Monte Carlo simulation of small animal SPECT studies with <sup>123</sup>I-labeled radiotracers using high and low energy photons in a modified version of SimSET

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Small-animal molecular imaging is a prominent field of research, and SPECT plays an important role within it. Evaluation of imaging systems and reconstruction algorithms in SPECT imaging often involves Monte Carlo simulations, and SimSET is a widely used code for emission tomography simulation. 123I is a frequently used radionuclide in human and small animal studies. It emits a main yield of 159 keV photons, which are used for imaging, but it also has a low contribution of higher energy photons, which generate non-negligible noise in SPECT studies. The aim of this work was to implement experiment-based high- and low-energy response functions for a small animal SPECT system, to incorporate them into SimSET and to carry out a preliminary validation of the simulator obtained for 123I labeled radiotracers. Experimental measurements with 99mTc were carried out to characterize 159 keV detection of 123I. Since most of the relevant high-energy (HE) lines of 123I are close to 511 keV, the detector response to HE photons was characterized with the 511 keV photons from positron-electron annihilation subsequent to 18F decay. Once the obtained response functions were incorporated into SimSET, experimental and simulated images of capillaries and small spheres containing 123I were generated at various distances. Efficiency and spatial resolution were estimated from these images. The values from simulated images are close to the experimental ones, with deviations of under 10% in efficiency and under 6% in spatial resolution.