PET/CT powers MPI improvements

There are several advantages to PET/CT for myocardial perfusion imaging (MPI) including upgrades in quantitative analysis and biomarker technology that could provide a more complete evaluation of cardiovascular disease, according to a review published in the Journal of Nuclear Cardiology.

Amit Pursnani, MD, from Massachusetts General Hospital and Harvard Medical School in Boston, and colleagues explored the value of PET/CT in applications of MPI. In contrast with conventional SPECT imaging, PET/CT MPI is associated with a lower radiation dose and provides opportunities for quantitative analysis, including calculations of absolute coronary flow reserve (CFR) and myocardial blood flow (MBF) made possible with developments using rubidium-82.

“There are several clinical scenarios in which assessment of absolute MBF or CFR evaluation may be particularly useful,” wrote Pursnani et al. “For example, in patients with multi-vessel CAD, there is potential for underestimation of ischemia using qualitative MPI assessment, as qualitative analysis of myocardial perfusion images relies on identification of relative differences in blood flow from rest to stress perfusion. Therefore, only the areas of most significant relative coronary flow impairment are visually apparent. Measurement of absolute MBF or CFR limits the risk of underestimating disease severity, as areas with low MBF or CFR will be identified as being abnormal regardless of the flow in other myocardial regions.”

Quantitative measurement of CFR with PET/CT could be especially beneficial in patients with diabetes with no prior history or suspected CAD and others with small- or multi-vessel coronary disease. Reduced CFR with PET has been shown to be an independent predictor of future cardiac events.

“The detection of reduced CFR in either of these scenarios could result in a change in clinical management,” wrote the authors. “In the patient with probable multi-vessel CAD, there is a lower threshold for consideration of invasive catheterization for confirmation of diagnosis and subsequent revascularization. In the latter case, aggressive risk factor modification and/or medical therapy should be contemplated.”

Additionally, MPI with PET/CT has been found to accurately assess myocardial ischemia and left ventricular function. Technological advancements in PET/CT imaging have made PET MPI attenuation correction possible using CT anatomical mapping.

“Although hybrid scanners were originally developed for CT attenuation correction and oncologic applications, there has been great interest in establishing whether non-contrast CT acquisition of calcium scoring data (which can be readily obtained during the same patient sitting) has an incremental benefit in risk stratification or cardiac event prediction,” the researchers wrote.

Coronary artery calcium scoring is considered a “powerful” indicator of intermediate cardiac risk. Elevated coronary calcium score in asymptomatic patients has been associated with successful implementation of preventative medical treatments and improved patient outcomes. In one study of 368 low to intermediate risk patients, only one patient was shown to have acute coronary syndrome with zero calcium score. Thoracic aortic calcium scoring also may provide added information about cardiac risk, especially in women.

A recent retrospective analysis of quantitative CFR, coronary and thoracic aortic calcium scoring using Rb-82 PET/CT MPI in 75 patients indicated that coronary calcium score directly correlated with thoracic aortic calcium score, and that there was an inverse relationship between these scores and CFR, which was the strongest predictor of ischemia. While sensitivity and specificity were found to be “modest,” independent negative predictive value was evaluated at 96 percent for arterial calcium scoring, 94 percent for thoracic aortic calcium scoring and 95 percent for CFR.

Further research, including large-scale comparative studies, are needed to evaluate the benefits of simultaneous PET/CT MPI for the detection of stenosis. “Moreover, the question remains whether a hybrid approach combining anatomic and functional parameters actually improves patient management and results in reduction of downstream cardiovascular events,” wrote the authors.