ADVANCES IN NONNUCLEAR IMAGING TECHNOLOGIES/CME ARTICLE The incremental value of coronary artery calcium scores to myocardial single photon emission computer tomography in risk assessment

For more than two decades functional imaging with myocardial perfusion single photon emission computer tomography (SPECT) has been generally accepted as a powerful tool for risk stratification in patients with suspected or known coronary artery disease. The visualizing of myocardial perfusion through myocardial SPECT is informative about hemodynamic changes in every stage of atherosclerosis, including early endothelial dysfunction, intramyocardial microvessel disease, and intermediate or high grade coronary artery stenoses. Furthermore, non-atherosclerotic changes in myocardial perfusion such as the occurrence of left bundle blockinduced left ventricular dyssynchrony, constrictive pericarditis, or autoimmune diseases can all be detected with myocardial SPECT. There is a vast literature indicating that the absence of perfusion abnormalities to myocardial SPECT predicts very low combined mortality, and nonfatal infarction rate of only about 1% per year, thus sparing many symptomatic patients the need for more invasive evaluation of their myocardial function. Otherwise, in those patients with myocardial perfusion abnormalities, individual risk increases with the size of the defect, irrespective of the nature of underlying structural or functional changes causing these perfusion defects. During the past decade, functional SPECT imaging has been augmented by the advent of CT techniques, which enable the exact and highly reproducible measurement of coronary vascular calcium burden. This structural finding marks the presence and extent of coronary atherosclerotic changes, at least as a later disease stage, characterized by the subsidence of inflammatory processes. Thus, optimal diagnostic information might be afforded by perfusion SPECT augmented by CT. In contrast to CT, perfusion SPECT delivers mainly functional information, and only scant structural detail. Consequently, the presence or type of atherosclerotic vessel wall changes and the stage of disease per se cannot be concluded at the basis of SPECT examination alone. In a clinical setting, the requirement for more detailed anatomical information is frequently met through additional diagnostic methods such as invasive coronary angiography, particularly as required for the planning and guiding of therapy. But even in patients without perfusion defects to SPECT, the occurrence of high atherosclerotic burden, reflecting an advanced stage of disease, could potentially influence therapy decisions. To date, it is not completely clear how CT-based morphological changes might independently influence patients’ individual risk. The present review summarizes the state-of-the-art for quantification of coronary artery calcium scoring, along with an account of the strengths and weaknesses of coronary calcium scoring for risk stratification in different clinical settings. We place particular emphasis on the potential incremental value of the CT technique as compared to myocardial perfusion imaging alone.