Andrew C.Y. To and Milind Y. Desai
Cardiac magnetic resonance imaging (MRI) and computed tomography (CT) are noninvasive imaging modalities with clinical utility in a wide array of cardiovascular diseases. Common indications, technical considerations, and specific clinical scenarios are reviewed, with attention to the essential knowledge base that might be expected of fellows completing a general cardiovascular medicine fellowship.
CARDIAC MRI
Recent advances in pulse sequence design and scanner hardware have permitted MRI to become a useful tool in the noninvasive assessment of cardiovascular diseases. MRI provides high-resolution anatomic images; quantitative assessment of ventricular function as well as myocardial mechanics, perfusion, and viability; quantification of intra- and extracardiac shunts; measurements of valvular velocities and gradients; and contrast-enhanced angiography without the use of ionizing radiation or nephrotoxic contrast agents. Table 14.1 summarizes the indications of MRI in cardiovascular medicine, in particular drawing attention to the advantages of MRI in the assessment of these conditions.
TABLE
14.1 Indications of MRI in Cardiovascular Diseases
Technical Considerations
MRI Physics
The nuclei of all atoms are composed of one or more protons. Protons have a small positive electric charge and spin at a rapid rate. The rapid spinning motion of a positively charged proton produces a small but measurable magnetic field that in a sense is similar to a tiny bar magnet. Normally, the magnetic fields of these protons are randomly oriented throughout the body. When they are placed within an MRI scanner, however, protons within the body align themselves with the external magnetic field of the scanner, just as a compass aligns with the earth’s magnetic field. By applying radiofrequency (RF) waves, a portion of these protons can be made to change their alignment to a more excited state. As these protons relax and return to their original alignment, they emit a signal that can be measured and used to generate a clinical image. Hydrogen (1H) is the most abundant atom in the body and forms the basis of MRI imaging.
Basic Imaging Sequences
Imaging in MRI depends on using gradient coils within the scanner to send RF pulses in specific patterns to stimulate the 1H protons within the body. As these protons relax, they emit signals that are detected by receiver coils placed over the body, perpendicular to the main magnetic field of the scanner. Signal localization within the body is achieved by applying varying frequency and phase gradients along orthogonal planes. Image contrast is determined by variations in relaxation time (T1, T2, and T2*) of the different atoms that make up the tissues or organs of interest.
Pulse sequences are combinations of different types of RF pulses that result in different “weighting” toward one type of relaxation versus another (e.g., T1 weighting, T2 weighting). T1-weighted images are the basis of delayed enhancement imaging where contrast between fibrosed tissues with retained gadolinium and normal tissues are highlighted. T2-weighted images are useful for evaluation of tissue edema after a myocardial infarction, myocarditis or certain inflammatory conditions, such as Takayasu arteritis. T2* weighting, though rarely used, is helpful in iron overload conditions such as hemochromatosis or hemosiderosis.
The most common types of pulse sequences used in cardiac MRI are as follows:
Spin echo or “black blood” images. These pulse sequences are designed so that flowing blood produces no signal and appears dark (Fig. 14.1). However, because of the time for the inversion pulses to saturate the signal of flowing blood and the T2 weighting, spin echo pulse sequences usually produce still images. These pulse sequences provide good tissue contrast and anatomic detail, making them useful for visualizing morphology. Myocardium has an intermediate signal intensity on spin echo images, whereas fat and cerebrospinal fluid have high signal intensity (i.e., they appear bright).
FIGURE 14.1 Axial spin echo image at the level of the pulmonary artery of a patient with an atrial septal defect (ASD) and Eisenmenger syndrome. Moving blood is black, whereas myocardium and fat have an intermediate and high-signal intensity, respectively. Note the prominence of the main and right pulmonary arteries, suggesting pulmonary arterial hypertension.
Gradient echo or “white blood” images. Gradient echo sequences, including steady-state free precession (SSFP) sequences, are commonly used for cine sequences, as the short repetition times permit acquisition of sufficient data throughout the cardiac cycle with high temporal resolution, at 20 to 50 ms (Fig. 14.2). Flowing blood appears bright on gradient echo sequences as fresh unsaturated spins of flowing blood have high signal intensities. White blood images are useful for visualizing cardiac function as well as turbulent flow due to valvular disease or intracardiac shunts.
FIGURE 14.2 Still frame from a gradient echo short-axis cine loop of the same patient in Figure 37.1. Note the bright, “white blood” appearance of the blood pool within the left and right ventricular cavities. The RV is severely enlarged and, on cine images, has moderately reduced systolic function. In addition, a small pericardial effusion is present.
Phase-contrast velocity-encoded imaging. As hydrogen nuclei move through a magnetic field gradient, they acquire a particular phase shift proportional to the velocities at which they are moving through the field gradient. Phase-contrast imaging is analogous to Doppler imaging in echocardiography and is used to quantify both the velocity and flow of blood through an area of interest. Blood flow is quantified by multiplying the cross-sectional area of a particular vessel (e.g., the ascending aorta) and the integral of the velocities in the region of interest. Accurate velocity and flow quantification allows the estimation of gradients across valves, regurgitant volumes, as well as shunt fractions such as Qp/Qs ratios (Fig. 14.3).
FIGURE 14.3 Flow curve obtained from phase velocity images of the pulmonary artery in a patient with surgically palliated tetralogy of Fallot. Forward and regurgitant volumes as well as regurgitant fractions can be calculated, which in this case suggests moderate pulmonic insufficiency.
Magnetic resonance angiography (MRA). MRI gadolinium-based contrast (most commonly gadolinium DTPA) is an extracellular agent that influences the magnetic property of adjacent tissue. Contrast shortens the T1 relaxation times and appears bright on T1-weighted sequences. It allows better visualization of the cardiac and extracardiac vasculature. Gadolinium-based contrast agents have the advantage of being nonnephrotoxic and have a very low incidence of adverse events compared with contrast dyes used in x-ray angiography or CT. However, the concern over nephrotoxic systemic fibrosis limits the use of gadolinium based contrast in patients with chronic renal failure.
Perfusion imaging. First-pass tracking of MRI contrast bolus allows the evaluation of myocardial perfusion. When performed with stress conditions, most commonly with vasodilator stress such as adenosine, but also dobutamine, ischemia and infarction can be assessed, similar to perfusion imaging protocols used in nuclear medicine.
Delayed enhancement imaging. In areas of scarred or fibrotic myocardium, there is an expansion of interstitial space where gadolinium is retained, after the initial contrast injection. The delayed wash-out of gadolinium makes fibrotic areas appear as bright or “hyperenhanced” when myocardium is imaged 10 to 15 minutes after contrast injection (Fig. 14.4). A special inversion pulse is applied prior to main read-out sequence timed to “null” the signal intensity of normal myocardium, so that the contrast with fibrotic tissues is maximized. Specific patterns of hyperenhancement correspond with certain cardiovascular diseases, as described below.
FIGURE 14.4 Delayed hyperenhancement short-axis image acquired 10 minutes after the injection of gadolinium DTPA in a patient with multivessel coronary artery disease. An optimal inversion time (TI) was used in this image to “null” (darken) normal myocardium and accentuate (white) areas where gadolinium has remained in the interstitium on a “delayed basis” (delayed hyperenhancement). Note the hyperenhanced area along the subendocardial anteroseptal wall (white arrow), as well as patchy areas of hyperenhancement in the midinferoseptal and inferolateral walls.
Cardiac MRI imaging is usually performed during periods of 10 to 15 second breath holds to minimize respiratory movement. Cardiac gating with electrocardiography is done using MRI-compatible electrodes, aggregating data from multiple cardiac cycles to generate a single image or a single cine loop. Respiratory gating or triggering can be used in pulse sequences that require data from more cardiac cycles than can be fitted within one breath hold. Free-breathing acquisitions are sometimes used for real-time imaging of the heart when patients are unable to hold their breath, although spatial and temporal resolutions are limited in these acquisitions.
MRI-compatible equipment is used for monitoring patient’s blood pressure, heart rate, cardiac rhythm, respiration, and oxygen saturation inside the scanner.
Selected Clinical Applications
Viability Assessment
Delayed hyperenhancement imaging delineates of areas of acute and chronic myocardial infarction. Areas of both acute and chronic myocardial infarction appear bright (hyperenhanced) on delayed images postgadolinium administration. Such areas typically begin at the subendocardium and extend at a variable depth toward the epicardium, depending on the transmurality of the infarction. These hyperenhanced areas occur within a coronary distribution, unlike other causes of hyperenhancement such as sarcoidosis or myocarditis.
Hyperenhancement in acute infarction is due to disruption of myocardial membranes, which permits the diffusion of contrast agent into the expanded extracellular spaces. Hyperenhancement in chronic infarction is the result of the expanded interstitial space from the collagenous scar tissue allowing contrast to accumulate with slow wash-out, thereby appearing bright on delayed images. Areas of acute infarction appear bright on T2-weighted spin echo images due to local tissue edema, permitting one to differentiate between an acute and chronic infarct.
In patients with coronary artery disease and left ventricular dysfunction, the distinction between viable and nonviable myocardium may be important in determining the strategy of revascularization. Patients with viable myocardium are more likely to have an improved left ventricular ejection fraction and survival after revascularization. The extent of transmural hyperenhancement by cardiac MRI has been shown to predict both improvement in myocardial contractility and survival after coronary revascularization in patients with ischemic cardiomyopathy, and is thus used as a measure of myocardial viability. Myocardial segments with 25% or less transmural hyperenhancement are considered viable, whereas segments with 75% or greater transmural hyperenhancement are considered nonviable. Segments with 25% to 75% of transmural hyperenhancement have an intermediate likelihood of functional recovery after revascularization. The likelihood of functional recovery of these intermediate segments is often determined by the number of adjacent segments with either nonviable (75% to 100% transmural hyperenhancement) or viable (0% to 25% transmural hyperenhancement) myocardium.
Coronary Artery Disease
Visualization of the coronary arteries by MRI is technically challenging because of the small size of the coronary arteries, vessel motion during ventricular systole, and limited acquisition time. Coronary imaging with either MRA or three-dimensional SSFP sequences is a useful tool in the assessment of anomalous origin of the coronary arteries (Fig. 14.5). However, due to the abovementioned limitations, assessment of coronary artery stenosis, smaller caliber vessels, and coronary stent patency remains difficult.
FIGURE 14.5 Gradient echo axial image of an anomalous left coronary artery arising off the right sinus of Valsalva (white arrow) and passing between the pulmonary artery and aorta. The right coronary artery (RCA) arises normally. Note the motion artifact in the lower half of the picture
Heart Failure
MRI is the “gold standard” technique for the evaluation of left and right ventricular volumes, mass, and ejection fraction. Standard gradient echo cine images accurately assess for volumes, systolic function, regional wall motion abnormalities, ventricular aneurysm, and pseudoaneurysm. Other MRI techniques including delayed enhancement and stress perfusion imaging differentiate between ischemic and nonischemic etiologies of heart failure.
Myocardial Disease
Dilated Cardiomyopathy MRI accurately quantifies the degree of ventricular dilatation and ventricular dysfunction in dilated cardiomyopathy. Typically, delayed enhancement images demonstrate either no fibrosis or midmyocardial fibrosis not fitting a coronary artery territory. Resting perfusion sequences are normal.
Hypertrophic Cardiomyopathy Hypertrophic cardiomyopathy is primarily assessed by echocardiography but MRI has definite incremental values because of its superior image quality and ability to demonstrate myocardial fibrosis. MRI excels in characterizing the morphology of the disease and quantifies the degree, extent, and distribution of hypertrophy. The complex relationship between the septal hypertrophy, left ventricular outflow tract, mitral valve leaflets, subvalvular apparatus, and papillary muscles is accurately demonstrated, so that atypical cases of hypertrophic cardiomyopathy with concurrent mitral valve leaflet abnormalities and papillary muscle abnormalities can be identified. The accurate visualization of anatomical relationships also makes MRI crucial for premyectomy or preablation planning. In addition, MRI may also have a role in screening for preclinical disease because of the sensitivity of the technique.
Myocardial fibrosis demonstrated on delayed hyperenhancement imaging has potential prognostic value. A distinct pattern of midmyocardial hyperenhancement is noted in hypertrophic cardiomyopathy, affecting most commonly the hypertrophied septum, but also right ventricular free wall insertion points into the interventricular septum and other areas. The presence of myocardial hyperenhancement correlates with clinical markers of sudden cardiac death, as well as detection of ventricular arrhythmia on Holter monitoring. Recent longitudinal studies suggest an association between hyperenhancement and sudden cardiac death.
MRI helps in the differential diagnosis of cases that mimic hypertrophic cardiomyopathy such as physiologic hypertrophy of an athlete’s heart and hypertensive heart disease. Fabry disease, an X-linked recessive disorder of lysosomal targeting enzymes, accounts for as much as 5% of all cases of presumed hypertrophic cardiomyopathy and can be identified by marked left ventricular hypertrophy and the distinct basal inferolateral delayed hyperenhancement with sparing of the subendocardium. Identification of these patients guides treatment, usually with intravenous α-galactosidase replacement therapy.
Restrictive Cardiomyopathy MRI can help distinguish among various etiologies of restrictive cardiomyopathy, as well as differentiate between restrictive cardiomyopathy and constrictive pericarditis.
Cardiac amyloidosis. Spin echo and gradient echo images demonstrate increased thickness of the left and right ventricular myocardium, and occasionally of the atrial walls and atrioventricular valves. Systolic function is usually preserved or mildly impaired, although abnormal diastolic relaxation may be evident on myocardial tagging. Delayed hyperenhancement images demonstrate a diffuse enhancement of the left and occasionally right ventricle (RV). Such hyperenhancement may be predominantly subendocardial, but usually the global pattern of myocardial hyperenhancement distinguishes amyloidosis from coronary artery disease. In addition, appropriate nulling of the myocardium with an inversion recovery prepulse on delayed hyperenhancement images may be difficult with amyloidosis due to both the global shortening of T1 relaxation time in the amyloid affected myocardium and the early wash-out of gadolinium, from the bloodstream.
Cardiac sarcoidosis. Cine gradient echo images may demonstrate normal or impaired left ventricular systolic function, often with regional wall motion abnormalities. The extent and distribution of sarcoid involvement of the heart vary. In the early stages of sarcoidosis, the myocardium may demonstrate patchy high signal intensity on T2-weighted black blood images as a result of localized inflammation, along with focal patchy areas of hyperenhancement in a noncoronary distribution. Patchy hyperenhancement corresponds to areas of noncaseating granulomas that are the typical histologic findings of sarcoidosis. In more severe cardiac sarcoidosis, areas of hyperenhancement are also seen, along with ventricular wall thinning and aneurysms, most commonly along the basal anteroseptal wall.
Hemochromatosis. These patients present as dilated cardiomyopathy on cine gradient echo images, with evidence of reduced myocardial contractility on cine-tagged images. Due to tissue iron accumulation leading to local field inhomogeneity, signal loss is seen in both the myocardium and the liver on T2*- weighted spin echo sequences.
Arrhythmogenic Right Ventricular Cardiomyopathy
Arrhythmogenic right ventricular cardiomyopathy (ARVC) presents with right ventricular enlargement and/or decreased right ventricular dysfunction. Right ventricular dysfunction can be diffuse in advanced ARVC, or limited to areas of “scalloping” or bulging of the right ventricular free wall with associated akinesis or dyskinesis. Small saccular aneurysms appear as nipple shaped projections off the right ventricular free wall and right ventricular outflow tract (RVOT). Fibrofatty infiltration can be seen on spin echo images as areas of high signal intensities, which suppresses fat-saturation sequences. These areas appear bright on delayed hyperenhancement images.
On the latest task force criteria on ARVC in 2010, cardiac MRI is one of the imaging modalities for documenting global/regional dysfunction/structural alterations (Table 14.2). While fibrofatty replacement of the myocardium is considered a major criterion for the diagnosis of ARVC if detected on biopsy specimens, diagnostic criteria for MRI have not been incorporated into task force recommendations as yet.
TABLE
14.2 Imaging Criteria for Diagnosis of ARVC
From Marcus FI, McKenna WJ, Sherill D, et al. Diagnosis of arrhythmogenic right ventricular cardiomyopathy/dysplasia: proposed modification of the task force criteria. Circulation. 2010;121:1533–1541, with permission.
Constrictive Pericarditis
MRI is excellent for both the morphologic and functional assessment of constrictive pericarditis and other pericardial disease. T2-weighted spin echo sequences demonstrate pericardial thickening, as well as the presence of pericardial effusion. The normal pericardium is typically <2 mm in thickness and is often thickest over the right ventricular free wall. In constriction, pericardial thickness is usually >3 to 4 mm. Pericardial calcification is more difficult to demonstrate on MRI compared to CT, because of signal loss on MRI in areas of calcifications. Indirect signs of constriction can be seen in atrial enlargement, systemic and pulmonary vein enlargement.
Functional assessment of pericardial diseases by MRI demonstrates the presence/absence of constrictive physiology, which includes diastolic bounce of the interventricular septum, diastolic restraint, tubular or conical-shaped narrowing of one or both ventricles, and pericardial tethering. These can be seen in standard cine gradient echoes. Myocardial tagging images can also demonstrate pericardial tethering to the adjacent myocardium, rather than the normal free sliding of the myocardium over the pericardium in healthy hearts. In addition, ventricular interdependence can be demonstrated on free-breathing gradient echo cine sequences where the interventricular septum is seen to shift toward the left ventricle during inspiration. The characteristic septal shift is often most prominent on the first heartbeat that follows the beginning of inspiration.
Aortic Disease
Aortic Dissection Using a combination of spin echo, gradient echo, and contrast-enhanced MRA images, the thoracic and abdominal aorta can be evaluated for dissection flaps, location of the true and false lumens, entry and reentry intimal tears, involvement of aortic branch vessels, and associated complications (e.g., pleural effusion, cardiac tamponade, aortic regurgitation).
Compared to transesophageal echocardiography, MRI has a similar sensitivity for detection (98% to 100%) but a superior specificity (98% to 100% compared to 68% to 77%) for the diagnosis of aortic dissection. The sensitivity and specificity of MRI and CT are similar. However, an MRI study of the aorta takes much longer than CT, which makes it better suited for nonemergency imaging (e.g., chronic dissection) or in the follow-up of patients after surgical repair.
Intramural Hematoma Noncommunicating intramural hematomas of the aorta are thought to be a form of aortic dissection without intimal rupture or tear. Clinical presentation is similar to typical aortic dissection. On MRI, it appears as a smooth crescentic or circumferential area of aortic wall thickening without evidence of false lumen blood flow.
Aortic Aneurysm A combination of spin echo, gradient echo, and contrast-enhanced MRA images describe the size, location, and extent of a thoracic or abdominal aortic aneurysm. In addition, presence of thrombus, accompanying dissection, involvement of aortic branch vessels, and growth rate can also be accurately documented. MRA sequences can potentially underestimate the size of the aneurysm if there is significant thrombus formation. Effacement of the sinotubular junction may be seen in aneurysms of the ascending aorta and indicate annuloaortic ectasia, conferring a higher risk of aortic rupture. Mycotic aneurysms of the aorta or its branch vessels can be identified by the typical saccular appearance, as well as increased signal intensity of the aneurysm wall on T2-weighted spin echo sequences due to the presence of localized inflammation.
Limitations of Cardiac MRI
Despite its increasing versatility and robustness, important limitations still remain in cardiac MRI imaging. The electromagnetic forces created by the MRI scanner can induce important thermal and nonthermal effects in some patients. Contraindications to MRI imaging must be identified before the patient enters the scanner (Table 14.3). Nonferromagnetic metallic devices, such as mechanical heart valves, sternal wires, and retained pacing wires after cardiac surgery, are safe to image, although they are often a source of image artifact. Internal orthopedic prostheses (e.g., artificial hip joints) are safe to image.
TABLE
14.3 Contraindications to MRI
Patient cooperation is critical to successful cardiac imaging. Patient movement during image acquisition results in degraded image quality, and anxious or claustrophobic patients may require oral or intravenous sedation. Breath holds of 10 to 15 seconds are used to limit respiratory movement artifact. Children and adult patients who are unable to breath hold can still be imaged, although acquisition times may be increased for signal averages, or spatial resolution is limited on free-breathing sequences. Arrhythmias are problematic due to image degradation and make flow quantification by phase velocity imaging unreliable.
Cost and availability are other limitations to the widespread use of cardiac MRI, although cost will decrease over time. Furthermore, MRI scanners are not portable, and acquisition times of 15 to 60 minutes make imaging of unstable patients difficult.
CARDIAC CT
Technologic advances have revolutionized the use of multidetector CT (MDCT) in cardiac imaging. Improvements in gantry rotation, increased detector rows, image acquisition protocols, and postprocessing algorithms have lead to vastly improved spatial and temporal resolution.
Indications
While there is considerable overlap in many of the clinical indications for cardiac MRI and CT, the techniques differ significantly. CT excels in superior spatial resolution and is the gold standard for assessing coronary artery stenoses and anomalous coronary arteries. Scan time is short and is more suited for emergency imaging. On the other hand, MRI excels in functional assessment with a higher temporal resolution, ability to quantify hemodynamics and characterize tissue, and is therefore more suited in imaging cardiomyopathy.
Technical Considerations
CT Physics
Data acquisition in CT depends on the measurement of transmitted x-rays after they pass through an object. An x-ray source is used to produce a collimated, fan-shaped x-ray beam. As this beam passes through a patient, photons are absorbed or scattered, thereby reducing transmission of x-ray to the detectors on the opposite side. This attenuation of x-ray correlates with the atomic composition and density of the objects through which the photons pass, as well as the energy of the photons themselves. Objects with a high attenuation, such as bone or metal, absorb most of the transmitted photons from the x-ray beam, whereas low-attenuation tissues such as lung allow most of these photons to pass through to the x-ray detectors.
X-ray detectors receive the attenuated signal and digitize the information so that a set of attenuation values can be calculated. From the x-ray measurements made as the x-ray tube rotates around the patient, specific mathematical reconstruction algorithms, most commonly filtered back-projection, generate the image. The raw data are “filtered” or preprocessed to minimize beam hardening and scattered radiation, after which “back-projection” is performed to create a set of axial images with specific density values from the raw data. Newer reconstruction algorithms have been increasingly employed, including iterative reconstruction, where a statistical model is employed to reconstruct the image from the raw data, with a relative insensitivity to noise.
CT densities are expressed as Hounsfield units (HU), which range in value from –1,024 to +3,071 HU. Although this full range of density values could be displayed as a gray scale from black (lowest) to white (highest), the human eye is incapable of distinguishing between small changes in densities within this scale. The image display is therefore adjusted using “window levels” and “widths” to optimize tissue contrast. The window level (or center) indicates the density value in the middle of the displayed gray scale. The window width determines the density values around the window level within the gray-scale display. Objects with a CT density above the window width are displayed as white; objects below the window width are displayed as black. In effect, the window level determines the tissue types visualized and the window width determines the image contrast.
The density of water is defined as 0 HU. The value of nonenhanced (i.e., no contrast) tissues, such as muscle and blood, range from –100 to +200 HU. Fat tissue has lower density values, whereas bone and calcium have higher density. Contrast-enhanced arterial blood, such as the coronary arteries, has a density level of +200 to +400 HU. For cardiac imaging, the window level is usually set between +250 and +300 HU, with a window width of 600 to 1,000 HU.
Electron-Beam CT
Electron-beam CT (EBCT) scanners, although decreasing in popularity, were the first technique developed to evaluate coronary calcium scoring. EBCT images are generated by scanning an electron beam at four tungsten coils positioned below the patient. Because there is no mechanical motion within the gantry, temporal resolution is excellent (50 to 100 milliseconds). Although they are still used in a limited number of practices, EBCT scanners have largely been replaced by more advanced MDCT imaging.
Multidetector CT
The MDCT scanner is the most common type of CT scanner used today and consists of a rotating assembly of x-ray tube mounted opposite a series of detectors on a gantry around the patient. The x-rays form a “cone beam” flowing from the source on one side to the detectors on the opposite side of the rotating gantry. In most CT imaging, including cardiac imaging, the patient is moved at a fixed speed, or pitch, through a constantly rotating gantry (spiral acquisition). Multiple detector rows allow the acquisition of multiple simultaneous parallel slices per gantry rotation, which occurs at speeds between 300 and 400 milliseconds per rotation. Current MDCT scanners acquire the entire cardiac image during a single breath hold. In spiral acquisitions, electrocardiogram (ECG) gating is done retrospectively, which means that data are collected throughout the entire cardiac cycle, but image is reconstructed only in mid-late diastole, according to the ECG signal. Dose modulation algorithms reduce the x-ray tube current during systole when data are generally not used for image reconstructions.
Newer scanners have the option of acquiring data with prospective ECG gating, where the table moves sequentially through the long-axis of the body and data are acquired within just over half of a complete gantry rotation, triggered by the ECG signal. This acquisition mode minimizes radiation dose. In the new generation scanners, several innovations further improve image acquisition and quality. In the new 320-slice scanner, the axial sequential acquisition mode covers the entire heart in half a gantry rotation. Alternatively, a dual source scanner utilizes two x-ray sources so that data are acquired within just over one-quarter of a gantry rotation, doubling the temporal resolution, hence minimizing motion artifact.
In cardiac CT, an additional data reconstruction technique known as multisegment reconstruction may be performed. This technique utilizes data from more than one cardiac cycle to construct a clinically useful image. Multisegment reconstruction improves temporal resolution at the cost of an increase in radiation exposure, because sampling occurs during more than one cardiac cycle.
Patient Selection and Preparation
Appropriate patient selection is an important part of maximizing the clinical utility of any diagnostic test. Coronary CT angiography (CTA), the most popular application for cardiac CT, is best suited for patients with an intermediate risk of coronary artery disease. Low-risk patients are better served with other noninvasive tests to avoid unnecessary contrast and radiation exposure. High-risk patients are better served by cardiac catheterization because of a high likelihood of significant coronary calcification that impact usefulness of the technique. In addition, patients with coronary stents or those with ongoing cardiac arrhythmias are not well suited for coronary CTA.
Unlike cardiac MRI, cardiac catheterization, or other imaging modalities, cardiac CT image acquisition cannot be easily repeated, because of radiation dose and limit in contrast administration. Appropriate patient preparation increases patient comfort and maximizes image quality. Each step of the scan should be explained to the patient, and breath holds should be practiced before the actual scan. Heart rate should be slowed to a target heart rate of 50 to 60 beats per minute prior to the scan—typically with oral Atenolol or Metoprolol (50 to 100 mg 1 hour prior to the scan) and intravenous Metoprolol (5 mg IV every 5 minutes up to a maximum dose of 30 mg) as tolerated. Calcium channel blockers can also be used to slow the heart rate in those intolerant of beta-blockers but are less efficacious. Finally, in patients undergoing coronary CTA, sublingual nitroglycerin spray or tablet is given immediately before the scan for coronary vasodilation.
Selected Clinical Applications
Coronary Artery Disease
Coronary CTA visualizes both luminal stenoses as well as wall abnormalities. Images are often interpreted in traditional orthogonal planes (axial, sagittal, and coronal) as well as oblique planes that follow the axis of the coronary arteries. Multiplanar reconstruction, MPR, uses straight or curved thin images from the three-dimensional volume of images to create a two-dimensional representation of a vessel. This technique is useful for following tortuous coronary artery segments and for visualizing an entire vessel simultaneously (Fig. 14.6). Maximum-intensity projection, MIP, selects the brightest voxel (three-dimensional pixel) from a three-dimensional image stack and displays them in the specified projections, similar to the way angiographic images appear on cardiac catheterization (Fig. 14.7).
FIGURE 14.6 Multiplanar reformation (MPR) MDCT image of the left anterior descending (LAD) artery. The vessel is viewed from the beginning of the left coronary artery ostium, through the left main, to the distal LAD. The bifurcation of the LCX and diagonal arteries are not shown in this view. Note the calcified and noncalcified plaque in the proximal and midportion of the vessel, causing mild stenosis.
FIGURE 14.7 MIP of an oblique axial MDCT image of the LAD. Note the calcified plaque in the proximal portion of the vessel. It is not possible to accurately quantify the degree of stenosis behind the calcified plaque.
CT identifies coronary artery stenoses caused by calcified or noncalcified plaque. Noncalcified plaques have low to intermediate attenuation and appear as defects in the vessel wall as outlined by contrast. Calcified plaques appear as high attenuation (i.e., bright) lesions that are often associated with calcium blooming artifacts. Blooming may give the appearance that the stenosis is more severe than actuality due to attenuation of the x-ray photons by calcium. When vessel or plaque calcification is significant, it is often impossible to quantify the degree of vessel stenosis, as the x-ray attenuation precludes assessment of the full vessel lumen (see Fig. 14.7). Coronary stenoses are classified as mild (<50% diameter stenosis), moderate (50% to 70% stenosis), or severe (>70% stenosis). Spatial resolution of the advanced CT scanners is 0.4 × 0.4 × 0.4 mm or less. In comparison, invasive coronary angiography provides a spatial resolution of <0.2 mm. Despite the limitation of spatial and temporal resolutions, current scanners have >90% sensitivity and specificity for the detection of >70% coronary artery stenosis compared with cardiac catheterization. The negative predictive value of a normal coronary CTA is typically over 98%. Despite recent advances, coronary CTA remains an evolving field with several important limitations. Although contrast within a stent may suggest stent patency, in-stent restenosis cannot be reliably quantified because beam-hardening artifact from the stent material that obscures the vessel lumen. Coronary artery bypass grafts can be evaluated for patency; however, the distal anastomosis is often difficult to visualize due to a combination of small vessel size and artifacts from calcium and surgical clips.
Coronary Calcium Scoring
Coronary artery calcification is a reliable, albeit somewhat limited, sign of coronary atherosclerosis. The prevalence and extent of coronary calcium increases with age in both men and women, although the onset of calcification seems to be delayed by about 10 years in women compared with men. Men tend to have higher calcium scores than women. Individuals of either gender with diabetes or renal insufficiency have increased coronary calcification. Coronary calcium scoring uses noncontrast EBCT or MDCT to quantify coronary calcification. Several algorithms are available. The Agatston score, the most commonly used method, assigns a calcium score based on the maximal HU number and the area of calcium deposits. Only areas of calcification ≥1 mm2 and >130 HU are included in this algorithm. Other algorithms quantify the volume and/or mass of coronary calcium. Coronary calcium scoring has been used to assess long-term cardiac risk, with incremental value over that of current risk assessment tools. The test is most useful in intermediate-risk populations, in whom a normal or abnormal score may reclassify individuals to a lower or higher risk group, respectively. However, it remains unclear how this score should be incorporated into current treatment pathways and the improvement of overall risk assessment over that of traditional risk factors remains small. Coronary plaque calcification does not correlate well with the degree of histopathologic stenosis, and the typical plaque rupture that leads to acute coronary syndromes does not always occur at the site of calcification. Some centers use calcium scoring prior to coronary CTA in elderly patients; patients with a calcium score >800 are thought to have excessive calcification and coronary CTA is aborted, saving the patient unnecessary contrast and radiation exposure. In addition, CT is widely used in patients undergoing sternotomy for repeat cardiac surgeries as it identifies structures such as coronary bypass grafts or the ascending aorta that may be adherent to the back of the sternum and therefore increase procedural risk.
Pulmonary Vein Assessment
Pulmonary vein isolation is an increasingly common treatment of atrial fibrillation. Cardiac CT is useful in preprocedural planning to delineate the number and location of pulmonary veins, as well as to evaluate for the development of postprocedural pulmonary vein stenosis, a known complication of the procedure. Pulmonary vein stenosis is often detected early as pulmonary vein wall thickening, sometimes with mediastinal lymph node enlargement; at a later stage, luminal narrowing or obstruction is present. Some centers incorporate the preprocedural anatomic data provided by cardiac CT into procedural left atrial electrophysiologic maps.
Aortic Disease
CT is well suited for the evaluation of aortic anatomy and pathology and is the test of choice for acute aortic dissection, transection, intramural hematomas, penetrating ulcers and aneurysmal diseases. Motion artifact of the aorta is uncommon because of its relative immobility, and most studies are adequate using older 16-slice scanners and 3-mm-thick slices. Imaging of the entire thoracic and abdominal aorta can be acquired in a single breath hold. Prospective or retrospective ECG gating can be utilized if anatomy of the aortic root and ascending aorta is important.
Limitations of Cardiac CT
Image Quality
A number of factors influence image quality by cardiac CT. Characteristics related to the MDCT scanner include detector row number and type, detector width, gantry rotation time, and tube output. Tube output varies according to the patient’s body habitus (higher output for larger patients) or clinical condition (e.g., lower output for evaluation of prosthetic valve motion). Other factors that affect image quality are determined by the patient or clinical conditions at the time of image acquisition (Table 14.4). Finally, streak artifacts result from metallic objects within the thorax (e.g., bypass graft clips and pacemaker wires) or high contrast concentration in the right atrium or ventricle. The latter can be minimized by the use of a saline flush immediately following contrast bolus injection so that contrast is cleared from the right side of the heart.
TABLE
14.4 Factors Influencing Cardiac CT Image Quality
Radiation Exposure
Cardiac CT uses ionizing radiation to image the heart and exposes the patient to a predictable amount of radiation depending on the body part and the protocol used (Table 14.5). The effective radiation dose is higher in women and obese patients because of their increased body fat. Radiation dose is also higher in patients with faster heart rates, which negates the effectiveness of dose modulation.
TABLE
14.5 Relative Radiation Exposure due to Medical Procedures
“Dose modulation,” or ECG-controlled tube current modulation, is a technique that limits radiation exposure while maintaining adequate image quality. The technique of ECG triggering adjusts the scanner tube current so that it is highest during ventricular diastole when cardiacmotion is minimized and imaging is desirable and lowest during systole when cardiac motion impairs image quality. Images acquired during systole are during periods of reduced tube output, resulting in noisier images, but often are acceptable, as systolic frames are typically used only in reconstruction of cine loops. Dose modulation can reduce the effective radiation dose by 35% to 45% when used appropriately. Dose reduction is best at slower heart rates because of the relative increase in the duration of diastole and overall shorter scan time. The new iterative reconstruction algorithm shows promise in reducing image noise and hence reducing the overall dose needed in order to maintain similar image quality. In general, CT scanning should be avoided in pregnant women because of teratogenicity and potential increase in childhood malignancy. Breastfeeding is a relative contraindication to contrast exposure.
Contrast Exposure
Most cardiac studies currently require between 75 and 100 mL of contrast dye, followed by 30 to 50 mL of a saline flush. Iodinated contrast agents used in CT imaging carry a 2% to 4% risk of contrast allergy and a variable risk of renal dysfunction after contrast exposure. In general, contrast nephropathy risk is negligible in patients with a serum creatinine ≤1.8 mg/dL with no predisposing factors to renal dysfunction. Factors that increase contrast nephropathy risk include increasing age, elevated serum creatinine level or a history of renal dysfunction, volume depletion, heart failure, and diabetes. Because of the presence of multiple comorbidities in patients undergoing cardiac CT studies, most centers use low-osmolar nonionic contrast agents. Patients with a history of contrast dye allergy should be premedicated with steroids and diphenhydramine several hours prior to their study.
Gadolinium based contrast agent used in MRI can be substituted for traditional CT contrast agents in those patients with a history of anaphylaxis with iodinated contrast dye. Disadvantages include a reduction in contrast attenuation and the higher cost of gadolinium.
EVALUATION OF CARDIAC MASSES BY CT AND MRI
CT and MR are able to visualize not only cardiac anatomy but also the surrounding mediastinal, pulmonary, and chest wall structures. The wide field of view, coupled with high spatial resolution, make these imaging modalities useful techniques in the evaluation of cardiac and paracardiac masses.
Both benign and, to a lesser extent, malignant masses have various anatomic and tissue characteristics by CT and MR that help to narrow the differential diagnosis of a cardiac mass (Table 14.6). Findings suggestive of malignancy include right ventricular or right atrial involvement, infiltration into surrounding structures (e.g., penetration through the pericardium), irregular borders, pulmonary or mediastinal involvement, and hemopericardium. Findings suggestive of a benign tumor are left-sided involvement along the interatrial septum, smooth borders, and lack of pericardial effusion. Contrast perfusion through the mass can be used to identify vascular tumors, both benign and malignant, and to differentiate tumors from thrombus.
TABLE
14.6 Evaluation of Cardiac Masses by MRI and CT
T1-W, T1-weighted spin echo images; T2-W, T2-weighted spin echo images.
SUGGESTED READINGS
MRI:
Hendel RC, Patel MR, Kramer CM, et al. ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 appropriateness criteria for cardiac computed tomography and cardiac magnetic resonance imaging: a report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group, American College of Radiology, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, American Society of Nuclear Cardiology, North American Society for Cardiac Imaging, Society for Cardiovascular Angiography and Interventions, and Society of Interventional Radiology. J Am Coll Cardiol. 2006;48:1475–1497.
Lima JA, Desai MY. Cardiovascular magnetic resonance imaging: current and emerging applications. J Am Coll Cardiol. 2004;44:1164–1171.
Nagueh SF, Bierig SM, Budoff MJ, et al. American Society of Echocardiography clinical recommendations for multimodality cardiovascular imaging of patients with hypertrophic cardiomyopathy: endorsed by the American Society of Nuclear Cardiology, Society for Cardiovascular Magnetic Resonance, and Society of Cardiovascular Computed Tomography; American Society of Echocardiography; American Society of Nuclear Cardiology; Society for Cardiovascular Magnetic Resonance; Society of Cardiovascular Computed Tomography. J Am Soc Echocardiogr. 2011;24:473–498.
Nagel E, van Rossum AC, Fleck E. Cardiovascular Magnetic Resonance. Darmstadt, Germany: Steinkopff-Verlag; 2004.
CT:
de Feyter PJ, Krestin GP. Computed Tomography of the Coronary Arteries. London: Taylor & Francis; 2005.
Gerber TC, Carr JJ, Arai AE, et al. Ionizing radiation in cardiac imaging: a science advisory from the American Heart Association Committee on Cardiac Imaging of the Council on Clinical Cardiology and Committee on Cardiovascular Imaging and Intervention of the Council on Cardiovascular Radiology and Intervention. Circulation. 2009;119:1056–1065.
Taylor AJ, Cerqueira M, Hodgson JM, et al. ACCF/SCCT/ACR/AHA/ASE/ASNC/NASCI/SCAI/SCMR 2010 appropriate use criteria for cardiac computed tomography. A report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, the Society of Cardiovascular Computed Tomography, the American College of Radiology, the American Heart Association, the American Society of Echocardiography, the American Society of Nuclear Cardiology, the North American Society for Cardiovascular Imaging, the Society for Cardiovascular Angiography and Interventions, and the Society for Cardiovascular Magnetic Resonance. J Am Coll Cardiol. 2010;56:1864–1894.
QUESTIONS AND ANSWERS
Questions
1. A 51-year-old man presents to your clinic for evaluation of progressive exertional dyspnea over the last 6 months. On physical examination, his heart rate is 85 beats per minute, his respiratory rate is 22, and his blood pressure is 108/65 mm Hg. His jugular venous pulse is visible 8 cm above the sternal angle at 45 degrees. The Point of Maximal Impulse (PMI) is sustained but normal in location. He has an S4 gallop and 1+ bilateral pedal edema. A PA and lateral chest x-ray are unremarkable. A transthoracic echocardiogram reveals normal left and right ventricular systolic function with mild left ventricular hypertrophy and abnormal diastolic function. A cardiac magnetic resonance imaging (MRI) with gadolinium contrast is obtained.
Still frame gradient echo image, four-chamber view.
Corresponding delayed hyperenhanced image, four-chamber view.
Based on these images, the next most appropriate clinical step is:
a. Endomyocardial biopsy
b. Fat pad biopsy
c. Initiate corticosteroid therapy
d. Surgical pericardial stripping
2. You are asked to see a 58-year-old woman in the emergency room who has presented with intermittent retrosternal chest pain without radiation lasting for <1 minute and a single episode of rest pain lasting 10 minutes today. She states she has been having these symptoms since shoveling snow 1 week prior to presentation. Her past medical history is significant for gastroesophageal reflux, for which she takes an over-the-counter H2 blocker infrequently. She takes no other medications. She was told at a health screening fair a few months ago that her cholesterol levels were high, but she has not seen her family physician about it. Physical examination is unremarkable. Electrocardiogram (ECG) reveals normal sinus rhythm with no ischemic changes. Initial laboratory evaluation, including a portable chest x-ray and cardiac enzymes, are within normal limits. A cardiac CT angiography (CTA) is obtained to further evaluate the etiology of her chest pain.
Curved multiplanar reformatted (MPR) image of the aortic valve (black arrow) and right coronary artery (RCA) (white arrow).
Based on this image, the next most appropriate step is:
a. Begin therapy with an angiotensin-converting enzyme (ACE) inhibitor, beta-blocker, and diuretic.
b. Begin therapy with a proton pump inhibitor.
c. Obtain a transthoracic echocardiogram to evaluate for aortic stenosis.
d. Refer the patient for a nuclear stress study.
e. Refer the patient for cardiac catheterization.
Left anterior oblique cranial projection of the RCA reveals a 60% to 70% stenosis in the midportion of the vessel (white arrow).
3. A 42-year-old man with diabetes and a family history of coronary artery disease undergoes coronary CTA after an equivocal exercise stress test. The following multiplanar reconstruction (MPR) image is obtained of the left main and left anterior descending (LAD) arteries
Curved MPR image of the LAD (white arrow).
Which of the following statements is true regarding the calcified plaque seen in the proximal LAD in this image?
a. Additional postprocessing should be performed to remove the calcium blooming artifact.
b. Coronary calcification may occur in the presence of atherosclerosis but is a nonspecific finding.
c. Coronary calcification tends to overestimate coronary artery stenosis due to the blooming artifact.
d. The degree of coronary calcification correlates well with the severity of stenosis in the underlying vessel.
4. A 60-year-old man with a history of hypertension and dyslipidemia presents to the hospital in acute pulmonary edema approximately 72 hours after probable onset of an anterior myocardial infarction. The patient is stabilized and a cardiac catheterization is performed, which demonstrates a diffusely calcified 90% lesion of the ostial–proximal LAD that is not amenable to percutaneous intervention. The RCA and left circumflex (LCX) artery demonstrate mild to moderate diffuse disease. A transthoracic echocardiogram demonstrates a left ventricular ejection fraction of approximately 15%. He is referred for coronary artery bypass surgery and a cardiac MRI is obtained to assess for myocardial viability.
Delayed hyperenhanced image obtained 15 to 20 minutes after gadolinium DTPA, two-chamber view.
Delayed hyperenhanced image obtained 15 to 20 minutes after gadolinium DTPA, three-chamber view.
All of the following would be appropriate except:
a. Surgical revascularization of the LAD artery
b. Medical therapy with an ACE inhibitor, beta-blocker, and diuretic
c. Medical therapy with aspirin and a statin
d. Consideration for implantation of a defibrillator
5. A 32-year-old woman is referred to your office for evaluation of occasional palpitations and increasing exertional dyspnea. She denies any history of fever, syncope, or neurologic deficits. Physical examination is unremarkable. A transthoracic echocardiogram demonstrates a poorly defined left ventricular mass, and a cardiac MRI is obtained for further evaluation of the lesion.
Still frame gradient echo image, three-chamber view.
Black blood (turbo spin echo) axial image depicting an intracardiac mass (white arrow).
Corresponding fat saturated black blood (T2-weighted STIR) axial image.
Both echo and MRI demonstrate normal systolic function and no valvular abnormalities. No other lesions are noted on the cardiac MRI study. A computed tomography (CT) scan of the chest, abdomen, and pelvis are otherwise normal. Given the signal characteristics on the above image, this lesion most likely represents:
a. Fibroma
b. Lipoma
c. Myxoma
d. Thrombus
e. Papillary fibroelastoma
6. In which of the following aspects is CT inferior to MRI?
a. Spatial resolution
b. Temporal resolution
c. Typical scan time
d. Characterization of calcium
e. None of the above
7. In which of the following does MRI not have incremental value over echocardiography in the evaluation of patients with hypertrophic cardiomyopathy?
a. Quantification of left ventricular septal thickening and left ventricular mass
b. Quantification of left ventricular outflow tract obstruction
c. Mitral valve anatomy
d. Papillary muscle anatomy
e. Presence of myocardial fibrosis
8. All of the following CT scanning properties are employed to minimize radiation exposure except:
a. Axial prospective ECG gating
b. ECG gated tube modulation
c. Iterative reconstruction
d. Filtered back projection
e. Alteration of tube voltage
9. A 77-year-old man with prior mitral valve repair with a C-shaped mitral annuloplasty ring and aortic valve replacement with a stented bovine bioprosthesis is referred for cardiac MRI to investigate for suspected ascending aortic aneurysm. He has a history of prior motor vehicle accident with a total right knee joint replacement, thoracic spinal fixation device. Which of the following is an absolute contraindication for him to undergo MRI?
a. Mitral annuloplasty ring
b. Bioprosthetic aortic valve
c. Prosthetic knee joint
d. Thoracic spinal fixation device
e. None of the above
10. Retrospective or prospective ECG gating is commonly used in cardiac CT to freeze cardiac motion. In which of the following clinical indication(s) is/are ECG gating crucial for optimal image quality?
a. Coronary artery stenosis
b. Pulmonary vein stenosis
c. Ascending aortic dissection/hematoma
d. a and c
e. a, b, and c
Answers
1. Answer A: Endomyocardial biopsy. Although the mildly thickened ventricular myocardium is consistent with several different etiologies of cardiomyopathy, the diffuse pattern of hyperenhancement throughout the left ventricle on delayed hyperenhanced black blood MRI images (second figure above) is typical of cardiac amyloidosis. Cardiac sarcoidosis, which might be an indication to begin corticosteroid or other immunosuppressive therapy, typically demonstrates patchy areas of hyperenhancement, along with ventricular wall thinning and aneurysms, most commonly along the basal anteroseptal wall. Cine gradient echo images in sarcoidosis may demonstrate normal or impaired left ventricular systolic function, often with regional wall motion abnormalities. Although there may be a role for immunosuppressive therapy in specific subtypes of amyloidosis, histologic diagnosis should be confirmed before therapy is initiated. There is no thickening of the pericardium, conical deformity of the ventricles, or atrial enlargement on these images to suggest constrictive pericarditis, making pericardial stripping inappropriate.
2. Answer E: Refer the patient for cardiac catheterization. The curved MPR image reveals a noncalcified atherosclerotic plaque in the mid-RCA associated with severe luminal stenosis, which can be compared to her corresponding coronary angiogram (see image below). The low attenuation characteristics of this lesion on coronary CTA suggest that it is a noncalcified plaque, unlike the higher-attenuation calcified plaque that occurs more proximally. Current CT technology does not allow precise quantification of coronary stenoses as is done with invasive angiography. Therefore, most lesions are graded as mild (<50%), moderate (50% to 70%), or severe (>70%) stenoses. There is no evidence of heart failure that would suggest therapy with ACE inhibitors, beta-blockers, and diuretics. The aortic valve leaflets appear thin and noncalcified, making aortic stenosis less likely. Cine CT images of the ventricles and aortic valve could be reconstructed, if desired, to assess ventricular function and leaflet mobility. Additional noninvasive testing is not indicated in this patient because of the abnormalities seen on coronary CTA.
3. Answer C: Coronary calcification tends to overestimate coronary artery stenosis due to blooming artifact. This is due to attenuation (absorption) of the x-ray photons by deposits of calcium, which is relatively dense compared to its surrounding tissues. Currently, this artifact cannot be removed by postprocessing techniques. The presence of coronary calcification does correlate with an individual’s overall atherosclerotic disease burden, but it does not predict the severity of stenosis of the underlying vessel. Calcification within an artery is a specific sign of atherosclerosis.
4. Answer A: Surgical revascularization of the LAD artery. Delayed hyperenhancement images demonstrate transmural scarring from the proximal to distal anterior and anteroseptal walls, as well as the apex and inferoapical segments. The transmural extent of hyperenhancement suggests a poor likelihood of recovery of myocardial function after revascularization (whether surgical or percutaneous), consistent with nonviable myocardium. Surgical revascularization would be high risk given his low ejection fraction, and unlikely to improve his long-term survival or ventricular function because of the nonviable myocardium in the infarctrelated territory. The other choices would be indicated given the clinical scenario.
5. Answer B: Lipoma. In the first and second images, an encapsulated mass is visible in the posterolateral wall of the left ventricle. In the third image, the mass has similar signal intensity as the nearby subcutaneous fat, suggesting a possible fatty nature. This is confirmed on the subsequent fat-saturated black blood axial image, in which a special pulse is given prior to acquisition of the image to suppress signal arising from fatty tissue. The mass now appears black due to loss of signal, as does the nearby subcutaneous fat, confirming the fatty nature of the mass.
The fatty content of the mass and the normal left ventricular systolic function are not consistent with a left ventricular thrombus. Papillary fibroelastomas do occur on the endocardium but most often (50%) occur on the aortic valve. They are not usually encapsulated and often demonstrate a “frond-like” appearance (similar to pompoms used by cheerleaders) and frequently have a stalk. Myxomas are most often located in the atria and are not characterized by this degree of fat content. Many myxomas have patchy, dark areas of low signal intensity on MRI because of calcification within the tumor.
6. Answer B: Comparing CT and MRI in cardiac applications, CT has clearly superior spatial resolution and canbe completed usually within one breath hold. MRI, on the other hand, excels in superior temporal resolution and tissue characterization. However, calcium appears in signal void areas on MRI, which makes it difficult to identify. Hence, “temporal resolution” is the correct answer.
7. Answer B: Cardiac MRI is an important adjunctive imaging modality for the evaluation of hypertrophic cardiomyopathy. It accurately quantifies the thickening of the left ventricular septum (a), as well as visualizes the mitral valve (c) and subvalvular anatomy (d) which are not uncommonly associated conditions that require surgical treatment at the time of myectomy. These are sometimes imaged sufficiently on echocardiography, although MRI has the definite advantage of not being limited by imaging planes and window. The presence of myocardial fibrosis (e) is associated with worse prognosis and is not characterized by echocardiography. Quantification of left ventricular outflow tract (b) obstruction, however, is best performed by Doppler echocardiography where real-time imaging and alignment with the left ventricular outflow tract at high temporal resolution enables the technique to obtain the most accurate gradient, which is at present difficult by MRI.
8. Answer D: Radiation dose exposure depends on the tube voltage (e), current, coverage, and pitch. In cardiac imaging, the window when full radiation is applied during the cardiac cycle is also important. Tube current modulation (b) reduces the amount of radiation applied during a portion of the cardiac cycle when data may not be used for reconstruction. Axial prospective ECG gating (a) restricts the tube current and data acquisition to a specified narrow window within the cardiac cycle, hence reducing the amount of radiation. Iterative reconstruction (c) is a new method of image reconstruction from the raw data obtained, which has reduced image noise hence allowing for a lower tube current for similar image quality. Filtered back projection (d) is the name of the most commonly used image reconstruction algorithm, therefore not a strategy for radiation dose reduction.
9. Answer E: Most prosthetic valves and annuloplasty rings are made of nonferromagnetic materials and can be safely scanned with MRI. While the stented prosthetic valves can give rise to significant susceptibility artifact in some MRI sequences, the effect of annuloplasty rings on image quality tends to be minimal. Most orthopedic devices such as prosthetic hip and knee replacements as well as internal spinal fixation devices are made of titanium or cobalt–chromium alloy and have no significant ferromagnetic properties. While large image artifact from thoracic spinal fixation devices may affect the image interpretation due to artifact, they are usually safe.
10. Answer D: ECG gating freezes cardiac motion and is important for cardiac imaging. The coronary arteries move rapidly during the cardiac cycle, especially the RCA, making cardiac motion artifact problematic in patients with higher heart rate, even when ECG gating is employed. Ascending aortic anatomy is best assessed with ECG gating, especially in cases of suspected aortic dissection or hematoma, where aortic root and ascending aorta motion artifact can sometimes mimic the appearance of intramural hematoma. ECG gating becomes less of an issue when the suspected pathology is in the aortic arch or the descending aorta. Motion of pulmonary veins during the cardiac cycle is relatively less compared because of its location in the base of the heart. While many scanning protocols incorporate ECG gating, pulmonary veins can often be adequately imaged in atrial fibrillation where a nongated spiral scan is employed.