Simulation-based reconstruction of absolute activities from the 99mTc/111In dual-isotope SPECT/CT: phantom experiments and imaging of neuroendocrine tumors

We investigate the quantitative accuracy of the reconstruction of absolute Tc-99m and In-111 activities from Tc-99m/In-111 dual-isotope SPECT studies. The separate reconstruction of two images is achieved by applying Monte Carlo simulation-based corrections for self-scatter and cross-talk between energy windows. For method evaluation, a series of Tc-99m/In-111 physical phantom experiments was performed using a clinical SPECT/CT camera. The containers were filled with different ratios of Tc-99m and In-111 activities to create cross-talk with varying severity levels. In addition, we illustrate the performance of our method by reconstructing images from four simultaneous Tc-99m/In-111 SPECT/CT studies of neuroendocrine patients. Similarly to the phantom experiments, clinical cases provide examples with different severity of cross-talk. Phantom experiments showed that Monte Carlo simulation-based corrections improved both quantitative accuracy and visual properties of Tc-99m and In-111 images. While the errors of absolute activities for both tracers in six containers ranged from 16% to 75% if no corrections for self-scatter and cross-talk were applied, these errors decreased to below 10% when images were reconstructed with the aforementioned corrections. These activities were measured using regions of interest larger than the true sizes of the containers in order to account for the spill-out effect. Analysis of patient studies confirmed that accurate simulation-based compensations improved resolution and contrast for both Tc-99m and In-111 images.