Myocardial blood flow quantification with SPECT and conventional tracers: a critical appraisal

Abstract

Quantification of absolute myocardial blood flow (MBF) and flow reserve (MFR) from rest and stress studies is a valuable technique in cardiac PET since well over two decades using shortand medium-lived tracers such as O-water, Rb-rubidium, N-ammonia or, more recently, F-labeled compounds such as FFlurpiridaz. However, due to complex logistics, their clinical use is limited as compared to the widely used single-photon imaging tracers Tc-labeled or Tl reaching annual world wide examination numbers in the two-digit million range. Actually, as one can see from these sheer numbers, the lack of non-invasive, absolute quantification of MBF did not stop the success of these agents at all. So, why bother with dynamic imaging and kinetic modeling at all? In general, the authors firmly believe that it represents the step from the assessment of a relative distribution of tracer uptake in an image to the physiologically relevant value of absolute myocardial blood flow and flow reserve. Independent of this conviction, other modalities such as MRI and CT try to duplicate this since many years. Finally, even invasive imaging in the cath lab found this to be a powerful method to assess in absolute rather than relative terms the hemodynamic relevance of a stenosis. But also in more subtle alterations of myocardial perfusion, sophisticated quantification allows a more truthful characterization of the functional capabilities of myocardial tissue (diabetes, chronic kidney disease, etc.). So, why did absolute myocardial perfusion assessment with PET did not bypass SPECT already? The answer is rather simple: PET cameras are more complex to design, produce, and operate than SPECT systems, and the same holds true for the production and distribution of the short-lived radiopharmaceuticals. Thus, blood flow quantification with SPECT would be a very welcomed addition to the armamentarium of nuclear cardiology. Unfortunately, due to the rapid kinetics of perfusion agents and the limited temporal resolution of SPECT cameras—which can lead to inconsistent projection data as the tracer distribution changes during the rotation of the heads—the technological hurdle is high. However, early feasibility studies using conventional photo-multiplierbased SPECT systems and recently a new generation of SPECT cameras using solid-state photon detectors showed potential to expand quantification of absolute blood flow and flow reserve also into the single-photon domain. It is worthwhile remembering that myocardial blood flow and flow reserve is a complex issue as illustrated by Johnson and Gould in a recent publication where they suggested a segmentation scheme for a scatter diagram if regional stress flow values are plotted against the flow reserve values. Such a diagram would allow the identification of different tissue types such as ‘‘definitive ischemia’’ or ‘‘moderately ischemia’’ indicating that a binary (‘‘black and white’’) answer does not give a proper picture of the state of the myocardium under investigation. Although such a sophisticated approach is See related article, pp. 1075–1088