Monte Carlo based estimation of detector response in a large solid angle Preclinical PET imaging system

Small animal PET imaging imposes high performance requirements on image resolution and system sensitivity. Scanners with larger solid angle, achieved by using smaller crystal ring diameter and longer axial field-of-view (FOV), have higher absolute sensitivity. The Inveon dedicated PET (DPET) system, the latest generation of commercial tomographs from Siemens Preclinical Solutions, Inc., is such a high sensitivity scanner. Its crystal ring diameter and axial extent is 16.1 cm and 12.7 cm respectively, which give a solid angle coverage of 62%. However, this geometry also accentuates inter-crystal penetration, especially in the axial direction and causes axial blurring. Axial and radial blurring recovery is crucial for high resolution small animal PET imaging. System response modeling in combination with iterative reconstruction algorithms like Maximum a posteriori reconstruction (MAP) can be used to recover the resolution loss. Blurring in both radial and axial directions were simulated in GATE with a planar strip of ¹⁸F source placed inside the Inveon scanner. Detector response for each possible line of response (LOR) was calculated and can be incorporated into iterative reconstruction algorithms. As the entrance ring difference (δ) increased, the recorded coincidences tended to shift to a larger ring difference. As the entrance radial offset (u) increased, the recorded coincidences tended to shift to a smaller radial offset. The blurring effect got larger as δ and/or u increased. A real ¹⁸F plane source printed on carbon paper was also imaged and compared with the simulation data as a validation. The coincidence counts recorded for each ring difference showed a very good agreement between simulation and experiment except for very large oblique angles. This discrepancy should be improved with the inclusion of the scanner end shields in future simulations.