Monte-Carlo modeling of the microPET R4 small animal PET scanner for coincidence-mode emission and singles-mode transmission data acquisition

We present an extension of the simulation tool GATE for singles-mode transmission tomography and its validation using experimental data from the microPET R4 scanner. Before using GATE to model transmission data, we first validated our simulated PET scanner by comparing experimental and simulated resolution and scatter fractions (SFs) for coincidence-mode sinogram data. Two levels of complexity were simulated for emission sources. The first level neglected beta⁺ interactions and only modeled the emission of perfectly collinear annihilation gamma-rays. For these data simulated FWHMs were on average 0.9 mm narrower and the simulated SFs were 9% lower than experimental values. The second type of simulation included beta⁺ interactions. When compared to experimental values, the simulated FWHMs for these data differed by less than 0.4 mm on average and SFs agreed to within 2%. For singles-mode transmission data, we have compared simulated and experimental data for three different sized water phantoms using ⁵⁷Co (ap124 keV gamma-emitter) and ⁶⁸ Ge (beta⁺-emitter) transmission sources. We have currently only modeled gamma⁺ interactions for the ⁶⁸ Ge source. The average difference between experimental and simulated SFs were 4.6% and 6.5% for the ⁵⁷Co and ⁶⁸ Ge sources, respectively. The simulated attenuation correction factors (ACFs) and effective attenuation coefficients were also in better agreement with experimental values for ⁵⁷Co than for ⁶⁸Ge data. Our results may be improved by including the effects of detector dead-time. Although some additional development is still planned, this new transmission tomography tool has already provided insight into the effects which degrade the accuracy of the ACFs obtained from singles-mode scans and may soon become useful by providing realistic simulated transmission data. These data could help improve the design a- - nd performance of PET transmission hardware and aide in the development of reconstruction and correction techniques