Excerpt

In the last decades, the clinical characteristics of the patients that have been investigated by myocardial perfusion imaging (MPI) have changed dramatically, with a progressive reduction of the average cardiovascular risk and of the pre-test probability of coronary artery disease (CAD).1 These data have been recently confirmed by the results of some accurate retrospective analyses coming from leading institutions in the field of cardiac imaging, showing that in the current clinical practice the majority of patients undergoing nuclear cardiac imaging with single-photon emission computed tomography (SPECT) mainly report atypical symptoms and show a strikingly lower evidence of positive test results than in the past.2 Interestingly, those data were independent of the patients’ clinical profile, with a disturbingly high prevalence of negative test results both in subjects with suspected and known CAD.2,3 Considering the strict interconnection between the accuracy of any ischemia test and the underlying prevalence of the disease, it is not surprising that the diagnostic capability of “standard” cardiac imaging methodologies has been decreasing substantially as compared to classical data. Those findings have been further confirmed by a comparative prospective cardiac imaging study for the diagnosis of patients with suspected CAD that demonstrated clearly that the accuracy of the major non-invasive functional imaging tests is, at best, in the range of 70%, a rather modest result that can barely influence patients’ management.4 Among the major non-invasive myocardial imaging strategies, conventional SPECT seems to suffer relatively more from the changing of the patients’ population, with relatively lower values of diagnostic accuracy than some other “competitors.”5,6 As an example, as reported in the recent “Cardiovascular magnetic resonance and single-photon emission computed tomography for diagnosis of coronary heart disease (CE-MARC)” trial, the sensitivity of MPI performed with traditional SPECT cameras in detecting obstructive CAD is significantly lower than that of cardiac magnetic resonance,5 although the clinical impact of such a difference is still disputed.7 Accordingly, an evolution of SPECT technology was urgently needed to maintain its long-standing status as an accurate and trusted strategy for the evaluation of CAD patients. The introduction of dedicated cardiac SPECT cameras equipped with cadmium-zinc-telluride (CZT) detectors has revolutionized nuclear cardiac imaging, as a technology that could allow a more accurate and faster evaluation of regional myocardial perfusion than standard Anger cameras.8 As a matter of fact, an increasing number of reports has demonstrated repeatedly that the increased photon sensitivity and spatial resolution of CZT pixelated crystals might overcome most of the technical limitation sodium-iodine (Na-I)-based devices, with superior diagnostic performances even in the most complex patients’ categories, such as those with multivessel CAD.9,10 Moreover, CZT technology might allow investigating what is commonly regarded as the “holy Grail” of coronary pathophysiology, namely absolute myocardial blood flow (MBF) and myocardial perfusion reserve (MPR).11,12 In fact, absolute assessment of MBF is difficult with standard SPECT cameras, mainly because of the inadequate properties of the currently available flow tracers that show a non-linear extraction fraction at higher flow rates, coupled with the unfavorable physical properties of the Na-I crystals that typically saturate at higher count rates. CZT cameras allow image acquisition in a non-rotating mode and therefore acquisition of time-activity curves, which entail the computation of absolute myocardial perfusion parameters.12,13 As shown by a wide number of reports, the computation of myocardial perfusion parameters allows obtaining some of the most accurate predictors of CAD, possibly individuating even subtle alterations of myocardial perfusion heterogeneity—as a possible result of sub-clinical and/or diffuse CAD, endothelial/microvascular dysfunction—that would be unapparent with a qualitative evaluation.13,14 …