Performance of an analytical positron range modelling approach in the context of whole body small animal and clinical PET

Monte Carlo simulation codes that model positron interactions along its tortuous path are expected to be accurate but slow. A simpler and potentially faster approach is to model positron range from analytical annihilation density distributions. We have implemented and validated such a method. 1D annihilation density distributions for different isotope-media combinations were fitted with Gaussian functions and described by simple look-up-tables of fitting coefficients. Together with a method developed for simulating positron range in heterogeneous media this allowed for efficient modeling of positron range. Previously, we focused on evaluating soft tissue-bone transitions and the performance of the modeling in small animal PET where positron range has a higher relative contribution to the spatial resolution than in clinical PET. Here, we extend the assessment to soft tissue-lung transitions in whole body small animal and clinical PET. The performance of the modeling was evaluated by comparing annihilation density distributions obtained in heterogeneous media with those produced by the Monte Carlo simulator GATE and by quantitatively analyzing the final reconstructed images of Monte Carlo simulated data for the small animal PET scanner microPET Focus 220 and the clinical PET scanner ECAT Exact HR+. Modelling heterogeneous media showed some limitations for soft tissue-lung transitions leading to systematic divergence from GATE at large positron range values depending on the isotope and the distance travelled before the transition occurs. The differences in the reconstructed point spread functions in water and lung showed that modeling medium heterogeneity is essential for small animal PET and would be beneficial for clinical PET. The level of agreement between the analytical model and GATE depends somewhat on the simulated scanner, but appears to be suitable for lower energy positron emitters, such as ¹⁸F or ¹¹C. However, the method for heterogen- ous media modelling could be used with any underlying positron range model.