Design Optimization of a Small-animal SPECT System Using LGSO Continuous Crystal and a Micro Parallel-hole Collimator

The aim of this study was to optimize the design of a monolithic LGSO scintillation crystal and micro parallel-hole collimator for the development of a small-animal single photon emission computed tomography (SPECT) system with compact size, low-cost and reasonable performance through Monte Carlo simulation. L0.9GSO crystals with surface area of 50 mm X 50 mm were investigated for the design optimization. The intrinsic detection efficiency, intrinsic spatial resolution, and intrinsic energy resolution of crystals were estimated for different crystal thicknesses and photon energies (using I-125 and Tc-99m sources). Two kinds of surface treatments (providing polished and rough surfaces) were compared by optical photon simulation. The standard deviation of the angle between a micro-facet and the mean surface was set to 0.1 and 6.0 for polished and rough surfaces, respectively. For comparison, the intrinsic performance of NaI(Tl) was also investigated. A multi-photomultiplier tube was designed with 16 X 16 anode pixels having size of 2.8 mm X 2.8 mm and pitch of 3.04 mm, and a 1.5 mm thickness glass window. The length of the micro collimator was also optimized. Finally, the performance of the SPECT system was assessed and an ultra-micro hot spot phantom image was obtained in simulation. The 1-mm-thick LGSO was sufficient to detect most incident photons from I-125 but a thickness of 3 mm was required for Tc-99m imaging. Polished crystal yielded better intrinsic spatial resolution (~540 μm) and lower light output than rough crystal. Energy resolutions of I-125 and Tc-99m were ~36.9% and ~19.1%. With the optimized collimator length, spatial resolution of ~1 mm and sensitivity of ~100 cps/MBq were achieved with a four-head SPECT system. A hot rod with a diameter of 1.0 mm was resolved in the SPECT image of ultra-micro hot spot phantom.

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