Effect of inter-crystal events inclusion on high resolution small-animal PET scanner performance

Le résumé fourni par la source

Inter-crystal scattering (ICS) events cause mispositioning of annihilation events, leading to contrast degradation of Positron-Emission-Tomography (PET) images. In Depth-of-Interaction (DOI) detectors for high resolution PET scanners the ICS events rate increases due to the smaller size of the crystals. This results in uniformly distributed counts of photon hits on flood histogram of the detected events (flood-map) in the detector, increasing the probability of crystal misidentification. DOI detectors with Anger-type calculation exhibit areas on the flood map not referable to any crystal. In our system, events in such areas represent ICS events and reach 25% of the total number of events detected, resulting in ~50% of the coincidence events being discarded, as they contain at least one event unassignable to any pixel. We developed a method based on most likely position to include such ICS events in the PET reconstructed images to improve the sensitivity of the scanner. The method is being developed in the context of the “Small animal proton Irradiator for Research in Molecular Image-guided radiation Oncology” (SIRMIO) project, which succeeded at developing a prototype in-beam PET system for image-guided small animal proton irradiation at clinical facilities. The scanner features 56 staggered 3-layer DOI detectors, each of them comprising a LYSO pixelated (0.9 × 0.9 × 6.67 mm3crystal size) scintillation block coupled to an 8 × 8 SiPM array, light-sharing method and Anger type calculation with a 4 channels output for crystal identification. The signal of a22Na source and the activation produced by a 30 MeV proton beam were measured. The events outside the crystal response areas were assigned to one of the closest pixels according to a probabilistic computation and later used for image reconstruction. The results support the feasibility of the method for online PET applications, resulting in a doubled sensitivity and a minimal spatial resolution degradation.

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