The Influence of Crystal Size and Material on Intercrystal Scattering and Parallax in PET Block Detectors: A Monte Carlo Study

Introduction:  In  this  study,  we  utilized  the  MCNP4C  Monte  Carlo  code  to  quantitatively  evaluate  the  influence of crystal size and material on intercrystal scatter and parallax effects.   Materials and Methods:  For each of the 5 selected crystals (BGO, LSO, LYSO, LuAP, GSO), transport of  511  keV  photons  originating  from  a  point  source  and  incident  on  the  central  crystal  of  a  block  detector  consisting of a 13 × 13 array of 4 mm × 4 mm × 20 mm  crystals was simulated. Functions such as position  detection accuracy (PDA) and point spread function (PSF) in different crystals were considered by changing  the parameters such as crystal thickness, crystal pixel size and gamma ray angle of incidence. The code was  validated against the simulated data published by Shao et al.   Results:  The  results  of  the  simulations  and  their  analysis  showed  that  increasing  the  crystal  pixel  size  increased  the  PDA  and  decreased  the  ICS  but  increasing  the  crystal  thickness  decreased  the  PDA  and  increased the ICS.  Discussion and Conclusion: By considering the attenuation properties of 511 keV photons, we conclude that  BGO  is  the  most  appropriate  crystal  for  reducing  the  intercrystal  scattering  phenomenon.  However,  the  higher light yield and lower decay time of LSO and LYSO makes them the most appropriate crystals for use  in  PET  systems.  The  obtained  functions  for  intercrystal  scattering  can  be  used  for  recovering  spatial  resolution and improving image quality in the image reconstruction process.