| In thallium-201 and technetium-99m dual isotope myocardial imaging, rest and stress images are acquired using the two isotopes to shorten the overall study time. In this technique the rest and stress images can be acquired either separately or simultaneously. The separate acquisition protocols are well established and documented. Simultaneous acquisition offers a number of advantages over separate acquisition, but downscatter and Pb x-rays due to Tc-99m photons degrade the quality of the Tl-201 data. In this work, we have developed improved acquisition and model-based compensation methods, with the goal of improving the quality of simultaneously acquired dual isotope images. To study photon interactions in both, the attenuator and the collimator, a Monte Carlo (MC) code was modified and validated. This MC code was used to study and parameterize the collimator Pb x-ray line source response function (XLSRF) as a function of the source distance and source energy. The parameterized XLSRFs were combined with the previously proposed effective source scatter estimation (ESSE) scatter model, to estimate the Pb x-ray contamination generated by scattered Tc-99m photons. The ESSE model was improved to more accurately model scatter in non-uniform attenuators and non-photopeak energy windows. An analytical projector incorporating the improved ESSE and Pb x-ray models was shown to estimate crosstalk accurately. We also determined acquisition parameters having an optimal tradeoff, as measured by the ideal observer signal-to-noise ratio, between maximizing detected counts and minimizing crosstalk. The compensation and optimal acquisition methods were evaluated using experimental phantom data. It was found that using the optimal acquisition parameters gave substantial improvements in Tl image quality with minimal degradation in Tc image quality; inclusion of a Tl energy window centered at 167 keV further improved Tl image quality. The model-based compensation (MBC) method was compared to a previously-proposed idealized triple-window compensation (TWC) method. It was found that both methods substantially improved Tl image quality and offered a similar tradeoff between lesion contrast recovery and image noise. However, TWC requires determination of scaling and blurring parameters for each patient while MBC requires no such patient-dependent parameters. |
