Characterization of the SIRMIO Spherical Small Animal PET toward Simultaneous Beam and Tracer Imaging

We developed a small animal in-beam positron emission tomography (PET) scanner for accurate image-guided proton irradiation at clinical facilities within the project “Small animal proton Irradiator for Research in Molecular Image-guided radiation Oncology” (SIRMIO)”. The PET scanner is composed of 56 3-layer depth-of-interaction (DOI) PET detectors arranged in a unique spherical shape with an inner diameter of 72 mm. Each DOI PET detector is composed of a LYSO scintillator block with a pixel size of 0.9 mm readout by an 8×8 SiPM array. A custom- made charge division circuit and an amplifier circuit board are used to reduce the 64 signals from the SiPM array to 4 signals. We employ the R5560 digitizer (CAEN, Italy). We succeeded to image the distribution of positron emitters generated by proton beams irradiated in PMMA targets with an energy range of 30-50 MeV using our novel SIRMIO beamline and the PET scanner at the Danish Centre for Particle Therapy (Aarhus, Denmark). Moreover, we are currently characterizing our scanner for imaging PET tracers, which could potentially enable us to image both the tumor and proton beam simultaneously with high resolution. In this study, we evaluated the performance of the count rate and the image resolution of a micro Derenzo phantom at the center pf the PET scanner. A Maximum Likelihood Expected Maximization algorithm was used for PET reconstruction. The count rate was linear with the activity in the investigated range up to 1.65 MBq. The peak-to-valley ratio along the radial direction was 1.5, 1.6, 1.6, 1.3 and 1.2 for the rods with diameters of 1.5, 1.2, 1.0, 0.9 and 0.8 mm, respectively, without any corrections. The 0.7 mm rod was not resolved. We confirmed that our PET scanner has 0.9 mm imaging and can be used for high resolution imaging both tumor and the beam simultaneously.