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

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.