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1 Both authors: Division of Cardiothoracic Radiology, Department of Radiology, Taubman Center, 1500 E Medical Center Dr., Ann Arbor, MI 48109-0326.
Received March 1, 2006;
accepted after revision June 1, 2006.
Address correspondence to A. K. Attili
(aattili{at}umich.edu).
Abstract
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The educational objective of this evidence-based self-assessment module is to use case examples to review the current evidence and the roles of CT and MRI in evaluating and managing patients with both congenital and acquired coronary artery disease.
Conclusion
In this educational module, we review the use of CT and MRI in the noninvasive diagnosis and management of patients with coronary artery disease.
Keywords: cardiac imaging • coronary artery disease • CT angiography • MRI
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TopAbstractINTRODUCTIONEDUCATIONAL OBJECTIVESScenario 1References |
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Description of Images
An axial image from CT angiography (Fig.
1A) shows the left main coronary artery arising from the right
sinus of Valsalva and having a common origin with the right coronary artery.
The left coronary artery then passes between the aorta and the pulmonary
outflow tract.
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| QUESTION 1 Which of the following statements about coronary artery anomalies is FALSE?
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Solution to Question 1
Congenital anomalies of the coronary arteries are infrequent, affecting
about 1% of the population, a percentage derived from cineangiography
performed for suspected obstructive disease
[1,
2]. Necropsy studies yield a
lower incidence [3]. Anomalies
of the origin and course of the coronary arteries can be classified into
malignant and nonmalignant forms. Malignant forms are associated with an
increased risk of myocardial ischemia and sudden death. The malignant
anomalies most often course between the pulmonary artery and the aorta, the
most common form being origin of the right coronary artery from the left sinus
of Valsalva. Anomalies of the left main coronary artery and the left anterior
descending artery arising from the right sinus of Valsalva with this course
are also associated with higher risk. The reason for the anomaly is either a
kink at the sharp leftward or rightward bend at the vessel ostium or a
pinchcock mechanism between the aorta and the pulmonary artery. The oblique
origin of the anomalous artery causes a slitlike orifice in the aortic wall
and is capable of collapsing like a valve
[4]. An origin of either the
left or the right coronary artery from the pulmonary artery
(Bland-Garland-White syndrome) must also be considered malignant and is
associated with myocardial ischemia and sudden death in childhood. Coronary
anomalies account for 19% of deaths in athletes, according to the Sudden Death
Committee of the American Heart Association
[5]. Approximately 59% of
patients with an anomalous left coronary artery arising from the right sinus
die before the age of 20 years, usually during or shortly after vigorous
exertion [6]. Until the
pathophysiologic mechanisms of ischemia and sudden death are clarified in
patients who die inexplicably, the presence of an anatomically variant
coronary pattern should be considered a potential but unproven risk factor for
sudden cardiac death. Option A, which is true, is not the best
response.
Sudden death is frequently the first manifestation in patients with ectopic coronary artery origin; however, warning symptoms such as chest pain and syncope may occur in a substantial proportion of these individuals. The diagnosis of coronary artery anomalies requires a high index of suspicion during the history and physical examination. Resting and stress 12-lead ECG can show normal findings and would not be reliable as a screening test in a large athletic population [7]. Option B, which is true, is not the best response.
Anomalies of the origin and course of the coronary arteries are often difficult to detect on catheter angiography. In comparison, the ability of MDCT to detect and characterize anomalies of the coronary arteries is higher [8]. In contrast to catheter angiography, which provides 2D data, MDCT provides 3D data sets that allow the anatomic course of the coronary arteries to be simultaneously displayed in relation to the mediastinal vessels and the cardiac chambers. MDCT is also noninvasive. Coronary MR angiography can also be used to identify anomalous coronary arteries with a higher accuracy than catheter angiography [9, 10]; however, the procedure takes longer to complete than MDCT angiography. MRI has the important advantages of being radiation-free and not requiring the use of an IV contrast agent. Option C, which is false, is the best response.
An epicardial segment of a coronary artery that courses through the myocardium is termed "myocardial bridging." Myocardial bridges (Fig. 1B) most commonly affect the left anterior descending artery and, less frequently, other left ventricular branches [11]. With myocardial bridging, the involved coronary artery is compressed in systole, particularly in deeper bridges. Thin bridges may cause little compression [12]. The clinical significance of myocardial bridges is uncertain. Generally, myocardial bridging is considered a benign condition because most coronary flow occurs during diastole, but the abnormality has been reported to be a cause of angina, ischemia, and infarction [13]. Option D, which is true, is not the best response.
Coronary artery fistula is a condition in which a communication exists between one or two coronary arteries and either a cardiac chamber, the coronary sinus, the superior vena cava, or the pulmonary artery. It more commonly involves the right coronary artery (60% of cases) than the left coronary artery (40% of cases). The drainage site of the fistula has a greater clinical and physiologic importance than does the artery of origin. The most common site of drainage is the right ventricle (45% of cases), followed by the right atrium (25%) and the pulmonary artery (15%) [14]. The fistula drains into the left atrium or left ventricle in less than 10% of cases. When the shunt leads into a right-sided cardiac chamber, the hemodynamics resemble those of an extracardiac left-to-right shunt; when the connection is to a left-sided cardiac chamber, the hemodynamics mimic those of aortic insufficiency. Myocardial perfusion may be diminished for that portion of the myocardium supplied by the abnormally connecting coronary artery. This situation represents a hemodynamic steal phenomenon and may lead to myocardial ischemia. Option E, which is true, is not the best response.
Conclusion
The patient in this clinical scenario is at risk of sudden death. A
coronary artery bypass graft using a saphenous vein graft to the left anterior
descending artery and an obtuse marginal artery was performed to protect the
patient from ischemia resulting from the ectopic origin and course of the left
main coronary artery. The patient did not have a recurrence of angina after
the procedure.
Scenario 2
Clinical History
A 29-year-old man presents to the emergency department with the acute onset
of chest pain. Cardiac enzymes are elevated, and pain is relieved by
nitroglycerine. Cardiac catheterization is performed at the time of admission.
MDCT is performed on an outpatient basis a year later to follow up the
abnormalities seen on conventional angiography.
Description of Images
A curved multiplanar reformatted image of the right coronary artery from CT
angiography (Fig. 2A) shows
multiple coronary artery aneurysms.
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| QUESTION 2 Regarding coronary artery aneurysms, which of the following statements is FALSE?
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The most commonly affected coronary artery segments are, in order of frequency, the proximal and mid portions of the right coronary artery, the proximal portion of the left anterior descending artery, and the proximal portion of the circumflex coronary artery. Aneurysms of the left main coronary trunk are unusual [16]. Option B, which is false, is the best response.
Kawasaki disease is an acute vasculitis of infancy and childhood. When it is left untreated, 15-25% of patients develop coronary artery aneurysms [17]. Medical treatment, including corticosteroids, reduces the incidence of coronary artery aneurysms in Kawasaki disease [17]. Option C, which is true, is not the best response.
Coronary angiography has been considered the standard reference technique for diagnosing coronary aneurysms, but it is invasive and expensive. However, if the aneurysm contains substantial thrombus, its true size may be underestimated on catheter angiography. CT provides a noninvasive approach for the diagnosis of coronary artery aneurysms. Thin-section axial images can provide primary diagnostic information. However, ECG-gated MDCT allows a more rapid and accurate delineation of the size and shape of aneurysms [18]. MDCT also enables high-quality 2D and 3D reformations, which may be valuable in showing spatial relations among the aneurysm, the great vessels, and the heart. MRI offers an alternative imaging technique for evaluating coronary artery aneurysms and obviates the radiation dose associated with MDCT. However, the spatial resolution of MRI is inferior to that of CT. Option D, which is true, is not the best response.
A documented association exists between coronary artery aneurysms and aneurysms in other vascular beds, probably owing to a common underlying pathogenetic mechanism of atherosclerosis. The association with abdominal aortic aneurysms is particularly well recognized [19]. Option E, which is true, is not the best response.
Conclusion
The clinical diagnosis in this patient was myocardial infarction resulting
from thromboembolism from the aneurysms. An extensive workup to determine the
cause of the coronary artery aneurysms, including Kawasaki disease, failed to
determine the cause, and the aneurysms were considered idiopathic.
| QUESTION 3 Which of the following statements is TRUE about coronary calcium scores?
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| QUESTION 4 Which of the following statements is FALSE about CT angiography technique?
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| QUESTION 5 Which of the following statements is TRUE of the applications of CT angiography?
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Scenario 3
Clinical History
A 40-year-old man presents to the outpatient cardiology department with
hyperlipidemia and atypical chest pain. He has a strong family history of
premature atherosclerotic heart disease.
Description of Images
ECG-gated 64-MDCT coronary angiography (Figs.
3A,
3B,
3C,
3D,
3E,
3F and
3G) was performed after
unenhanced CT to detect coronary calcium, and showed a focal, eccentric, mixed
calcified and soft plaque causing less than 25-30% stenosis of the proximal
vessel.
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The Agatston score (AJ score) is determined as follows [25]: Calcified lesions are identified by applying a threshold of 130 H on all slices covering the entire coronary tree (usually acquired in a prospective ECG-triggered mode for clinical purposes) and ignoring structures smaller than 1 mm2 to exclude noise from the evaluation. The area and the maximum attenuation value of each calcified lesion are determined, and a weighting factor is applied to each lesion depending on the maximum attenuation value measured. The AJ score for each lesion is calculated as the product of the area of the lesion multiplied by the weighting factor. The total calcium score is determined by summing the scores of the lesions for all arteries in all images. The dependence of the AJ score on maximum CT attenuations and the nonlinear weighting factor used lead to a high interscan variability [26]—that is, a low reproducibility. The volume score provides an alternative quantification method for coronary calcium, and studies indicate that it is better with respect to reproducibility [27]. A region of interest is drawn around each calcified lesion, and for each lesion the number of voxels in the volume data set that lie above a certain threshold is multiplied by the volume of one voxel. A nonlinear weighting factor, as in the AJ score, is not applied, which improves reproducibility. The calcium mass score aims at determining the absolute calcium mass and is the only scoring method that provides a truly quantitative measure for the amount of calcium (e.g., in milligrams of hydroxyapatite). The calcium mass of a lesion is directly proportional to the mean attenuation value multiplied by the volume of the lesion. The calculation of the mass score corrects for linear partial volume effects and does not apply a nonlinear weighting factor, producing increased reproducibility. The AJ score is the least reproducible among the three scoring methods [28]. Option B is not the best response.
The introduction of EBCT in the mid 1980s made quantification of coronary calcium possible. A limitation of EBCT is the fixed 60-mAs setting. Image noise increases and degrades image quality in larger patients when the tube current is fixed. Signal-to-noise ratio is crucial in distinguishing small calcifications from noise [29]. With MDCT, tube current can be adjusted to improve the signal-to-noise ratio, thus helping to distinguish small calcifications from image noise. However, the measurement precisions of EBCT and MDCT are similar [30]. Option C is not the best response.
The absence of detectable calcium has a high negative predictive value for ruling out obstructive coronary artery disease [31]. This is the most important application of the coronary artery calcification examination. A negative predictive value of 98% has been reported for coronary chest pain or myocardial infarction in patients with acute symptoms and nonspecific ECG results [32]. Furthermore, negative results on CT calcium scanning carry prognostic information with a low probability of future coronary artery disease-related events [33]. It must be stressed, however, that although negative CT findings for calcium do imply a low likelihood of significant luminal obstruction, the presence of noncalcified atherosclerotic plaque remains a possibility. Option D is the best response.
Most acute coronary syndromes result from rupture of an inflamed vulnerable plaque consisting of a thin fibrous cap and a lipid-rich core. The pattern of plaque calcification has been correlated to plaque morphology in a histomorphologic study of patients who died suddenly of severe coronary disease. The greatest amount of calcium was found in healed ruptures [34]. Calcification may be seen as an attempt of the arterial wall to stabilize itself because calcified and fibrotic plaques are much stiffer than lipid-rich lesions. Option E is not the best response.
Solution to Question 4
It is important to optimize the heart rate to obtain high-quality results
with ECG-gated MDCT angiography. A slow, regular heart rate increases the
portion of the cardiac cycle spent quietly in diastole and is ideal for image
quality. Heart rates greater than 65 beats per minutes (bpm) increase motion
artifacts and reduce the image quality of portions of the coronary arteries,
particularly the right coronary artery. Segment visibility and sensitivity for
detecting stenosis decrease with increasing heart rates
[35]. Premedication with
ß-adrenergic receptor blocking agents (ß-blockers) is recommended to
reduce the heart rate before CT angiography
[36], particularly in patients
with heart rates greater than 65 bpm. Contraindications for ß-blocker
therapy include asthma, atrioventricular conduction block, heart failure,
diabetes, and Raynaud syndrome. Option A, which is true, is not the best
response.
Retrospective ECG gating is used for coronary CT angiography performed on an MDCT scanner [36]. With this method, the scanning data and ECG tracing are recorded simultaneously but independently. Retrospective ECG gating allows the scanning data to be acquired throughout the cardiac cycle for subsequent reconstruction during specified periods of the cycle. The entire heart is imaged as a volume for subsequent 3D manipulation. Because the individual coronary vessels have different motion patterns, performing individual reconstruction for each vessel with regard to its position in the cardiac cycle may optimize coronary segment visualization. Atrial contraction during end-diastole causes a rapid motion of the right coronary artery and the left circumflex coronary artery because of their positions in the atrioventricular groove. The right coronary artery is best seen early in diastole at 40% of the R-R interval, the left circumflex artery is best seen in mid cycle, and the left anterior descending artery is best seen at 60-70% of the R-R interval [37]. Option B, which is false, is the best response.
Isoosmolar nonionic iodinated contrast medium is used for CT angiography. Seventy to one hundred twenty milliliters of contrast medium is injected at the rate of 3.5-4 mL/sec through an 18- to 20-gauge needle placed in an antecubital vein [36]. The newer 16- and 64-MDCT scanners allow a smaller total volume of contrast medium to be used. Either a fixed delay, a test bolus protocol, or a bolus tracking technique can be used to synchronize scan timing with coronary artery enhancement. The use of a saline bolus chaser is recommended to diminish beam-hardening contrast artifact in the right ventricle that obscures the right coronary artery. A saline bolus also facilitates rapid delivery of the entire contrast volume [38]. Option C, which is true, is not the best response.
Sixteen-MDCT with standard protocols for coronary CT angiography (120 kV, 400 mAs, 12x0.75 mm collimation) results in an effective radiation dose of 8.1 mSv for men and 10.9 mSv for women [39]. This dose is higher than that of selective conventional coronary angiography (3-5 mSv). The use of ECG-pulsed tube current modulation results in a significant reduction in dose. Option D, which is true, is not the best response.
Reliable coronary artery imaging is best performed in patients when they are in normal sinus rhythm. Heart rate alterations and irregularities, such as premature ventricular contractions (PVCs) during the scan acquisition, move anatomic data from where it is expected in the cardiac cycle to incorrect phases, causing section gaps and apparent pseudostenosis in the reconstructed vessels. Postprocessing ECG editing techniques are now available to edit out PVCs and restore image quality when mild heart rate irregularities occur during the scan acquisition. These techniques consist of visually identifying cardiac motion artifact and manually modifying the scan data to delete, insert, or reposition the data that appear incorrectly by placing different phase data into the correct phase, so that the number and position of the corresponding temporal windows are aligned with the least residual motion. Editing is typically limited to one or two beats because greater manipulation will lead to gaps in the scan data. Improved diagnostic accuracy of MDCT for detecting coronary stenosis has been shown by using ECG editing [40]. Option E, which is true, is not the best response.
Solution to Question 5
ECG-gated CT angiography can be used to assess stent patency on the basis
of contrast enhancement in the stent because an unenhanced distal coronary
artery lumen usually reflects critical in-stent stenosis or complete
occlusion. However, assessment of the stent lumen for nonocclusive narrowing
due to neointimal hyperplasia remains challenging. Recent improvements in
spatial resolution with the latest generation of MDCT scanners have improved
the assessment of the stent lumen. However, improved spatial resolution can
only partially compensate for metallic artifacts arising from stent struts,
which exaggerate the actual size of the stent and obscure subtle in-stent
abnormalities of the lumen. The clinical value of CT after stent placement is
therefore largely limited to the detection of stent occlusion
[41]. Option A is not the
best response.
Coronary arteries are small and they move rapidly. Thus, imaging of the coronary arteries requires high spatial and high temporal resolution. Invasive, catheter-based coronary angiography has a temporal resolution ("shutter speed") of approximately 6 milliseconds and a spatial resolution of approximately 0.25 mm [42]. CT has undergone tremendous technical development since the first generation of MDCT scanners. Gantry rotation speed has increased rapidly, resulting in improved temporal resolution. The current 64-MDCT scanners allow an isotropic resolution of 0.4x0.4x0.4 mm at a gantry rotation speed of 330 milliseconds [43]. By applying a half-scan algorithm (only data from a 180° gantry rotation is used for image reconstruction), acquisition time can be reduced to 165 milliseconds. Thus, although they are improved, the temporal and spatial resolutions of CT angiography are still inferior to those of conventional angiography. Option B is the best response.
Calcium deposits in the coronary arteries affect the X-ray beam, leading to beam-hardening and partial volume artifacts. As a result, calcified plaque appears larger than it actually is, thereby increasing the apparent severity of lumen narrowing and making accurate assessment of stenosis difficult. The latest generation of MDCT scanners, with improved spatial resolution and reduction of partial volume effects, improves diagnostic accuracy and potentially reduces the problems caused by calcification. Nevertheless, extensive calcification prevents assessment of many coronary artery segments and results in false-positive or false-negative interpretations of significant stenosis [44, 45]. Some investigators have proposed performing low-dose unenhanced scanning in all patients before CT angiography. Some patients with extensive calcification can be unsuitable for coronary CT angiography for this reason. Option C is not the best response.
There is considerable interest in the use of MDCT for evaluating chest pain in the emergency department setting. MDCT has the potential to provide a fast and comprehensive evaluation for causes of chest pain, including cardiac and noncardiac causes such as aortic disorders and pulmonary embolism. However, only limited published data exist on the use of MDCT for triaging patients with chest pain who present to the emergency department [46, 47]. In a recent publication, White et al. [46] showed a sensitivity and specificity of 83% and 96%, respectively, for the establishment of a cardiac cause of chest pain using 16-MDCT in an emergency setting. The published accuracies for 64-MDCT scanners have surpassed those of 16-MDCT scanners. Leschka et al. [48] showed a negative predictive value of 99% for 64-MDCT compared with invasive angiography in assessing significant coronary stenosis. The routine application of this technique in clinical practice in the emergency setting will need to await further studies relating to the effect on patient outcome and cost-effectiveness. Option D is not the best response.
CT coronary angiography is a diagnostic tool and does not provide the option for immediate interventional treatment. Thus, the clinical application of CT angiography in patients with a high pretest probability for coronary artery disease, such as an older patient with typical anginal chest pain, is of limited value [49]. If the likelihood of an intervention is reasonably high, the patient should proceed directly to invasive angiography rather than CT. Routine screening of asymptomatic individuals by coronary CT angiography will not be beneficial because treatment of asymptomatic stenosis is generally not expected to alter the patient's prognosis. Option E is not the best response.
Conclusion
This patient with atypical chest pain, hyperlipidemia, and a family history
of atherosclerosis had a total Agatston score of 4. The plaque shown on CT
angiography in the left anterior descending coronary artery was causing less
than 30% stenosis and did not warrant surgical intervention. Therefore,
lifestyle modification, aspirin, and cholesterol-lowering medication (statins)
were recommended, and no active intervention in the form of angioplasty or
surgical revascularization was contemplated.
Scenario 4
Clinical History
A 70-year-old man is diagnosed with paroxysmal ventricular tachycardia. His
medical history is significant for an episode of chest pain that required
emergency admission. MRI of the heart, including a delayed contrast-enhanced
sequence, is performed. Review of the cine images with the patient resting
revealed a dyskinetic inferior wall.
Description of Images
Delayed contrast-enhanced short-axis MR imaging
(Fig. 4A) shows transmural
enhancement of the inferior wall of the left ventricle that extends to the
inferior septum.
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| QUESTION 6 Which of the following statements is TRUE about the short-axis and two-chamber long-axis delayed contrast-enhanced MR images?
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| QUESTION 7 Which of the following statements is TRUE of delayed contrast-enhanced MRI?
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| QUESTION 8 Which of the following statements about delayed contrast-enhanced MRI is FALSE?
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Solution to Question 6
Identification of irreversibly injured myocardium from dysfunctional but
viable and potentially salvageable myocardium is of crucial importance for the
management of cardiac patients. Revascularization of an infracted area by
percutaneous coronary intervention or coronary artery bypass grafting is
deemed justified only if functional recovery after the intervention is
predictable, or if late outcome and patient well-being can be improved.
Reversible myocardial dysfunction can be acute or chronic and is treatable
with angioplasty or bypass surgery in many cases. The term "hibernating
myocardium" is used to describe viable myocardium in a state of
persistent but potentially reversible dysfunction resulting from chronic
coronary artery stenosis. "Stunned myocardium" is dysfunctional
but viable myocardium after an acute ischemic episode with early reperfusion
[50].
Delayed contrast-enhanced MRI is an excellent tool for evaluating
myocardial viability [51,
52]. Dysfunctional,
predominantly viable segments will have no or minimal (
25% transmural
extent) hyperenhancement, whereas predominantly scarred segments will show
greater than 75% transmural extent of hyperenhancement. In the present
scenario, transmural enhancement in the inferior wall and inferior septum
represents scarred or nonviable tissue. Option A is not the best
response.
Although the coronary artery blood supply to myocardial segments is variable, it is appropriate clinical practice to assign individual myocardial anatomic segments to specific coronary artery territories [53]. The right coronary artery generally supplies the inferior wall of the left ventricle and the inferior basal septum in a right-dominant system, as seen in the present case, whereas circumflex territorial enhancement usually involves the lateral wall of the left ventricle. Option B is not the best response.
The extent of transmural infarction in patients with chronic ischemic heart disease is an important predictor of functional recovery after revascularization [51, 52]. Dysfunctional segments with extensive enhancement (> 75% of wall thickness) on delayed contrast-enhanced MRI are unlikely to exhibit functional recovery after percutaneous or surgical revascularization. Option C is the best response.
Perfusion of ischemic myocardium may not be completely restored after myocardial infarction despite successful reperfusion of infarct-related territory because of microvascular injury: the "no-reflow" phenomenon [54]. When microvascular damage is extensive, delayed contrast-enhanced MRI can show a hypoenhanced zone in the hyperenhanced infarcted zone. The presence of microvascular obstruction after myocardial infarction is a predictor of an adverse outcome, with a higher incidence of left ventricular remodeling, congestive heart failure, malignant arrhythmias, and death [55]. Option D is not the best response.
The hyperenhancement is transmural, involving the subendocardium, and in the territory of the left circumflex coronary artery. Such a pattern is indicative of an infarction due to coronary artery ischemic disease. Nonischemic myocardial infiltrative diseases such as sarcoidosis typically produce patchy midwall enhancement that spares the subendocardium [56]. Option E is not the best response.
Solution to Question 7
The delayed contrast-enhanced MRI technique for the detection of myocardial
viability relies on the extracellular distribution of gadolinium chelates in
the myocardium. In regions with increased extracellular space (e.g.,
infarction and fibrosis), higher concentrations of gadolinium accumulate with
concomitant slower clearance and a higher signal on delayed enhanced sequences
[57]. Option A is not the
best response.
The typical pulse sequence for myocardial delayed enhancement is an inversion recovery-prepared segmented gradient-echo sequence exhibiting an increased signal intensity of infarcted tissue that is superior to other imaging techniques, such as the spin-echo technique [58]. Option B is not the best response.
"Nulling" of the normal myocardium is critical if areas of hyperenhancement are to be properly displayed. The gradient-echo technique used consists of an inversion prepulse chosen so that there is no or little longitudinal magnetization in the normal myocardium. Selection of the appropriate inversion time is crucial. In clinical settings, this is usually performed visually by applying a 2D inversion sequence with variable prepulse delays (200-300 milliseconds, in steps of 25 milliseconds) or a Look-Locker sequence[59]. The optimal time to inversion is the delay with the best visual suppression of myocardium. The typical signal intensity is a dark normal myocardium, a slightly brighter blood pool, and a very bright infarct. Option C is the best response.
Both 2D and 3D sequences for delayed contrast-enhanced MRI are possible. The 3D sequences have the advantages of being able to cover the entire ventricle in a single breath-hold, with a higher signal-to-noise ratio. However, 3D sequences are subject to more motion artifact because of the larger number of k-space lines that must be acquired compared with the 2D sequences. In a comparison of the one-breath-hold 3D inversion recovery gradient-echo MR sequence with a multiple-breath-hold 2D inversion recovery gradient-echo MR sequence for the detection of nonviable myocardium, a high level of agreement was found for the presence of hyperenhancement, whereas agreement was poor for the transmural extent of hyperenhancement that could be attributed to the blurred appearance of the 3D MR images [60]. Option D is not the best response.
Motion-related artifacts during acquisition of the delayed enhancement images may be caused by either respiratory or cardiac motion [61]. Image acquisition is typically performed in breath-hold at a time corresponding to the period of diastolic diastasis. In general, MRI motion artifacts occur in the phase-encoding direction. If an artifact is noted during the acquisition, the phase-encoding direction can be changed to see if it recurs. Most problematic are artifacts related to the motion of the heart; for example, caused by the patient starting to breathe out at the end of the acquisition. This can create a cardiac-shaped overlay on the actual image acquisition and may hamper proper image analysis. It is useful to realize that an artifact does not respect anatomic borders; usually a contour can be found outside the heart, revealing the nature of this imaging finding. Option E is not the best response.
Solution to Question 8
Late enhancement is specific for regional myocardial damage that is not
necessarily due to a myocardial infarction. Delayed enhancement can also be
seen in patients with myocarditis, hypertrophic cardiomyopathy, dilated
cardiomyopathy, sarcoidosis, and other infiltrative and storage myocardial
diseases [56]. Subendocardial
sparing of enhancement and distribution that do not conform to a coronary
artery territory favor a nonischemic cause. Option A, which is false, is
the best response.
The presence, location, and transmural extent of healed Q-wave and non-Q-wave myocardial infarction can be accurately determined on contrast-enhanced MRI [62]. Delayed contrast-enhanced MRI can show a pattern of subendocardial late enhancement and sparing of the subepicardial layer in patients with an absence of Q waves on ECG. Option B, which is true, is not the best response.
The spatial resolution of delayed contrast-enhanced MRI (e.g., 1.4x1.9x6 mm) is approximately 60-fold greater than that currently available with SPECT. Although SPECT and delayed contrast-enhanced MRI detect transmural myocardial
infarcts at similar rates, delayed contrast-enhanced MRI systematically detects subendocardial infarcts that are missed by SPECT [63]. Option C, which is true, is not the best response.
Ventricular thrombus formation is a potentially dangerous complication in patients with ischemic heart disease. With contrast-enhanced MRI, thrombi appear as black, well-defined structures surrounded by bright contrast-enhanced blood. Contrast-enhanced MRI shows significantly more ventricular thrombi in ischemic heart disease than do transthoracic echocardiography and cine MRI alone [64]. The technique is particularly superior for detecting small mural thrombi or thrombi trapped in endocardial trabeculations. Option D, which is true, is not the best response.
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Conclusion
This older patient with arrhythmia was referred for cardiac catheterization
because of a history of coronary artery disease and positive findings on
delayed contrast-enhanced MRI. Coronary angiography revealed complete
occlusion of the right coronary artery. Angioplasty or surgery was not
performed because of total infarction of the right coronary artery territorial
supply. A pacemaker was implanted.
Scenario 5
Clinical History
A 68-year-old man with suspected coronary artery disease presents with
atypical chest pain and hyperlipidemia. Echocardiography is technically
unsatisfactory so the patient is sent to MRI for adenosine stress perfusion
imaging.
Description of Images
Basal short-axis views from MRI stress perfusion imaging with the patient
at rest (Fig. 5A) and at
stress (Fig. 5B) show two
inducible subendocardial defects of the basal anterior and lateral walls.
Video of first-pass stress perfusion MRI (Fig. S5C)—three short-axis images (apical, mid, and basal)—shows the dynamic of contrast enhancement and washout in the ventricular cavities
| QUESTION 9 Which of the following statements about adenosine stress perfusion MRI is FALSE?
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as well as the normal and ischemic myocardium. Note the persistent subendocardial perfusion defects visible as dark areas in the basal anterior and lateral walls. These images can be viewed in the supplementary material for this article at www.ajronline.org.
Solution to Question 9
Myocardial oxygen demand is greater in the subendocardium than in the
subepicardium, with a greater flow and oxygen extraction in the inner layers.
In addition, the oxygenated blood supply begins in the subepicardium and ends
in the subendocardium. Hence, the subendocardial layers are much more
vulnerable to ischemia. Therefore, myocardial necrosis begins in the inner
layers with variable transmural spread, depending on the severity of the
ischemia [66]. In the cascade
of ischemic myocardial events, subendocardial perfusion defects are the
earliest findings, followed by transmural perfusion defects. Option A,
which is true, is not the best response.
Quantification of myocardial perfusion using PET is useful for the detection and localization of coronary artery disease [67]. Myocardial perfusion MRI has several advantages over PET. Perfusion MRI has higher spatial resolution, does not expose the patient to radiation, and eliminates attenuation problems related to the anatomy, such as overlying breast shadows, an elevated diaphragm, and obesity. The high spatial resolution of MRI allows separate visualization of the endocardial layer of the left ventricle, a distinct advantage over PET [68]. Option B, which is false, is the best response.
Pharmacologic stress is used in studies of myocardial perfusion. Adenosine,
dipyridamole, or dobutamine infusion can induce increased blood flow to the
myocardium by vasodilation or increased oxygen demand
[69]. Maximized blood flow is
needed to accentuate differences in perfusion between myocardial regions
supplied by normal and diseased arteries. With rapid imaging after a
first-pass bolus injection of an MR contrast agent, differences in perfusion
are readily identifiable. The choice of imaging sequences to evaluate
myocardial perfusion include T1-weighted spoiled gradient-echo, echo-planar
imaging, and balanced steady-state free precession sequences
[68,
70]. The most widely used
pharmacologic agent for stress perfusion MRI is adenosine, which causes
vasodilation by activation of the 
2-adrenergic receptors.
Adenosine has a short half-life and a good safety profile; however, minor side
effects such as flushing, warmth, or headache are common. Severe side effects
are rare but include myocardial infarction, high-degree atrioventricular
block, and bronchospasm [71].
A history of second- or third-degree atrioventricular block and the presence
of chronic obstructive pulmonary disease or both are contraindications for
adenosine stress testing. Option C, which is true, is not the best
response.
Cardiac syndrome X is characterized by typical angina, abnormal exercise test results, and normal coronary arteries. Microvascular dysfunction may be a causative factor. In patients with syndrome X, cardiovascular MRI shows subendocardial hypoperfusion during the IV administration of adenosine [72]. Other applications of perfusion MRI include emergency evaluation of chest pain, assessment of myocardial perfusion after coronary revascularization, and the assessment of collateral perfusion in patients with coronary artery disease [73, 74]. Option D, which is true, is not the best response.
The risk to the patient during administration of adenosine is low. Although side effects of a minor nature such as flushing and dyspnea are common, severe side effects such as myocardial infarction, heart block, and bronchospasm are rare, as mentioned previously [71]. Even though the risk is small, continuous monitoring of the heart rate and rhythm during the scan is essential. Emergency precautions—such as having personnel trained to follow evacuation procedures and personnel trained for resuscitation—should be followed. The safety and feasibility of performing adenosine cardiac MRI has been well shown, including using mobile cardiac MRI systems [75]. Option E, which is true, is not the best response.
Conclusion
This patient with atypical chest pain was referred for cardiac
catheterization after positive findings on stress perfusion MRI. Coronary
arteriography showed triple-vessel stenotic coronary artery disease, with the
most significant lesions involving the left anterior descending artery and the
left circumflex coronary artery. The patient underwent coronary bypass
surgery.
Scenario 6
Clinical History
A 60-year-old man with a history of coronary artery disease presents with
occasional mild chest pain 3 years after surgery that placed multiple bypass
grafts (right coronary, obtuse marginal, and left anterior descending
arteries). However, the patient is able to continue an active lifestyle and
exercise regimen. Stress echocardiography showed inducible ischemia in the
territory of the distal left anterior descending artery. CT angiography is
performed to evaluate bypass graft patency.
Description of Images
A 3D volume-rendered image from CT angiography
(Fig. 6A) shows the origin and
course of a left internal mammary graft to the left anterior descending artery
and a saphenous vein graft to the right coronary artery.
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ECG-gated MDCT with submillimeter resolution is emerging as a reliable noninvasive method to evaluate coronary artery bypass grafts. The sensitivity and specificity of 16-MDCT for the detection of graft occlusion using data pooled from multiple studies are 99% and 98%, respectively [77]. Significant graft stenosis without occlusion is detected with slightly less accuracy. MDCT has technical limitations, and portions of grafts may not be ideally displayed. Metallic vascular clips may cause beam-hardening and reconstruction artifacts, limiting the ability to assess portions of bypass grafts. However, many MDCT examinations are completely successful in ruling out significant graft disease, thus sparing the patient the more invasive catheter angiography. These technical limitations are becoming less a problem with improvements in MDCT technology. Option B, which is true, is not the best response.
| QUESTION 10 Which of the following statements regarding postoperative cardiac imaging is FALSE?
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True aneurysms of bypass grafts are unusual but do occur. Pseudoaneurysms at the distal anastomotic site develop early in the clinical course and are more common that true aneurysms of grafts, which occur later because of atherosclerosis [78, 79]. Patients with graft aneurysms can be asymptomatic or can present with chest pain or symptoms resulting from compression of adjacent mediastinal structures. MDCT is a valuable tool for the diagnosis and preoperative localization of aneurysms and pseudoaneurysms [80]. Option C, which is true, is not the best response.
Reoperation after previous coronary artery bypass surgery is a challenge because of the potential for injury to patent coronary grafts, the aorta, or the right ventricle. Dense adhesions often cause distortion of normal anatomy and make safe dissection difficult. Injury to preexisting grafts during sternal reentry is associated with significant mortality and morbidity rates [81]. ECG-gated MDCT with 3D reformatted imaging is superior to chest radiography and catheter angiography for defining the position of patent grafts and vital structures in relation to the sternum [80, 82]. Preoperative mapping with CT of patent coronary grafts and other vital mediastinal structures reduces the morbidity of the reoperation through modification of the surgical approach [82]. Option D, which is true, is not the best response.
Extended coverage of the thorax is needed to evaluate the origin and course of coronary bypass grafts in their entirety. In assessing reoperative cardiac surgery patients with left internal mammary artery (LIMA) or right internal mammary artery (RIMA) grafts, CT is performed from the thoracic inlet to the apex of the heart [80]. Option E, which is true, is not the best response.
Conclusion
This patient with a history of multiple coronary bypasses who has
occasional mild chest pain was found to have an occluded saphenous vein graft
to the obtuse marginal artery. The left internal mammary graft to the left
anterior descending artery and the saphenous vein graft to the right coronary
artery were patent. The risks of surgical revascularization in this relatively
asymptomatic patient were considered to outweigh the benefits. The patient was
advised to continue aggressive medical treatment, including ß-blockers,
aspirin, and lipid-lowering agents. Dietary modification and exercise were
also advised.
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TopAbstractINTRODUCTIONEDUCATIONAL OBJECTIVESScenario 1References |
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