Absolute Myocardial Blood Flow and Flow Reserve Assessed by Gated SPECT with Cadmium–Zinc–Telluride Detectors Using ^99mTc-Tetrofosmin: Head-to-Head Comparison with ¹³N-Ammonia PET

Abstract

Recent advances in SPECT technology including cadmium-zinctelluride (CZT) semiconductor detector material may pave the way for absolute myocardial blood flow (MBF) measurements by SPECT. The aim of the present study was to compare K 1 uptake rate constants as surrogates of absolute MBF and myocardial flow reserve index (MFRi) in humans as assessed with a CZT SPECT camera versus PET. Methods: Absolute MBF was assessed in 28 consecutive patients undergoing adenosine stress-rest myocardial perfusion imaging (MPI) by 99m Tc-tetrofosmin CZT SPECT and 13 N-ammonia PET, and MFR was calculated as a ratio of hyperemic over resting MBF. Results from both MPI methods were compared, and correlation coefficients were calculated. The diagnostic accuracy of CZT MFRi to predict an abnormal MFR defined as PET MFR less than 2 was assessed using a receiver-operator-characteristic curve. The definition of coronary artery disease (CAD) was originally based on the presence of anatomic luminal narrowing greater than 50%. Over the past decades, however, many advances in imaging techniques have enhanced our pathophysiologic understanding of CAD. It has been recognized that many factors beyond luminal narrowing may determine whether or not an anatomic coronary lesion induces ischemia. Because this cannot be comprehensively addressed by coronary angiography alone, nuclear myocardial perfusion imaging (MPI) has been well established as a tool for providing proof of ischemia. This is mandatory for prognostically relevant target lesion revascularization in chronic stable CAD. Because revascularization of a non-flow-limiting coronary stenosis is not beneficial to the patient, neither from a prognostic nor from a symptomatic point of view, nuclear MPI has been suggested as a gatekeeper for invasive coronary angiography (1-3). MPI studies with PET have demonstrated that absolute myocardial blood flow (MBF) and flow reserve (MFR) provide incremental diagnostic (4) and prognostic (5) information over relative perfusion alone. Quantitative assessment of MBF is difficult with standard SPECT for which detectors need to rotate around the patient and for which unfavorable properties of the currently available flow tracers hamper accurate MBF quantification because of nonlinear extraction fraction with roll-off at higher flow values. The recent advances in SPECT technology including semiconductor detector material (cadmium zinc telluride [CZT]) allow acquisition in a nonrotating mode and therefore acquisition of time-activity curves. This may enable absolute MBF and MFR estimation by SPECT as recently shown in experimental animal models (6), which may pave the way for its implementation in clinical routine (7). More knowledge on the impact of the tracer properties on the values obtained from quantification is crucial.The aim of the present study was a head-to-head comparison in humans of indices of absolute MBF and MFR as assessed with 99m Tc-tetrofosmin on a CZT SPECT camera versus MBF and MFR measured with 13 N-ammonia on a PET scanner. MATERIALS AND...

Single Photon Emission Computed Tomography (SPECT) Myocardial Perfusion Imaging Guidelines: Instrumentation, Acquisition, Processing, and Interpretation

“…Data are acquired in a list mode enabling dynamic SPECT radiotracer myocardial blood flow imaging in a tomographic mode. 16,17 Indirect photon detection with solid-state photodiodes (Cardius, Digirad, Suwanee, GA). Another approach is to couple pixelated scintillation crystals to solid-state photodiodes instead of PMTs.…”

“…The feasibility of this approach has been demonstrated in a large animal model, 155 but at this time SPECT MBF and MFR measurements in humans are very new. While the results have been very encouraging, investigations with both conventional rotating systems 156,157 and non-rotating cardiac cameras 15,17,158,159 have been limited to single-center feasibility studies. While the benefits of PET-based MBF/MFR are clear, [160][161][162][163] no published study has currently evaluated the incremental diagnostic and prognostic value of SPECT MBF and MFR measurements; more investigations are ongoing in this area.…”

Absolute myocardial blood flow vs relative myocardial perfusion: Which one is better?

“…Figure 1 demonstrates representative time-activity curves and k 1 polar maps obtained from attenuation and scatter-corrected images at rest and during dipyridamole stress with LAD occlusion. In a human study, 19 MBF and MFR obtained by CZT-SPECT with CT attenuation correction were compared to those obtained by N-13 ammonia PET. While resting MBF was similar, hyperemic MBF was lower for CZT compared to PET, yielding significantly lower MFR for CZT-SPECT compared to PET.…”

The tools are ready, are we?

“…8 Additional critical weakness of measuring MBF using Tc-99m-labeled tracers is its low extraction fraction. By the first human study comparing MBF data from dynamic CZT acquisition with data from PET Nkoulou et al 9 showed that median MBF at rest was comparable between CZT and PET, whereas it was significantly lower at stress in CZT than PET. This resulted in median myocardial flow reserve values of 1.32 by CZT, which is very low as compared to the usual cutoff value of 2.0 by many PET-based studies.…”