Adding calcium scoring to myocardial perfusion imaging: Does it alter physicians’ therapeutic decision making?

Abstract

While the paucity of studies relating cardiovascular imaging to patient outcomes has drawn much attention, the complex nature of cardiovascular disease makes it extremely hard to study how a test performed today can lead to better outcomes in the future. Realizing this, a simpler question is whether imaging testing leads to changes in patient or physician behavior. In other words, does the test lead to initiation of new therapies that would not have otherwise been considered? Among symptomatic patients with known or suspected coronary artery disease (CAD), myocardial perfusion imaging (MPI) provides important information about the presence and extent of ischemia or infarction and can be used to predict the presence of hemodynamically significant obstructive CAD. This information can then be used in deciding between medical therapies and revascularization. Positron emission tomography (PET) is a superb technique to assess myocardial perfusion. While PET has many advantages over SPECT techniques, such as improved image resolution, robust attenuation correction and ability to measure absolute myocardial blood flow, it is limited in its ability to identify pre-clinical atherosclerosis and severe multi-vessel atherosclerosis (balanced ischemia). On the other hand, coronary artery calcium (CAC) scoring identifies whether coronary calcifications are present or absent, and reliably quantifies the burden of calcified coronary atherosclerosis (though not necessarily representing obstructive CAD). The exam is simple, does not require intravenous contrast and can be performed with a relatively low radiation dose (*1-2 mSv) on the majority of CT scanners as well as on hybrid SPECT/CT and PET/CT platforms. In patients undergoing MPI, CAC scoring can be used to better define the presence or absence of calcified atherosclerosis, thereby resulting in improved risk assessment. In highlighting the complementary information provided by these tests, several recent studies have showed that approximately 21-47% of patients with normal MPI have extensive coronary calcifications (i.e. CAC [ 400). Among patients who are otherwise considered low to intermediate risk, such findings could be used to reclassify their risk, thereby indicating that more aggressive therapies should be considered. The increased availability of hybrid SPECT/CT and PET/CT platforms has facilitated the acquisition of CAC score together with MPI during a single setting. One of the arguments for such an approach is that the identification of CAC can be used to identify the presence and extent of atherosclerosis. On the other hand, when extensive calcifications are found, perfusion imaging can be used to determine the physiologic significance of plaque. However, in contrast to screening cohorts undergoing CAC scoring, patient cohorts undergoing combined MPI and CAC at the same setting have a higher pretest likelihood of CAD as they are more likely to be symptomatic patients referred for an evaluation of ischemia. Patients referred for PET MPI may have an even higher risk, due to their reduced functional capacity and/or presence of obesity. However, even in such patient populations, the presence of severe calcified coronary atherosclerosis provides substantial incremental prognostic value. Therefore, the logical next question is whether the improved diagnosis of coronary atherosclerosis by CAC score leads to changes in From the Noninvasive Cardiovascular Imaging Program, Departments of Medicine (Cardiovascular Division) and Radiology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA. Reprint requests: Sharmila Dorbala, MD, Noninvasive Cardiovascular Imaging Program, Departments of Medicine (Cardiovascular Division) and Radiology, Brigham and Women’s Hospital, Harvard Medical School, 70 Francis Street, Boston, MA 02115; sdorbala@ partners.org. J Nucl Cardiol 2010;17:168–71. 1071-3581/$34.00 Copyright 2009 by the American Society of Nuclear Cardiology. doi:10.1007/s12350-009-9185-7