Investigation of the effect of crystal dead layer on system performance in single photon emission tomography

SPECT (Single Photon Emission Tomography), which is frequently used in nuclear medicine today, is an effective diagnostic imaging device. It is based on the principle of counting the gamma rays emitted by the radiopharmaceutical substance given to the patient's body and creating cross-sectional images by processing them in the computer unit. The quality of the images obtained is very important for the correct diagnosis of diseases. The best image quality is only possible with regular quality control tests of the device. One of the factors affecting image quality in SPECT applications is the dead layer that forms over time in the scintillation crystal, one of the detector components. In this study, the effect of dead layers in NaI (Tl) scintillation crystal used in SPECT on system performance was investigated. For this purpose, the GATE simulation code based on GEANT4, a simulation package use the Monte Carlo method, was used. In the first step, the standard SPECT model was simulated and SPECT performance parameters (Sensitivity, spatial resolution, energy resolution, contrast) at low 140 keV (for 99mTc), medium 167 keV (for 201Tl) and high 364 keV (for 131I) gamma energies used in nuclear medicine were analyzed. Then, dead layers of different thicknesses were formed on the simulated crystal and the effect of the dead layer on the performance parameters was evaluated for the same energies. As a result, it was observed that the increase in the dead layer of the scintillation crystal resulted in deterioration of the image quality assessment parameters of the SPECT system. Keywords: Contrast, Dead layer, GEANT4, Monte Carlo, Single Photon Emission Tomography