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 {\mu}m) 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.

💡 Research Summary

The paper presents a comprehensive design‑optimization study for a compact, low‑cost small‑animal single‑photon emission computed tomography (SPECT) system that employs a monolithic LGSO (lutetium‑gadolinium orthosilicate) scintillation crystal coupled with a micro parallel‑hole collimator. Using Monte Carlo simulations (GEANT4) and optical photon tracking, the authors evaluated intrinsic detection efficiency, spatial resolution, and energy resolution for LGSO plates of 50 mm × 50 mm with three thicknesses (1 mm, 2 mm, 3 mm) under two commonly used radionuclides: I‑125 (27 keV) and Tc‑99m (140 keV). Two surface‑finish models were examined: a polished surface (micro‑facet standard deviation σ = 0.1°) and a rough surface (σ = 6.0°). The polished finish yielded superior intrinsic spatial resolution (~540 µm FWHM) but lower light output, whereas the rough finish increased light yield at the expense of spatial resolution. Energy resolutions of ~36.9 % for I‑125 and ~19.1 % for Tc‑99m were obtained with the polished crystal, which are acceptable for quantitative animal imaging.

A multi‑photomultiplier tube (PMT) readout was designed with a 16 × 16 pixel array (pixel size 2.8 mm × 2.8 mm, pitch 3.04 mm) and a 1.5 mm thick glass window, matching the high light yield of LGSO (~30,000 photons/MeV). The micro collimator consisted of 100 µm diameter holes separated by 50 µm walls, forming a parallel‑hole geometry. Collimator length was varied from 5 mm to 15 mm to explore the trade‑off between spatial resolution and system sensitivity. Simulations identified a 10 mm collimator length as optimal, delivering an overall system spatial resolution of approximately 1 mm full‑width at half‑maximum (FWHM) and a sensitivity of about 100 counts per second per megabecquerel (cps/MBq) for a four‑head configuration.

Performance was validated with an ultra‑micro hot‑spot phantom containing rods of 1.0 mm, 0.5 mm, and 0.25 mm diameters. The optimized system clearly resolved the 1.0 mm rod, demonstrating that the design meets the target sub‑millimeter resolution while maintaining reasonable count statistics. For comparison, a NaI(Tl) crystal of equivalent dimensions was simulated; LGSO showed comparable or slightly better sensitivity for Tc‑99m and superior spatial resolution, while also offering greater radiation hardness and longer operational life.

Key conclusions include: (1) a 1 mm thick LGSO crystal is sufficient for low‑energy I‑125 imaging, whereas a 3 mm thickness is required to achieve high detection efficiency for Tc‑99m; (2) polished crystal surfaces improve intrinsic spatial resolution despite reduced light output; (3) a 10 mm micro parallel‑hole collimator balances resolution and sensitivity for small‑animal SPECT; and (4) the four‑head system can reliably resolve 1 mm features, positioning it competitively with existing commercial small‑animal SPECT scanners that typically offer 1–2 mm resolution and 50–150 cps/MBq sensitivity.

The study provides quantitative design guidelines for future low‑cost SPECT prototypes, highlighting the interplay between crystal thickness, surface finish, and collimator geometry. It also suggests next steps such as building a physical prototype for experimental verification, extending the design to multi‑isotope imaging, and integrating advanced reconstruction algorithms to further enhance image quality. The optimized LGSO‑based system holds promise for preclinical molecular imaging, drug development studies, and detailed physiological investigations in small animal models.