Return of dual-isotope SPECT myocardial perfusion imaging? Not so fast….

Myocardial perfusion imaging remains the most frequently used non-invasive modality for the detection and risk stratification of patients with obstructive epicardial coronary artery disease (CAD). From its inception more than 40 years ago, the mode of stressing, choice of radioactive tracers, and detection technology, have evolved: From exercise stress alone to a choice of four pharmacological stressors, from thallium-201 (Tl201) to technetium-99m (Tc-99m) tracers, from planar to SPECT imaging, from single head to dual-head cameras, from Na-I crystals to CZT solid state cameras. Despite the long track record of diagnostic accuracy, prognostic robustness and ubiquitous availability, continued success for the modality cannot be taken for granted. With competition from the emergence of other non-invasive modalities (stress echocardiography, cardiac CT angiography, cardiac MRI) and with documented changes in the clinical manifestations of CAD, the demands on the safety, efficiency, and convenience of myocardial perfusion imaging continue to grow. In the long absence of new tracers (Tc-99m-based agents were introduced more than 25 years ago!), the recent advent of solid state high-efficiency SPECT cameras allows for a re-assessment of by now ancient imaging protocols. Obvious and justified criticism of the length of the traditional rest-stress sequence as well as growing awareness of radiation doses to the patients and to the medical personnel can now be addressed with imagination and inventiveness. One of the novel tested approaches is described in the current issue of the Journal by authors from Grenoble, France. The authors propose a fast test, which was utilized in 1,336 patients, but analyzed in only 214 patients. Using the GE Discovery NM 530 high-efficiency SPECT camera (GE Healthcare), a mean dose of 82 MBq (2.2 mCi) of Tl-201 was injected at peak stress, 5 minutes stress imaging followed and a mean dose of 314 MBq (8.5 mCi) of Tc-99m was injected after stress imaging and 5 minutes rest imaging followed 2 minutes later. Thus, the whole stress-rest sequence was completed in 22.4 ± 4 minutes. Correlation with coronary angiography in 104 patients with high CAD prevalence was good. Normalcy rate in 110 patients with low pretest probability was also high. A similar protocol and tracer dosage (2-2.5 mCi of Tl-201 and 8-10 mCi of Tc-99m) was reported in 2009 by Berman et al using a different high-efficiency CZT cardiac camera (D-SPECT, Spectrum Dynamics, Israel) in 374 patients. Image quality, radiation dose, and study length were analyzed, but not correlation to angiography. There are pros and cons of both Tl-201 and Tc-99m as myocardial perfusion agents, even though Tc-99m agents are currently used in the vast majority of SPECT myocardial perfusion imaging studies. Tl-201 has a rapid blood clearance half-time (\30 seconds), a high first pass extraction ([88%) and better linearity between flow und uptake. Tc-99m sestamibi, on the other hand, has a longer blood clearance half-time ([1 minute), smaller first pass extraction (50%-60%), and clinically non-significant redistribution. These differences provide a higher contrast between regions of normal perfusion and abnormal perfusion using Tl-201 and result in reduced defect size with Tc-99m. However, higher Tc-99m energy (140 keV of Tc-99m compared to 70-80 keV of Tl-201) and shorter Tc-99m half-life Reprint requests: Milena J. Henzlova, MD, Mount Sinai Medical Center (Mount Sinai Heart), New York, NY; milena.henzlova@ mssm.edu J Nucl Cardiol 2015;22:523–5. 1071-3581/$34.00 Copyright 2014 American Society of Nuclear Cardiology.