Purpose: To characterize the performance of the PET subsystem of the first Reflexion X1 machine installation using the NEMA NU-2 2018 standard.
Methods: The X1 machine consists of two symmetrically opposing 90° arcs of PET detectors incorporated into the architecture of a ring-gantry linear accelerator rotating up to 60 RPM to aim radiotherapy beams using PET emissions of the tumors. Each PET arc has 32 detectors arranged in 2 rings with 90cm inner diameter and 5cm axial extent. Each detector is made up of 12x6 Lutetium-based crystals with a dimension of 4.14x4.14x20mm3. Sensitivity assessed using 2M coincidence events collected for each position. Spatial resolution tests utilized six point-sources arranged horizontally 1, 10 and 20cm away from the center axis, three in and three off the iso-plane, measured simultaneously for 10 mins. Count loss assessed using 20 frames were scanned, 2M coincidence events each, with a 15 min break between scans. Image quality assessed using a phantom filled with FDG (4:1 sphere-to-background ratio).
Results: X1 PET Subsystem sensitivity: 182.4 and 160.9 cps/MBq at the center and off-center positions, respectively. Spatial resolution: average FWHM values of 4.3, 5.1, and 6.7mm for the point sources at 1, 10, and 20cm off center, respectively. For count loss, max NECR: 2625.5 cps, max true coincidence rate: 5557.4 cps, and scatter fraction: 39.8%. Image-quality contrast values: 29.6%, 64.9%, 66.5%, 81.8%, 81.2%, and background variability: 14.8%, 12.4%, 10.3%, 8.8%, 8.3%, for the 13, 17, 22, 28, 37 mm sphere sizes, respectively.
Conclusion: When operating in an imaging mode, spatial resolution and image contrast were equivalent to those of typical diagnostic systems, while sensitivity and count rate were lower due to design of the system which requires a far smaller detector area than a typical diagnostic system. Clinical efficacy when used in BgRT remains to be validated.
Funding Support, Disclosures, and Conflict of Interest: Zhiqiang Hu, Manoj Narayanan, Peter Olcott, Matthew Bieniosek, Thomas Laurence and Shervin M. Shirvani are employees of Reflexion Medical, Inc. There is a master research agreement between Reflexion Medical and Stanford University.
PET, Radiation Therapy, Image-guided Therapy