Introduction: It is a common protocol to use ²⁰¹Tl for the rest and ^99mTc for the stress cardiac SPECT imaging. Theoretically, both types of imaging may be performed simultaneously using different energy windows for each radionuclide. However, a potential limitation is the cross-contamination of scattered photons from ^99mTc and collimator X-rays into the ²⁰¹Tl energy window. We used a middle energy window method to correct this cross-contamination.
Material and Methods: Using NCAT, a typical software torso phantom was generated. An extremely thin line source of ^99mTc activity was placed inside the cardiac region of the phantom and no activity in the other parts.  The SimSET Monte Carlo simulator was used to image the phantom in different energy windows. To find the relationship between projections in different energy windows, deconvolution theory was used. We investigated the ability of the suggested functions in three steps: Monte Carlo simulation, phantom experiment and clinical study. In the last step, SPECT images of eleven patients who had angiographic data were acquired in different energy windows. All of these images were compared by determining the contrast between a defect or left ventricle cavity and the myocardium.
Results: We found a new 2D kernel which had an exponential pattern with a much higher center. This function was used for modeling ^99mTc down scatter distribution from the middle window image. X-ray distribution in the ²⁰¹Tl window was also modeled as the ^99mTc photopeak image convolved by a Gaussian function. Significant improvements in the contrasts of the simultaneous dual ²⁰¹Tl images were found in each step before and after reconstruction. In comparison with other similar methods, better results were acquired using our suggested functions.
Conclusion: Our results showed contrast improvement in thallium images after correction, however, many other parameters should be evaluated for clinical approaches. There are many advantages in simultaneous dual isotope imaging. It halves imaging time and reduces patient waiting time and discomfort. Identical rest/stress registration of images also facilitates physicists’ motion or attenuation corrections and physicians’ image interpretation.