Despite an abundance of medical, interventional, and surgical therapies of proven efficacy, cardiac disease remains the leading cause of death for both men and women in the United States. People with acute myocardial infarction who get to the hospital promptly generally survive, but they may be left with a scarred myocardium, setting them up for congestive heart failure. How much of the myocardium is unreactive myocardium, how much is nonviable, and how much is hibernating? Which of these patients might benefit from treatment, and which treatment would be best for a particular patient? Is the heart responding to treatment? What of the persons with no cardiac symptoms or risk factors who suffer myocardial infarction or sudden cardiac death: could they be identified in time to institute preventive treatment? If so, how intensive should that treatment be?
These are the questions today’s cardiac imagers are being called on to answer. The options for evaluation are increasing, and some of the traditional techniques are being supplanted.
Because it requires no radiation, allows ready assessment of motion, and is cheaper, faster, and more convenient than other modalities, ultrasound has long been popular for cardiac studies in patients ranging from fetuses to the elderly.1,2 New developments are increasing its utility. After a long development curve, real-time three-dimensional imaging has become available, promising even faster and more detailed studies and better communication with patients about the nature of their disease.3,4 These images may be ideal for surgical planning and, according to physicians at the Noninvasive Cardiac Imaging Laboratory at the University of Chicago, are “already beginning to have a profound impact on the way we care for patients.”3 Ultrasound contrast agents permit examination of the myocardial microvasculature to assess the viability of akinetic segments and to determine the success of revascularization.5,6 Increasingly, quantitative techniques are being applied to stress echocardiography, reducing the subjectivity that has been problematic, especially in less expert hands.7 Harmonic imaging for analysis of the kinetics of individual segments,8 tissue tracking,9 and color Doppler imaging to measure myocardial strain10 improve evaluation of wall motion abnormalities. Among other applications, transesophageal echocardiography permits better determination of the thromboembolic risks in patients with atrial fibrillation.8 Intracardiac echocardiography is being now studied, with one observer expecting that its use for electrophysiologic and some interventional procedures in patients with congenital heart disease “probably will become standard clinical practice in the near future.”11
The value of nuclear medicine techniques in assessing heart disease is attested to by an abundance of clinical studies and is increasing with the introduction of new radiopharmaceuticals12 and possibly contrast agents that mark areas of hypoxia or apoptosis.13 In 2002, approximately 6 million stress myocardial perfusion SPECT studies were performed in the United States. Today, rest and stress scintigraphy is widely used for risk assessment,14,15 determination of prognosis after myocardial infarction, prediction of the therapeutic benefit of revascularization, identification of areas of ischemia in patients with unstable angina, and follow-up of interventions. The recent ability to measure left ventricular volumes and ejection fraction has now increased the prognostic power of SPECT.16 For patients unable to tolerate sufficient exercise, adenosine stress has become an increasingly valuable tool, recently reported to have excellent prognostic value in a population of more than 5,000 patients.17 In patients who cannot exercise and in whom adenosine is contraindicated because of chronic obstructive pulmonary disease with bronchospasm, pharmacologic stress using dobutamine is available. The overall sensitivity, specificity, and accuracy of dobutamine stress testing in published studies were 85%, 72%, and 83%, respectively.18
|Figure 1. Coronary MR angiogram (a) demonstrating anatomy of the left main, left anterior descending and circumflex coronary arteries. Long axis view of the left ventricle (b) showing an anterior aneurysm with enhancement after gadolinium administration, due to a prior myocardial infarction with scar formation. Images courtesy of David A. Bluemke, MD, PhD, Johns Hopkins Medical Institutions, Baltimore.|
Because of its higher sensitivity and specificity, its demonstration of tissue metabolism, and its ability to quantitate myocardial blood flow, PET with 18F-deoxyglucose is becoming the standard examination for myocardial viability at many institutions,19-21 at least in patients with equivocal ultrasound or scintigraphy studies or those who are candidates for surgery.22
“By its nature, PET has higher resolution than SPECT,” says Daniel Berman, MD, director of cardiac imaging, Cedars-Sinai Medical Center, Los Angeles, and professor of medicine at UCLA School of Medicine. “A PET scan done at rest and for blood flow measurements under dipyridamole or adenosine stress is likely to be more accurate in detecting significant coronary disease than is stress echocardiography or stress SPECT.”
CT Comes on
Computed tomography is a relative newcomer to cardiac imaging. If the heart rate exceeds 65 bpm, the 300-msec acquisition time of earlier scanners is too slow for applications such as angiography, given that the rest period of the right and left coronary arteries is as short as 66 msec. However, new scanners, with acquisition times of 50 msec, are capable of coronary angiography except in the presence of large calcium burdens. The introduction of 16-slice CT scanners has improved cardiac capabilities further. Among the emerging possibilities are virtual angioscopy and integrated evaluation of ischemic heart disease with coregistered displays of CT and MR images.23
An early application of coronary CT is still controversial, namely measuring calcium as a means of detecting coronary atherosclerosis.
“Scanning for coronary artery calcification, either by the latest-generation multislice CT scanner or by electron beam CT, is accurate,” Berman says. “The presence of calcium means atherosclerosis almost 100% of the time. Catheter angiography, the traditional means of detecting coronary atherosclerosis, produces only a luminogram, and recent work has clearly shown that plaque buildup on the outer portion of the vessel can be significant without any luminal narrowing. Thus, a normal coronary angiogram does not prove a patient is not at risk for cardiac events, whereas the number of patients who die of heart disease with normal calcium scans is extremely small.”
Calcium scoring is effective in selecting patients for medical management. However, in patients with high scores, it is not clear whether there is need for further risk stratification to determine whether a patient needs bypass surgery or angioplasty.
“Calcium scans, nuclear medicine, and echocardiography are emerging as gateways to the catheterization laboratory,” Berman reports. “If an asymptomatic patient has an abnormal calcium scan and a normal nuclear medicine test, he or she probably needs intensive medical therapy and does not need an angiogram.”
A report at this year’s Society of Interventional Radiology meeting suggests another way in which calcium scanning may be useful. Michael F. Mastromatteo, MD, and colleagues at Beth Israel Deaconess Medical Center in Boston, surveyed patients after they were found to have calcium scores above the 75th percentile. Three quarters of them reported obtaining additional cardiac testing and making changes in their diet and exercise, while the percentage receiving statin drugs more than doubled.24
MRI Has arrived
The biggest news in cardiac imaging unquestionably is the expanding utility of MRI, which depicts structure, function, perfusion, and viability “with an overall capacity unmatched by any other single modality.”25 Ventricular chamber volume, filling velocity and volume, and myocardial strain can all be analyzed.26 An early study from the National Heart, Lung and Blood Institute suggests that resting cardiac MRI is suitable for triage of patients admitted to the emergency department with chest pain,27 and contrast-enhanced MRI is superior to SPECT in detecting submyocardial infarcts.28 Intracellular contrast media and media specific for necrosis are entering clinical trials.29
The greater capabilities of MR suggest it will displace some traditional studies.
“A PET study of myocardial viability takes about 4 hours,” notes David A. Bluemke, MD, PhD, clinical director of MRI and associate professor of radiology at the Johns Hopkins Medical Institutions. “MR obtains the same information in 30 minutes with higher resolution, and there is no need for physical or pharmacologic stress. In fact, some studies suggest that MR is substantially better at detecting nonviable areas than are PET and thallium scintigraphy. Also, with MRI, you do not have radioisotopes.”
Although at present, MR studies are not possible in patients with a pacemaker or an implanted defibrillator, some pacemaker manufacturers have developed devices that are MR compatible, and Bluemke expects that this contraindication will become less common over the next 3 to 5 years.
A longstanding goal of cardiovascular imaging is the noninvasive assessment of plaque. Available CT methods depict only calcium, whereas MRI may permit direct examination of noncalcified plaque as well. A study from Johns Hopkins Medical Institutions that was presented at the annual meeting of the American Heart Association last fall confirmed a reduction of the aortic plaque burden in patients receiving statin therapy as measured by surface and transesophageal MRI.30
“In the past, people did those studies with coronary angiography,” Bluemke comments, “but that may well not show any difference if all the change happens outside the vessel lumen. If these results of the trials using MRI are replicated in larger studies, it could have a major impact on heart disease treatment.”
There also is hope that MRI will permit detection of vulnerable plaque, but at the moment, that is still an investigational use. Bluemke and colleagues at six medical centers are in the early stages of a study of 1,000 clinically normal individuals that will assess the ability of MR to relate the appearance of plaque directly to factors such as cholesterol, coronary calcium, and genetics and serologic evidence of inflammation.
Berman also sees a great potential for MRI. “Cardiac MRI has the potential to provide a dramatic change in the quality of the information physicians have about the heart,” he says. “After a patient has a myocardial infarction, the myocardium concentrates gadolinium differently from normal myocardium, and delayed enhancement seen on a contrast-enhanced scan is the most accurate test currently available to define myocardial scarring. MRI also can give you rest and stress perfusion studies and provides the most accurate measurements of ventricular function and myocardial mass. It may be the first means of showing stress-induced subendocardial ischemia. When MRI angiography is sufficiently refined, this modality is going to be a one-stop shop for cardiac imaging.”
The days when echocardiography, scintigraphy, and angiography were all that cardiac imagers needed are clearly gone. As Berman says, “Yes, we have those modalities. But we have a cardiac CT scanner, and we have also purchased an MRI scanner specifically for cardiac use.” Clearly, such capability will soon be commonplace.
Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.
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- Lang R, Sugeng L. A fantastic journey: 3D cardiac ultrasound goes live. Radiol Manage. 2002;24:18?22.
- Marx GR, Sherwood MC. Three-dimensional echocardiography in congenital heart disease: a continuum of unfulfilled promises? No. A presently clinically applicable technology with an important future? Yes. Pediatr Cardiol. 2002;23:266?285.
- Villanueva FS. Myocardial contrast echocardiography in acute myocardial infarction. Am J Cardiol. 2002;90(suppl 10A):38J?47J.
- Zoghbi WA. Evaluation of myocardial viability with contrast echocardiography. Am J Cardiol. 2002;90(suppl 10A):65J?71J.
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- Borges AC, Kivelitz D, Walde T, et al. Apical tissue tracking echocardiography for characterization of regional left ventricular function: comparison with magnetic resonance imaging in patients after myocardial infarction. J Am Soc Echocardiogr. 2003;16:254-262.
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- Arano Y. Recent advances in 99mTc radiopharmaceuticals. Ann Nucl Med. 2002;16:79?93.
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- Hachamovitch R, Berman DS, Shaw LJ.Incremental prognostic value of myocardial perfusion single photon emission computed tomography for the prediction of cardiac death: differential stratification for risk of cardiac death and myocardial infarction. Circulation. 1998;97:535-43.
- Hachamovitch R, Hayes S, Friedman JD, et al. Determinants of risk and its temporal variation in patients with normal stress myocardial perfusion scans. What is the warranty period of a normal scan? J Am Coll Cardiol. 2003;41:1329-40.
- Sharir T, Berman DS, Waechter PB et al. Quantitative analysis of regional motion and thickening by gated myocardial perfusion SPECT: normal heterogeneity and criteria for abnormality. J Nucl Med. 2001;42:1630-8.
- Berman DS, Kang X, Hayes SW, et al. Adenosine myocardial perfusion single-photon emission computed tomography in women compared with men. Impact of diabetes mellitus on incremental prognostic value and effect on patient management. J Am Coll Cardiol. 2003;41:1125-33.
- Elhendy A, Bax JJ, Poldermans D. Dobutamine stress myocardial perfusion imaging in coronary artery disease. J Nucl Med. 2002;43:1634?1646.
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- Mastromatteo MF, Welty F, Clouse ME. Coronary artery calcium scores greater than 75th percentile: follow-up indicates significant lifestyle changes [abstract 135]. Presented at: Society of Interventional Radiology 28th Annual Meeting; March 27-April 1, 2003; Salt Lake City.
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- Paelinck BP, Lamb HJ, Bax JJ, van der Wall EE, de Roos A. Assessment of diastolic function by cardiovascular magnetic resonance. Am Heart J. 2002;144:198?205.
- Kwong RY, Schussheim AE, Rekhraj S, et al. Detecting acute coronary syndrome in the emergency department with cardiac magnetic resonance imaging. Circulation. 2003;107:531?537.
- Wagner A, Mahrholdt H, Holly TA, et al. Contrast-enhanced MRI and routine single photon emission computed tomography (SPECT) perfusion imaging for detection of subendocardial infarcts: an imaging study [see comment]. Lancet. 2003;361:374?379.
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- Warren WP, Gautam S, Lima JA. Simvastatin reduces aortic atherosclerotic plaque volume measured by combined transesophageal and surface MRI in patients with documented cardiovascular disease [abstract 2587]. Presented at: Annual Scientific Sessions, American Heart Association; November 17-20, 2002; Chicago.