Intrinsic dual-energy processing of myocardial perfusion images

We have developed a software-based method for processing dual-energy 201 Tl SPECT emission projection data with the goal of calculating a spatially dependent index of the local impact of γ-ray attenuation. We refer to this method as intrinsic dual-energy processing (IDEP). Methods: IDEP exploits the differential attenuation of lower energy emissions (69-83 keV) and higher energy emissions (167 keV) resulting from the decay of 201 Tl to characterize the relative degree of low-energy γ-ray attenuation throughout the myocardium. In particular, IDEP can be used to estimate the relative probability that a low-energy γ-ray emitted from a particular region of the myocardium is detected during the acquisition of SPECT projection data. Studies on phantoms and healthy human volunteers were performed to determine whether the IDEP method yielded detection probability images with systematic structure visible above the noise of these images and whether the systematic structure in the detection probability images could be rationalized physically. In patient studies, the relative regional detection probabilities were applied qualitatively to determine the likely effects of attenuation on the distribution of mapped photon emissions. Results: Measurements of the detection probability in uniform phantoms showed excellent agreement with those obtained from computer simulations for both 180° and 360° acquisitions. Additional simulations with digital phantoms showed good correlation between IDEP-estimated detection probabilities and calculated detection probabilities. In patient studies, the IDEP-derived detection probability maps showed qualitative agreement with known nonuniform attenuation characteristics of the human thorax. When IDEP data were integrated with the findings on the emission scan, the correlation with coronary anatomy (known in 6 patients and hypothesized on the basis of clinical and electrocardiographic parameters in 5 patients) was improved compared with evaluating the mapped emission image alone. Conclusion: The IDEP method has the potential to characterize the attenuation properties of an object without use of a separate transmission scan. Coupled with the emission data, it may aid coronary diagnosis.