The spillover fraction of thallium photons in the I-123 energy window: Implications for post-exercise dual isotope simultaneous [ 123i] BMIPP and thallium imaging

Abstract

Background: β-Methyl-ω-[p- 123I-phenyl]pentadecanoic acid (BMIPP) is a radiolabeled branched-chain fatty acid analog under clinical investigation in the United States for the detection of myocardial “ischemic memory”. In a recent Phase II clinical trial, BMIPP images acquired within 30 hours of an exercise-induced ischemia by Tl-201 SPECT, showed fatty acid disturbances at rest that were similar in extent and severity to the thallium perfusion defects on stress. The clinical relevance of these data is in the potential for uncoupling the treadmill portion of cardiac SPECT from BMIPP injection and imaging. This sets the stage for post-treadmill dual isotope simultaneous imaging of the ischemic metabolic signal (BMIPP) with rest perfusion signal (Tl-201). The current study, therefore, was undertaken to determine the spillover fraction of Tl-201 photons in the I-123 energy window of BMIPP. Methods: Spillover was calculated by acquiring SPECT data in both Tl-201 and I-123 energy windows for a phantom filled only with Tl-201. The phantom arrangement consisted of a water filled thorax phantom (300 × 230 mm) with a central cylindrical compartment (50 mm diameter) to simulate the heart. The background and heart regions had activity concentrations of 5.3 kBq/ml and 104.9 kBq/ml respectively. All image data were acquired on a dual-head DST gamma camera (SMV, Twinsburg, OH) with low-energy high-resolution collimators. Two separate SPECT acquisitions were performed immediately after one another using the Tl-201 (73 keV 30% + 167 keV 20%) and the I-123 (159 keV 20%) energy windows. The resulting pair of images were analyzed by placing regions-of-interest (ROIs) over corresponding parts of the heart compartment. The spillover fraction was defined as the ROI counts in the I-123 window divided by the ROI counts in the Tl-201 window. Results: The spillover fraction was 0.14 +- 0.01. Conclusion: The spillover fraction of Tl-201 photons in the I-123 energy window is only 14%. If confirmed in the clinical setting, post-treadmill dual isotope simultaneous imaging of BMIPP and thallium may become an attractive alternative to exercise myocardial perfusion SPECT.