Monte Carlo modeling of cascade gamma rays in⁸⁶Y PET imaging: preliminary results

Abstract

86Y is a PET agent that could be used as an ideal surrogate to allow personalized dosimetry in 90Y radionuclide therapy. However, 86Y also emits cascade gamma rays. We have developed a Monte Carlo program based on SimSET to model cascade gamma rays in PET imaging. The new simulation was validated with the GATE simulation package. Agreements within 15% were found in spatial resolution, apparent scatter fraction (ratio of coincidences outside peak regions in line source sinograms), singles and coincidences statistics and detected photons energy distribution within the PET energy window. A 20% discrepancy was observed in the absolute scatter fraction, likely caused by differences in the tracking of higher-energy cascade gamma photons. On average the new simulation is 6 times faster than GATE, and the computing time can be further improved by using variance reduction techniques currently available in SimSET. Comparison with phantom acquisitions showed agreements in spatial resolutions and the general shape of projection profiles; however, the standard scatter correction method on the scanner is not directly applicable for 86Y PET as it leads to incorrect scatter fractions. The new simulation was used to characterize 86Y PET. Compared with conventional 18F PET, in which major contamination at low count rates comes from scattered events, cascade gamma-involved events are more important in 86Y PET. The two types of contaminations have completely different distribution patterns, which should be considered for the corrections of their effects. Our approach will be further improved in the future in the modeling of random coincidences and tracking of high energy photons, and simulation results will be used for the development of correction methods in 86Y PET.

Physics of pure and non-pure positron emitters for PET: a review and a discussion

“…Finally, a possible but time-consuming solution is a full Monte Carlo simulation of the emission and interaction of all radiation in the prompt gamma radioisotopes. Fast-simulation packages for PET like SimSET have been used [ 46 ]. GATE Monte Carlo simulation was successful for a complete simulation of non-pure radioisotopes, and it was used as a support for estimating suitable functions modeling the tails of the sinograms.…”

Quantitative imaging of 124I and 86Y with PET

“…Bading et al [ 36 ] examined the degradation of image resolution, when 124 I was used comparatively with 18 F in a microPET R4 small animal scanner (Concorde/Siemens, Knoxville, TN, USA). In the scanner’s centre the FWHM was 2.3 mm for 124 I and 1.9 mm for 18 F. Using a Discovery STE PET/CT scanner Zhu and El Fakhri [ 37 ] studied the spatial resolution of 18 F and 86 Y with a line source placed centrally in air and reported values of 5.56 and 6.06 mm. Gregory et al reported a resolution degradation of 0.7 mm for 124 I compared to 18 F for the Gemini Dual GS PET/CT scanner [ 28 ].…”

Impact of Region-of-Interest Delineation Methods, Reconstruction Algorithms, and Intra- and Inter-Operator Variability on Internal Dosimetry Estimates Using PET

“…The output sinograms covered a 15.7cm axial field of view (FOV). The number of transaxial and angular bins were 128 and 140, respectively and 192 was the number of axial slices (1 cm slice size) [22].…”