The performance of Lu_1.6Y_0.4SiO₅ (LYSO) continuous scintillator for PET applications

Abstract

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Objectives: Scintillating materials are the most widely applied radiation detectors in photon counting modalities. Their characteristics play a significant role to the quality of the results in medical image applications. In positron emission tomography (PET), apart from pixelated detectors, continuous crystals are also of high interest due to their cost efficiency and good sensitivity. This work deals with the study of a continuous crystal in truncated pyramid shape, a configuration that minimizes the image compression effect at the crystal borders. Methods: The performance characteristics of such crystals have been studied using Monte Carlo simulation. The detector material examined here had the following chemical composition: Lu_1.6Y_0.4SiO₅:Ce (commonly known as LYSO) whose physical properties (i.e. fast decay time: 40 ns, high density: 7.1 g/cm³, high effective atomic number: 63) indicate high gamma ray detection efficiency and demonstrate the scintillator’s suitability for coincidence detection of annihilation radiation (radiation energy: 511 keV). The crystal’s area was 40×40 mm² for the irradiated surface, 50×50 mm² for the output side (i.e. side of light emitted) with a thickness of 9.3 mm. Results: Data are reported on (a) the crystal’s detection properties for gamma rays (i.e. interacted gamma rays, absorbed gamma rays, single scatter, multiple scatter etc) and (b) the point spread function (PSF) as a function of the depth of interaction (DOI). Estimations based on Monte Carlo model showed: (a) approximately 52% of the incident gammas undergo inter-crystal scattering; (b) 71% of the scattered events undergo more than one interactions; (c) the proportionality between the full-width at half maximum (FWHM) of the PSF and the DOI takes a maximum and minimum value at the crystal’s irradiated surface and back side, respectively. Conclusions: This Monte Carlo based study can be proven very useful if combined with a high energy resolution data acquisition scheme for these detectors to help building models for the correct positioning of the detected gammas not only in the X-Y positions but also in the Z coordinate, that represents the DOI information.