Thrombi

Intracavitary cardiac thrombi probably represent the most common masses seen in cardiac imaging. In patients with atrial arrhythmias or other predisposing factor for atrial blood flow stasis (eg, mitral stenosis), they can frequently be seen along the posterolateral wall of the left atrium or within the left atrial appendage.119 In patients with cardiomyopathy, ventricular thrombi can form in areas of slow blood flow such as ventricular aneurysm or regions with dysfunctional myocardium. On CT, thrombi generally exhibit homogeneous hypodense attenuation after contrast administration.120 Care should be taken to distinguish slow flow with incomplete opacification, which may be seen on first-pass imaging, from true thrombi, which in general will remain hypodense on delayed imaging after contrast administration. (see Fig 5–12)

Vegetation

Intracardiac masses related to infective endocarditis are usually diagnosed with echocardiography in the appropriate clinical setting. However, in cases where the diagnosis is uncertain, CT imaging can aid in excluding a cardiac tumor. In general, vegetations exhibit CT imaging characteristics similar to thrombi. Because vegetations are often nonvascular, they generally do not enhance with contrast administration. Although distinction from cardiac tumor is possible, tissue differentiation between thrombi and vegetations is generally not possible with CT, and accurate diagnosis relies on incorporating the patient's accompanying clinical features and the effect on cardiac structures (eg, location and/or evidence of destruction) (Fig. 5–18).

Myocardial Evaluation

Myocardial structural assessment, including evaluation for viability and prognosis based on the presence of inflammation, scar, and diffuse fibrosis, is usually performed by MRI and/or nuclear cardiac imaging techniques. MDCT does not currently provide sufficient spatial or contrast-to-noise information to allow for quantification of such processes. However, often cardiac CT is the method of choice in the assessment of left ventricular (LV) structure and function because of patient care logistics, convenience, or contraindications to alternative imaging modalities.

Chamber Size and Functions

Multiple studies have documented the diagnostic value of cardiac CT to evaluate LV size and global and regional LV function.121-124 The use of retrospective ECG gating allows for collection of imaging data for reconstruction throughout the cardiac cycle, which can then be used for evaluation of wall motion and function. In routine clinical practice, functional analysis is most commonly performed to evaluate the left ventricle and aortic valve. Precise evaluation of LV function is important for diagnosis, therapy, and follow-up of patients with cardiovascular diseases.

Endocardial excursion and the extent and timing of systolic myocardial wall thickening are used for LV functional assessment of cardiac CT images. Wall motion may be reported as normal, hypokinetic, akinetic, dyskinetic, or aneurysmal. Quantitative functional evaluation relies on methods to segment the ventricular cavity. Definition of the intraventricular cavity is dependent on the software vendor used. Commonly, either a Hounsfield threshold cutoff value or endocardial contouring of the intraventricular cavity is performed to distinguish the cavity from myocardium. Ejection fraction, stroke volume, and cardiac output can then be derived from the calculated ventricular volumes. In general, when compared with cardiac magnetic resonance (CMR), cardiac CT demonstrates excellent agreement for assessment of LV ejection fraction and regional wall motion.122,123

It is important to note that newer, more recent low-dose coronary CTA (CCTA) protocols often use prospective triggering methods with collection of imaging data only during diastole at a substantially lower radiation exposure. These CCTA techniques preclude quantitative and qualitative functional analyses due to lack of additional phases through systole.

Myocardial Viability

The ability to distinguish dysfunctional but viable myocardium from nonviable tissue after acute or chronic ischemia has important implications for the therapeutic management of patients with CAD.125,126 Image-based characterization of myocardial scar morphology can identify those patients with hibernating myocardium who may achieve functional systolic recovery with revascularization.127 The assessment of myocardial viability and infarct morphology with delayed contrast-enhanced MRI has been well validated over the past several years and is considered the noninvasive imaging gold standard for detection of myocardial fibrosis.

The recent advent of MDCT technology has expanded its potential for a more comprehensive evaluation of cardiovascular diseases. Although hypoattenuation in the noncontrast scan (due to fatty degeneration of the infarcted area) or during the contrast-enhanced coronary angiography scan has been shown to demonstrate areas of previous myocardial infarction, it is largely underestimated by MDCT.128,129 Delayed MDCT myocardial imaging can accurately identify and characterize morphologic features of acute and healed myocardial infarction, including infarct size, transmurality, and the presence of microvascular obstruction and collagenous scar.130,131 Infarcted myocardial tissue by MDCT is characterized by well-delineated hyperenhanced regions (Fig. 5–19), whereas regions of microvascular obstruction by MDCT are characterized by hypoenhancement on imaging early after myocardial infarction.130,131 The mechanism of myocardial hyperenhancement and hypoenhancement in acutely injured myocardial territories after iodinated contrast administration is similar to that proposed for delayed gadolinium-enhanced MRI.125 Under conditions of normal myocyte function, sarcolemmal membranes serve to exclude iodine from the intracellular space. After myocyte necrosis, however, membrane dysfunction ensues, and iodine molecules are able to penetrate the cell. Because 75% of the total myocardial volume is intracellular, large increases in the volume of distribution are achieved, which results in marked hyperenhancement relative to the noninjured myocytes. The mechanism of hyperenhancement of healed myocardial infarction or collagenous scar is thought to be related to an accumulation of contrast media in the interstitial space between collagen fibers and thus an increased volume of distribution compared with that of tightly packed myocytes. The low signal intensity of microvascular obstruction regions despite restoration of normal flow through the infarct-related artery is explained by the death and subsequent cellular debris blockage of intramyocardial capillaries at the core of the damaged region. These obstructed capillaries do not allow contrast material to flow into the damaged bed, which results in a region of low signal intensity compared with normal myocardium. In minutes to hours, contrast material is able to penetrate this "no reflow" region, and the necrotic myocytes that reside in that myocardial territory then become hyperenhanced as iodine is internalized by the cell. In weeks, the microvascular obstruction area is replaced by collagenous scar tissue, and the former dark areas now become bright. Because the transmurality of delayed enhancement predicts functional recovery after revascularization,127 the better spatial resolution of MDCT, as compared with CMR, may influence the accuracy of viability assessment, but thus far, no study has tested this hypothesis.

Myocardial Ischemia

The use of CT imaging for evaluation of myocardial perfusion has been documented in the past by investigators using electron beam CT.132 However, it was not until the development of 64-slice CT systems that the combination of coronary angiography with stress-induced myocardial perfusion assessment could be reliably performed.133,134 Currently, the greatest limitation to CT coronary angiography is the presence of severely calcified coronary segments, stents, or other artifacts that limit luminal visualization. Patients with calcified arteries tend to be older and/or have advanced CAD. Their studies are challenging from a diagnostic viewpoint because vulnerable plaques and stenotic lesions may be hidden underneath large amounts of calcium accumulated in the outer portions of atherosclerotic plaques encompassing one or more segments. Although progress in multidetector technology has improved our ability to study such patients, greater coverage and improved temporal resolution are unlikely to eliminate the problem, which is in large part intrinsic to the pathogenesis of atherosclerosis—namely, plaques grow outwardly first and tend to accumulate calcium as part of the healing process, therefore creating a natural shield to x-ray penetration. This is a particular limitation to the CT evaluation of older persons, patients with advanced CAD, and patients with prior coronary artery bypass graft surgery or multiple stent implantations, as well as patients with diseases such as chronic renal failure that accelerate plaque calcification. Furthermore, coronary anatomic information is much more valuable when combined with a functional test, because decisions regarding treatment are based on the detection of myocardial ischemia.135 The poor correlation between anatomic modalities such as invasive coronary angiography136,137 and MDCT138 with stress perfusion tests underscores the fact that one cannot substitute for the other.

Myocardial perfusion measurements by MDCT are derived from the upslope differences in contrast enhancement between the ischemic and remote areas (Fig. 5–20). Current-generation, 64-detector scanners still have limited coverage of the heart, resulting in the base of the heart being scanned earlier in time than the apex, making comparisons in signal intensities between the two areas problematic. In this regard, the introduction of wide-coverage MDCT technology that would allow the entire heart to be imaged in one gantry rotation (Fig. 5–21) combined with the capability of programming such gated image acquisitions to occur only during specific portions of a given cardiac cycle has created a brand new horizon of possibilities to reduce radiation exposure enough to enable the performance of combined angiography and myocardial perfusion assessment during stress, which associated with the angiographic and delayed enhanced images should provide a comprehensive cardiac assessment. Also, with more coverage, scan times will likely decrease, potentially reducing the amount of contrast material per scan. Such techniques would be ideal for the assessment of patients with chest pain who also have calcified coronaries, as well as for the follow-up of patients with advanced heart disease after coronary artery bypass surgery or multiple stent implantations. Using iodine molecules as a tracer, current research has demonstrated that by measuring the concentration in time of the tracer in the myocardium, MDCT can determine absolute blood flow in different regions of the myocardium.133 Recent attention to patients with chest pain but no obstructive epicardial CAD (syndrome X) has demonstrated that in a substantial proportion of these individuals, microvascular processes can be identified by perfusion reserve measurements in association with traditional CAD risk factors such as hypercholesterolemia, hypertension, and smoking, as well as with diabetes.139 The possibility of quantifying epicardial coronary plaque while also assessing microvascular disease during maximal vasodilatation enables coronary MDCTA to characterize macrovascular atherosclerosis as well as microvascular dysfunction secondary to atherosclerosis or other disease processes. The capability of quantifying myocardial blood flow by contrast-enhanced MDCT could represent a "quantum leap" in our ability to assess and characterize the entire process of cardiac atherosclerosis.

Valvular Heart Disease

Valvular heart disease is an important part of cardiology. Often detected by physical examination, imaging studies can help clarify the affected valve, define the valvular anatomy, and assess the degree of valve dysfunction and its effect on cardiac chamber morphology and function. The cardiac valves are normally very thin, pliable, mobile structures. Hence, imaging techniques to assess valve disease need to have high spatial and temporal resolution. Traditionally, transthoracic echocardiography with its acceptable spatial resolution and high temporal resolution with near real-time imaging is the imaging modality of choice in the assessment of valve disease. Quantification of valvular stenosis and valve area, valvular regurgitation, and effective orifice area and an assessment of ventricular function can be easily performed. Transesophageal echocardiography can further define valvular anatomy and dysfunction in patients with infective endocarditis or in patients with poor acoustic windows. Although cardiac CT can provide high spatial resolution of valvular anatomy, moderate temporal resolution limits assessment of valvular flow and function.

Native Valves

Information on valve morphology, leaflet abnormalities, and valve motion is possible with CT (Fig. 5–22) but remains inferior to echocardiographic techniques.140,141 However, information from CT can be helpful in inconclusive cases. Valvular characteristics such as leaflet morphology (thickening, calcifications, and poor leaflet coaptation), congenital lesions, and abnormalities of the valvular and subvalvular apparatus can be demonstrated on CT.142 Severity of the stenosis can be assessed by planimetry of the orifice area during valve opening,143-145 and the severity of regurgitant lesions can be assessed by measuring the effective regurgitant orifice area by planimetry during valve closure.142 In addition, measurement of aortic valve Agatston calcium score has been shown to correlate with severity of aortic valve gradient by echocardiography.146,147 Indirect characteristics such as dilation of chambers or vascular structures (eg, dilated left ventricle and/or dilated aortic root in aortic regurgitation) and increased ventricular wall thickness (eg, right ventricular hypertrophy in pulmonic stenosis) provide further information on the severity of the secondary hemodynamic impairment.

Prosthetic Valves

Prosthetic heart valves and annuloplasty rings are implanted every year around the world and have become extremely useful devices in cardiac disease (Fig. 5–23). Prosthetic valves can be divided into two main classes: mechanical prostheses and biologic or tissue valves. Mechanical valves are classified into three major groups: caged-ball, tilting-disc, and bileaflet valves. Today, the most widely used mechanical valve is the bileaflet valve, although older caged-ball valves are still seen in patients for follow-up. Tissue valves include heterografts, homografts, and autografts. CT has not been extensively studied for the evaluation of prosthetic valve dysfunction. As with echocardiography, artifacts related to the metal used in the implants may limit accurate evaluation for prosthetic valve function. However, cardiac CT assessment of prosthetic valves, particularly mechanical valves, may provide additional information to that provided by echocardiography, leading to a more complete assessment of valvular function and hemodynamics.148

Infective Endocarditis

Infective endocarditis involves the endocardial surface, most commonly affecting the heart valves, but also mural endocardium. Characteristic vegetations are comprised of platelets, fibrin, inflammatory cells, and microorganisms. In general, vegetations exhibit CT imaging characteristics similar to thrombi and generally do not enhance with contrast administration. Abnormalities in valve structure, including the presence of prosthetic valves, are often predisposing factors for the development of endocarditis. In addition to the presence of vegetations, CT imaging can aid in the identification of perivalvular extension, myocardial or perivalvular abscess formation, fistula formation, and pseudoaneurysms, particularly along the intervalvular fibrosa (see Fig. 5–18). In particular, prosthetic valve endocarditis can be difficult to evaluate with echocardiography due to acoustic shadowing related to the prosthesis, and in those situations, CT imaging has the ability to provide important additional information.142

Coronary Artery Disease

A comprehensive assessment of CAD should include information on coronary artery anatomy and functional information regarding the hemodynamic relevance of lesions in order to guide revascularization procedures. At present, there is much debate regarding the optimal management of patients with stable CAD, and several guidelines recommend evidence of functional ischemia prior to revascularization of stenoses.149-151 Although there has been some experience with the use of MDCT for the evaluation of myocardial perfusion for assessment of functional significance of coronary artery lesions, at present, current MDCT systems still have limited coverage of the heart and prohibitive radiation doses to use for this indication. As such, standard noninvasive evaluation of hemodynamic significance of coronary lesions can be performed by stress nuclear perfusion, stress echocardiography, or stress CMR techniques. Information on segmental perfusion or wall motion response to either a physiologic or pharmacologic stressor can then be used to evaluate functional consequences of coronary artery lesions found on MDCT.

Congenital Cardiovascular Disease

Echocardiography and invasive left and right cardiac catheterization are the primary cardiac imaging modalities used to evaluate congenital cardiovascular disease. However, echocardiography is limited by the small field of view, limited acoustic windows, and operator dependence. Cardiac angiography is limited by overlapping of adjacent vascular structures, difficulty in demonstrating the systemic and pulmonary vascular systems simultaneously, invasive nature of the procedure with risk for complications, relatively high risk of ionizing radiation, and use of iodinated contrast material.

CT has been used in the anatomic and structural evaluation of congenital cardiovascular disease. With the wide field of view, information on intracardiac and extracardiac morphology, including systemic and pulmonary vascular anomalies, can be readily evaluated. Technologic developments in CT with high temporal and spatial resolution have further allowed for evaluation using multiplanar reformations, which are essential because each vascular structure has its own axis, and the cardiac heart axes are also unique to each individual. Furthermore, acquisition of multiphasic information on CT allows for assessment of cardiac chamber size and function, which is particularly useful for quantitative evaluation of structures not easily evaluated with echocardiography such as the right ventricle, or evaluations that may not be possible with CMR due to the presence of ferromagnetic implants such as closure devices or pacemakers. At present, MDCT evaluation is primarily limited to evaluation of anatomic structural disease, and further assessment of hemodynamic significance of anatomic lesions (eg, shunt quantification) requires additional data from either echocardiography, CMR, or invasive cardiac catheterization in a multimodal imaging approach depending on the patient's underlying disease.

Secondary Assessment for Cardiac Structure and Function

Echocardiography is the primary imaging modality of choice for the evaluation of cardiac structure and function. It is a cost-effective technique, widely available, portable, does not expose the patient to ionizing radiation, and provides the necessary information for management of patients with cardiac disease. However, there are several limitations with echocardiography, including operator dependence, narrow field of view, limited cardiac views dependent on limited acoustic windows, and often poor acoustic windows in patients with emphysema or large body habitus. In addition, acoustic shadowing due to high attenuators such as calcifications or metallic implants (eg, valve replacement) can further limit visualization of intracardiac structures. As a result, the complex geometry and orientation of certain cardiac structures may be difficult to fully evaluate with echocardiography.

CMR imaging has often been used as an alternative noninvasive imaging modality for evaluation of cardiac structure and function. Similar to echocardiography, CMR does not expose the patient to ionizing radiation and allows complete evaluation of cardiac and vascular structures in any field of view with a wider field of view when compared with echocardiography and can provide anatomic, functional, and hemodynamic information. In addition to anatomic structure, CMR can provide more information on tissue characterization (eg, myocardial scar or intracardiac mass) than other available techniques, which is important in patient management. CMR is evolving into the reference standard for quantification of global and regional right and left ventricular volumes and function, evaluation of valvular anatomy, and assessment of cardiovascular mass. However, the limited availability, contraindications in certain patients (eg, pacemakers), and artifacts in patients with certain metallic implants (eg, atrial septal defect closure devices) limit its widespread use.

Cardiac CTA is most commonly used for anatomic evaluation of CAD and congenital heart disease. Although it is not the primary imaging modality for assessment of cardiac structure and function, it can be used as a supplement to echocardiography and CMR imaging. In certain cases, cardiac CTA can provide useful information on right and left ventricular size and function, evaluation of valvular disease, and assessment of intracardiac mass, particularly when there are limitations or contraindications to other available techniques, as discussed earlier. An important limitation with cardiac CTA is the need for ionizing radiation. In addition, for assessment of cardiac function, imaging must be performed throughout the cardiac cycle, leading to a substantial radiation dose penalty compared with other cardiac CT techniques that limit radiation dose to certain periods of the cardiac cycle. Therefore, CTA in this setting should be performed only in selected patients in whom the necessary information is not obtainable with other imaging techniques such as CMR.

Case 1

A 70-year-old woman with hypertension presents for evaluation of occasional chest pain not related to exertion. Current medication was amlodipine. The patient was normotensive with a blood pressure of 127/82 mm Hg and heart rate of 70 bpm. Total cholesterol was 190 mg/dL, with high-density lipoprotein of 55 mg/dL and low-density lipoprotein of 130 mg/dL. Framingham risk score was calculated to be 5%. The patient was referred for exercise nuclear stress test. She exercised for 10 minutes on the Bruce protocol with no complaints during stress and no ischemic changes on ECG. Single photon emission CT imaging performed at rest and stress demonstrated small area of mild ischemia in the basal inferior wall (Fig. 5–24). However, attenuation artifact in this same area limited the specificity of these findings. The patient was referred for coronary CTA, which demonstrated long segments of severe stenoses involving the proximal and mid right coronary artery, which was confirmed on x-ray angiography (Fig. 5–25). The patient was treated with aggressive medical management without revascularization.

Case 2

A 55-year-old man with lone atrial fibrillation presents for evaluation of left atrial and pulmonary vein anatomy prior to pulmonary vein isolation procedure. Transesophageal echocardiography performed for evaluation of left atrial thrombus revealed an abnormal cystic structure noted in the interatrial septum (Fig. 5–26) without significant enhancement after injection of agitated saline. Cardiac CT imaging performed for further definition of left atrial anatomy revealed evidence of atrial septal aneurysm with unusual configuration, which corresponded to the cystic abnormality visualized on the echocardiogram (Fig. 5–27).

We gratefully acknowledge Martha Helmers for her hard work in preparing the final figures for this chapter.