Investigation of the effect of crystal dead layer on system performance in Single Photon Emission Computed Tomography: A Monte Carlo simulation study

One of the factors affecting image quality in SPECT (Single Photon Emission Computed Tomography) applications is the dead layer that forms over time in the scintillation crystal. In this study, we aimed to investigate the effect of dead layers in the NaI(Tl) scintillation crystal used in SPECT on system performance. The GATE simulation code, which is based on the GEANT4 toolkit and is widely recognized for its application in nuclear imaging simulations, was employed for this investigation. In the first step, the standard SPECT model was simulated, and the SPECT performance parameters (sensitivity, spatial resolution, energy resolution, and contrast) for low 140 keV (for 99 m Tc), medium 167 keV (for201Tl), and high 364 keV (for131I) gamma energies have been analyzed. Then, various thicknesses of dead layers on the crystal were simulated, and the effect of the dead layer on the performance parameters was evaluated for the same energies. The results indicate reduction in SPECT sensitivity of approximately 33.5%, 27.1%, and 21.7% for low, medium, and high energies, respectively, as the thickness of the crystal's dead layer increases from 0.19 mm to 1.52 mm (equivalent to 2% to 16%). Small deteriorations in spatial resolution and energy resolution of the SPECT have been determined for 140 keV, 167 keV, and 364 keV gamma energies. The dead layers slightly influenced the contrast of SPECT. The results of this study show that the image quality assessment parameters of the SPECT system deteriorated as the thickness of the scintillation crystal's dead layer increased.