Ultrasound, EBCT, and MRI compared
All have predictive value for stroke, heart disease
At least two major studies are under way comparing electron-beam computed tomography (EBCT) and carotid ultrasound for similarities and differences in their respective predictive powers. (See article on using carotid ultrasound to forecast stroke and heart attack risk, p. 42.)
There are a few obvious differences. An EBCT scan can cost the patient a total of $400 while ultrasound costs about half that. While ultrasonography uses ultrasound, EBCT uses X-ray. Ultrasonography is the cheaper technology, EBCT looks directly at the heart, and ultrasound examines the carotid arteries.
The predictive abilities of magnetic resonance imaging (MRI) are also under investigation. One study is tracking 5,000 individuals over 10 years with all three modalities, MRI, ultrasound, and EBCT, and studying functionality. "That’s a twist on the ultrasound," says Daniel O’Leary, MD, radiologist-in-chief at New England Medical Center in Boston. "Usually you look at the carotid wall and determine if it’s been injured and thickened. But you can also use ultrasound to look at the wall as it functions, looking at the elasticity of the vessel."
The fact that MRI can detect the nature of atherosclerotic plaque makes that technology especially valuable for prediction, although the imaging is expensive. (See chart comparing costs, below.)
| Compared Costs of Diagnostic Tests | |
| Noninvasive tests | |
| EBCT |
|
| MRI |
|
| Ultrasound |
|
| Stress EKG |
|
| Stress Echo |
|
| Thallium Stress |
|
| PET Scan |
|
| _____________________________________ | |
At the 71st scientific sessions of the American Heart Association last fall in Dallas, researchers presented findings on extracted plaques obtained from 19 patients with coronary artery disease (CAD). Plaques with a high-lipid content and ". . . with the consistency of peanut butter . . ." are more likely to rupture than harder, more calcified plaques, said the researchers, and MRI can detect the difference between the two consistencies, therefore aiding in risk stratification of patients.
"X-ray angiograms only detect lumen blockage," one study presenter points out. Vessels that don’t completely stenose may be more dangerous than those that completely block blood flow, he says, suggesting that intravascular imaging be performed just prior to angioplasty or thrombolysis to assess risk of plaque rupture.
EBCT can screen for CAD as well by detecting calcified plaque. The amount of calcium in the coronary arteries predicts the amount of plaque and blockage. (See February 1998 Cost Management in Cardiac Care, p. 15, for three EBCT images showing no, moderate, and severe calcification of the carotid artery.)
A recent report demonstrates the accuracy of EBCT compares well with that of angiography.1 Researchers performed EBCT in 125 patients who were scheduled to undergo angiography. Compared with that modality, the sensitivity of EBCT in detecting high-grade or complete stenoses was 92%, its specificity was 94%, its positive predictive value was 78%, and its negative predictive value 98%.
The team suggests EBCT may become a useful tool to rule out high-grade coronary artery stenoses or occlusions in, for example, patients with a low likelihood of obstructive disease, and those who have undergone balloon angioplasty or bypass grafting. In addition to the lower cost of an EBCT scan, it takes about five minutes to complete. By contrast, an angiogram can cost from $3,000 to $5,000, depending upon conditions.
There are possible disadvantages of using EBCT to screen for CAD, notes an editorial near the investigators’ report. There is the potential for patients with positive results to seek "a gathering snowball" of follow-up tests and the potential for patients with negative results to become complacent and even ignore symptoms of ischemia.
Cardiologists from the Mayo Clinic in Rochester, MN, have come up with an algorithm for the identification of severe CAD in symptomatic patients, which would greatly facilitate clinical triage in patients with no previous diagnosis of disease.2 The cornerstone of the algorithm is the combination of certain risk factor and EBCT calcium scores. "Patients with low scores on our index are unlikely to have severe coronary disease," reported one of the researchers, "while patients with high scores are more likely to have severe disease and should probably undergo further tests."
The investigators studied just under 300 patients with suspected CAD who underwent angiography as well as EBCT and concluded that calcium scanning in conjunction with risk factor assessment can be used to rule in or rule out angiographically severe disease in symptomatic patients. It may also, they said, facilitate individual decision-making concerning the urgency of coronary catheterization.
References
1. Achenbach S, Moshage W, Ropers D, et al. Value of electron-beam computed tomography for the noninvasive detection of high-grade coronary-artery stenoses and occlusions. N Engl J Med 1998; 339:1,964-1,971.
2. Schmermund A, Bailey KR, Rumberger JA, et al. An algorithm for noninvasive identification of angiographic three-vessel and/or left main coronary artery disease in symptomatic patients on the basis of cardiac risk and electron-beam computed tomographic calcium scores. J Am Coll Cardiol 1999; 33:444-452.
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