Stress/rest myocardial perfusion imaging (MPI) is commonly used for the diagnostic assessment of patients with suspected coronary artery disease (CAD) and for aiding clinical management decisions, such as the need for myocardial revascularization versus medical therapy. The latter use is facilitated by the well-documented relationship between the extent and severity of stress-induced myocardial perfusion abnormalities and the risk for subsequent adverse cardiac events. Despite these utilities, stress/rest MPI imaging has an important limitation: while myocardial perfusion abnormalities are useful for assessing the hemodynamic significance of obstructive coronary stenosis, many patients with normal stress/rest MPI studies have a substantial magnitude of subclinical coronary atherosclerosis, which cannot be detected because they are not flow-limiting in magnitude.1,2 Coronary artery calcium (CAC) scanning is a complementary test that can detect subclinical atherosclerosis.
Over the years, CAC has been primarily used for screening purposes among asymptomatic individuals with risk factors for CAD. However, there is now growing recognition that CAC scanning could also serve as a clinical aid for improving the work-up and management of patients who are candidates for stress/rest MPI.3,4
A number of features make CAC scanning make it an appealing test. The presence of CAC is a specific marker of CAD in an individual patient. Even very small amounts of CAC, with scores ranging 1–10, places patients at increased risk compared to patients with normal CAC scans.5,6 Patho-anatomic studies conducted by Rumberger et al in the 1990s demonstrated a proportional relationship between the amount of CAC and total atherosclerotic burden.7 Since then, hundreds of outcome studies have consistently demonstrated a strong proportional relationship between the magnitude of CAC abnormality and the frequency of subsequent cardiac events. Most of these studies have been performed in asymptomatic populations. However, more recent studies have also demonstrated a similar proportional relationship between CAC scores and adverse outcomes among patients with chest pain symptoms. For example, in the recent Prospective Multicenter Imaging Study for Evaluation of Chest Pain (PROMISE) trial, a proportional relationship was also noted between the degree of CAC abnormality and the frequency of subsequent clinical events among patients with chest pain symptoms.8
For these reasons, there has been growing interest in evaluating the combined use of stress/rest MPI and CAC scanning for diagnostic and prognostic assessment. MPI using positron emission tomography (PET) is particularly well suited for this use because most PET scanners used for cardiac imaging today are hybrid PET/CT scanners, permitting both the anatomic assessment of coronary atherosclerosis and functional assessment of myocardial perfusion in a single study. In addition, hybrid SPECT/CT camera systems are now also increasingly available, although they presently constitute only a small percentage of the SPECT-MPI scanners. At the time of hybrid PET or SPECT imaging, the concomitant low dose non-ECG-gated CT scan is used to perform attenuation correction for the radionuclide images. This permits visual estimation of CAC ...