Excerpt

Myocardial perfusion imaging (MPI) using single-photon emission computed tomography (SPECT) has long been the most common non-invasive test for management of patients with known or suspected coronary artery disease (CAD). The diagnostic and prognostic applications of SPECT MPI are traditionally based on visual and semi-quantitative assessment of the extent and severity of perfusion abnormalities, besides functional variables.1 However, standard semi-quantitative evaluation of myocardial perfusion is unable to assess subtle and/or diffuse alterations of myocardial blood flow (MBF) regulation. Myocardial flow reserve (MFR), obtained by the ratio between hyperemic and baseline MBF, is a well-validated index for the assessment of blood flow impairment, integrating the hemodynamic effects of epicardial coronary stenosis, diffuse atherosclerosis, and microvascular dysfunction.2-4 Cardiac position emission tomography (PET)/computed tomography (CT) imaging allows absolute quantification of MBF and MFR over a wide range of perfusion tracers in animal models and humans. A preserved MFR is associated with low probability of coronary atherosclerosis and low risk of cardiac events; on the contrary, a reduced MFR is a significant marker that can help in stratifying CAD risk.5-8 However, its diffuse application in the clinical scenario is limited by the technical complexity and elevated operative costs.9 Previous studies investigated the feasibility of MFR quantification using dynamic scintigraphy and ^99mTc tracers in humans.10-14 Nevertheless, the dynamic collection of tomographic data, necessary to obtain MBF and MFR accurate measurements, is limited using conventional SPECT systems. The poor temporal resolution of conventional scintillation crystals does not allow to collect an adequate number of counts during changes in radiotracer concentration. In addition, varying angular projections and the slow rotation of the gantry limit the collection of dynamic data. Recent advances in SPECT technology, related to the introduction of newly dedicated cardiac cameras equipped with cadmium-zinc-telluride (CZT) detectors and software-based resolution recovery, might overcome most of the limitation of traditional systems. In CZT cameras, spatial and temporal resolution are greatly improved by performance of detectors and count density, respectively.15 Ben-Heim et al16 first evaluated the feasibility of dynamic tomographic imaging for quantification of regional and global MFR in humans, using a CZT camera (D-SPECT, Spectrum Dynamics, Israel). Ninety-five consecutive patients were evaluated by dynamic CZT SPECT imaging with ^99mTc-sestamibi at rest and at peak vasodilator stress. The dynamic images were reconstructed, and a 2-compartment kinetic model was applied to calculate global and regional MFR index. Global MFR index was found to be higher in patients with normal imaging results than in those with abnormal MPI. Moreover, in 16 patients undergoing invasive coronary angiography, MFR index was also found to be lower in vascular territories supplied by obstructed arteries, showing a stepwise reduction with increasing extent of obstructive CAD.16 …