Development of an Optimized SPECT Module for BNCT Applications with Novel Pinhole Collimator and DOI Estimation with ANN

Boron Neutron Capture Therapy (BNCT) is a targeted radiotherapy modality that couples neutron irradiation with selective drug delivery. After administration of a 10Benriched compound that preferentially accumulates in tumour tissue, the patient is exposed to thermal neutrons, triggering the 10B(n,α)7Li reaction. The resulting high-LET charged particles release their energy within micrometer-scale ranges, concentrating the absorbed dose at the cellular level while sparing surrounding healthy tissue. In the majority of capture events, the resulting 7Li nucleus de-excites by emitting a 478 keV prompt γ ray, which can be detected externally and used for dose localization and real-time monitoring with single-photon emission computed tomography (SPECT). In this work, we present the results of an experimental tomography study conducted with a BNCT-SPECT prototype system at the LENA neutron facility in Pavia, Italy. In this study, we introduce a new collimator and a new 3D gamma-ray position estimator. The system employs the BeNEdiCTE detection module, based on a 5 cm × 5 cm × 2 cm LaBr3(Ce+Sr) crystal coupled to an 8×8 matrix of Near Ultraviolet High-Density silicon photomultipliers (SiPMs). A redesigned lead pinhole collimator with higher geometric efficiency improves peak identification at 478 keV, reduces the module dimensions, and enables more robust acquisitions without advanced data processing. A compact, hardware-oriented artificial neural network (ANN) is introduced to simultaneously estimate the interaction coordinates (x, y, z) in the monolithic scintillator. The ANN is trained on a full experimental dataset and preserves good x and y performance around 3mm FWHM, while providing depth-of-interaction (DOI) estimation with 5mm FWHM resolution, enabling parallax-error compensation for large acceptance-angle geometries. We demonstrate a successful 3D reconstruction of two vials filled with boric acid (7371 ppm of 10B) placed 1.4 cm apart using four partial projections, confirming a system spatial resolution under 1 cm.