OpenTOPAS-SPECT: an extension for full-physics clinical and preclinical SPECT simulation

Single-photon emission computed tomography (SPECT) simulations must resolve collimator transport, detector energy deposition, and low-efficiency projection formation within one model. General transport codes leave users to build the imaging components, whereas dedicated SPECT codes offer efficient camera models with less freedom to represent new systems. We developed OpenTOPAS-SPECT to bring projection-domain SPECT simulation into the declarative parameter-file workflow of OpenTOPAS. Approach: We added parametric parallel-hole, converging, pinhole, multi-pinhole, and slit-slat collimators; sodium-iodide and cadmium-zinc-telluride detector configurations; energy-resolved and forced-detection scorers; native decay-chain sources; and an evidence-graded library of 14 camera presets. We tested preset construction, multi-view consistency, and variance reduction, and benchmarked projection-domain quantities against measured, independent-code, and analytic references. Main results: At 140.5 keV and 10 cm from the collimator face, the Siemens Symbia low-energy high-resolution configuration gave 7.49 × 10-5 counts per decay, or 0.82 of the measured 9.1 × 10-5. GE collimator sensitivities were 1.08 to 1.15× the SIMIND range midpoints, while the 364.5 keV low-to-high-energy sensitivity ratio was 59.5 versus 67.7. Planar point-spread widths were 4% to 9% below analytic geometric resolution from 5 to 30 cm. In this Symbia LEHR point-source geometry, forced detection reached matched precision 6.9× faster than full transport and reproduced its absolute sensitivity to 1% without calibration. Across 20 configurations (five collimators, 113 keV to 1 MeV), forced detection stayed within 4% of full transport while septa carried up to a fifth of the signal, but fell 7% to 25% below it where they carried 25% or more. The release-gating suite ran all 23 example decks and built all 14 presets. Significance: OpenTOPAS-SPECT provides reusable SPECT components within the OpenTOPAS parameter system, with bounded projection-domain benchmarks for the first stable public release. The evidence supports the tested configurations and observables; reconstructed-image accuracy and broader system coverage remain untested.