Use of Inter-Detector Scattering Event for Sensitivity Improvement for an In-Beam Small Animal PET

We have developed a small-animal in-beam positron emission tomography (PET) scanner as part of the “Small Animal Proton Irradiator for Research in Molecular Image-Guided Radiation Oncology” (SIRMIO) project, enabling accurate image-guided proton irradiation at clinical facilities. The scanner comprises 56 three-layer depth-of-interaction (DOI) LYSO detectors of 0.9 mm -pitch, arranged in a compact unique spherical configuration (72 mm inner diameter). Using our custom SIRMIO beamline, we successfully imaged positron emitters generated in PMMA targets by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3 0 - 5 0 ~ M e V}$</tex> protons. Sensitivity is critical for in-beam PET, and prior work has shown that triple-coincidence events including inter-detector scattering can enhance it. Here, we quantify the sensitivity gain and introduce a simple geometric method to estimate the first-interaction (FI) position in inter-detector scatter events: the shallower DOI layer between interactions <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$A$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$B$</tex> is assumed to be the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$F I$</tex>, and when depths coincide, we compare the expected scattering angles to resolve the FI. Monte Carlo simulations demonstrate a sensitivity increase of 14-18% (equivalent to adding 8-9 detectors) in our geometry, with FI estimation accuracies at the isocenter of 72 % (DOI-depth criterion) and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{9 1} {\%}$</tex> (angular criterion). Point-source imaging simulations show that reconstructions using our geometric FI estimation closely approach the image quality obtained with true interaction ordering. We expect that using the inter-detector scattering method is beneficial for in-beam PET imaging of small animals with the SIRMIO scanner.