Monte Carlo Modeling of the Effects of Cascade Gamma Photons in 2D PET Images Obtained With the Isotopes: Br

Aim: The aim of this work was to determine by Monte Carlo Modeling (MCM) the effects of including gamma photons ( ), emitted in cascade with positrons, in 2D PET images of isotopes that decay both by Positron Emission (PE) and Electronic Capture. These isotopes have the advantage of having longer half-lives than conventional PET imaging isotopes, allowing for PET diagnostic and therapeutic applications where imaging over a period of several days is necessary. Materials and Methods: For this study we have considered the decay by PE, in which the positrons and the cascade photons are emitted by the same nucleus. The isotopes considered were the Br, the Y and the I. For the MCM we have altered SimSET in order to simulate a simplified PE decay scheme of these isotopes; considering only positrons with relative abundances >0.5%, but still accounting for more than 95% of the positrons emitted by each isotope (Br:97.9%, Y:96.2%, I:98.7%). Also, only -photons with abundances >1.0% were considered. The coincidences between the two positron annihilation photons (511C) and the and -511 coincidences (Cascade Coincidences, CC) were binned into different sinograms, as were the scattered-511C and scattered-CC. To simulate 2D PET images of a cylindrical phantom filled with water and 5 linear inserts containing radioactivity, we have used scanner geometry parameters corresponding to the GE DST PET/CT. Results: For this simulation geometry we have found that, for the three isotopes considered, the CC contribution to the image is spatially uniformly distributed. We have found that for Br the total number of true-coincidences in the images consisted of 7.9% of true-CC and 92.1% true-511C. The number of counts in the scatter images was composed of 36.2% scattered-CC and 63.9% of scattered-511C. The corresponding percentages for Y were: true-CC=6.6%, true-511C=93.5%, scattered-CC=48.7% and scattered511C=51.4%. For I we found the respective values: true-CC=2.8%, true511C=97.2%, scattered-CC=16.3% and scattered-511C=83.7%. Conclusions: Hence, MCM of CC originated from PE decay shows that the percentage of scattered-CC included in the images is significantly high and its correction is crucial to attain image quantitative accuracy. Although the results show that the percentage of true-CC included in the images is <10%, this component of the CC must also be corrected to achieve image quality. The MCM work in progress includes the assessment of the impact of CC in image spatial resolution, signal-to-noise ratio, contrast and radioactivity recovery factors, both in 2D and 3D PET images.