PET/CT Imaging Correlations and Outcomes
Abstract & Commentary
By Michael H. Crawford, MD
Source: Schenker MP, et al. Interrelation of coronary calcification, myocardial ischemia, and outcomes in patients with intermediate likelihood of coronary artery disease: a combined position emission tomography/computed tomography study. Circulation. 2008;117:1693-1700.
The introduction of hybrid pet/ct scanners has raised the issue of the benefits of adding CT coronary calcification measurements to stress testing in intermediate risk patients. Thus, Schenker and colleagues from the Brigham and Women's Hospital in Boston assessed the relationship between coronary calcium and stress-induced ischemia in 621 consecutive patients undergoing stress 82-rubidium, PET perfusion imaging, and coronary calcium scoring on a hybrid PET/CT scanner. Intermediate risk patients without known coronary artery disease were being tested for clinical reasons, and the majority had chest pain or dyspnea.
Results: The frequency of an ischemic PET study increased as the coronary calcium score increased. With a calcium score of zero, it was 16%; < 400 27%; 400-999 48%; and > 1000 49%. Thus, the negative predictive value of a calcium score of zero was 84%. Using a calcium score > 400 as the break point in a multivariate model employing clinical characteristics, calcium score was the strongest independent predictor of a positive PET scan for ischemia (OR 2.91, 95% CI 1.9-4.5). The only other significant predictor was sex (OR men vs women = 2.3). After > 500 days of follow-up, 33 deaths and 22 myocardial infarctions were observed. A positive PET predicted these hard events (OR 2.34, CI 1.36-4.03), as did a calcium score > 1000 (OR 1.88, CI 1.01-3.55). Increasing calcium scores predicted events in those with and without myocardial ischemia. Schenker et al concluded that increasing calcium scores generally predict a higher likelihood of discovering PET ischemia, but a score of zero does not completely exclude a positive PET result. Also, increasing calcium scores predict events regardless of the PET results.
Commentary
CT coronary calcium measurements have largely been advocated for low-risk, low pre-test likelihood of disease patients without symptoms, to detect preclinical atherosclerosis so that risk factor reduction can be advised or intensified. In such situations, the negative predictive value of a negative calcium scan can be as high as 99%. There has been interest in moving this test to those with atypical symptoms to quickly rule out coronary artery disease (CAD) as a cause and discharge the patient from the Emergency Department, but this is a slippery slope as this study shows. Once a patient reaches the intermediate risk range, which has been defined as a pre-test likelihood of disease of 15-85%, then this study would suggest that the negative predictive value of calcium scan is only 84%. Thus, in symptomatic patients with some risk factors, a calcium score of zero does not exclude CAD. So is this test worth doing in symptomatic patients?
A strength of this study was that all the patients got PET perfusion scanning at the same time. When the PET results were used to divide the patients into PET positive for ischemia and negative, calcium scans added incremental prediction of events in both groups. Thus, stress perfusion imaging is of little value for detecting atherosclerotic lesions which may rupture in the future. Consequently, Schenker et al tout the benefits of this new hybrid imager for giving you the best of both worlds. However, they do not discuss the costs in terms of charges or the radiation exposure.
The study population differs from most others in the literature because they had an estimated pre-test probability of disease of 60%, which is on the high end of the intermediate group (15-85% likelihood). Mean age was 60; over half had dyslipidemia; three-quarters had hypertension; one-third had a positive family history; one-quarter were diabetic; 15% were smokers; and all were symptomatic. Mean calcium score was 429; 29% had a positive PET scan and, over the 500-day study period, 9% had an event. This raises the question of how calcium scoring would perform in the low-intermediate-risk group (15-50% likelihood). The low-intermediate group is the one that causes problems for physicians. Accordingly, many are pushing for coronary CT angiography for this group. The ACC/AHA guideline viewed CCTA as an alternative to stress testing. Calcium scoring is not currently considered valuable in anyone you believe needs a stress test, and this study does not convince me otherwise. In those with a negative PET stress scan and an intermediate likelihood of disease, I would still intensify or start risk factor control. The only reason I can see to do a coronary calcium score is if the patient was not convinced that they need risk factor control and desired the test.
The introduction of hybrid pet/ct scanners has raised the issue of the benefits of adding CT coronary calcification measurements to stress testing in intermediate risk patients.Subscribe Now for Access
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