Image-mode performance characterisation of a positron emission tomography subsystem designed for Biology-guided radiotherapy (BgRT)

Abstract

Objectives: In this study, we characterise the imaging-mode performance of the positron emission tomography (PET) subsystem of the RefleXion X1 machine using the NEMA NU-2 2018 standard.

Methods: The X1 machine consists of two symmetrically opposing 90⁰ arcs of PET detectors incorporated into the architecture of a ring-gantry linear accelerator rotating up to 60 RPM. PET emissions from a tumour are detected by the PET detectors and used to guide the delivery of radiation beam. Imaging performance of the PET subsystem on X1 machine was evaluated based on sensitivity of the PET detectors, spatial resolution, count-loss performance, image quality, and daily system performance check.

Results: PET subsystem sensitivity was measured as 0.183 and 0.161 cps/kBq at the center and off-center positions, respectively. Spatial resolution: average FWHM values of 4.3, 5.1, and 6.7 mm for the point sources at 1, 10, and 20 cm off center, respectively were recorded. For count loss, max NECR: 2.63 kcps, max true coincidence rate: 5.56 kcps, and scatter fraction: 39.8%. The 10 mm sphere was not visible. Image-quality contrast values were: 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.

Conclusions: When operating in an imaging mode, the spatial resolution and image contrast of the X1 PET subsystem were comparable to those of typical diagnostic imaging systems for large spheres, while the sensitivity and count rate were lower due to the significantly smaller PET detector area in the X1 system. Clinical efficacy when used in BgRT remains to be validated.

Advances in knowledge: This is the first performance evaluation of the PET subsystem on the novel BgRT machine. The dual arcs rotating PET subsystem on RefleXion X1 machine performance is comparable to those of the typical diagnostic PET system based on the spatial resolution and image contrast for larger spheres.

Figure 1.

Layout of the PET arcs in the gantry (dimensions are in mm)

Figure 2.

PET image of a uniform cylinder (a Jaszczak phantom with inserts removed).

Figure 3.

PET Count-loss performance measured according to NEMA NU-2 2018 (Due to the computer hardware constraint associated with data processing, the counting performance of the Reflexion PET will saturate when the count rate is beyond certain number. What is reported is the performance numbers that are achievable with the Reflexion PET.)

Figure 4.

PET image quality evaluation- NEMA NU-2 2018

Figure 5.

SPC test for 77 days (a) Uncorrected energy resolution (red line indicate the pass mark <15%) and (b) Time resolution (red line indicate the pass mark pass <1000ps)