The Benefit of Time-of-Flight in Digital Photon Counting PET Imaging: Physics and Clinical Evaluation

Objectives: The introduction of solid state digital photon counting (DPC) technology enables the 3^rd generation time-of-flight (TOF) PET. This study evaluates the benefits and advantages of the DPC TOF by physics tests and phantom experiments as well as its translation to improve clinical PET.

Methods: A solid state DPC Vereos (Philips) PET/CT system (dPET) was used in this study and compared to a conventional PMT TOF PET system (cPET) (Gemini TF, Philips). Benefits of TOF were evaluated in three categories: A. Physics testing using a 3.7 MBq ²²Na point source together with two 30cm long water cylinder phantoms filled with ~740 MBq ¹⁸F was performed for TOF timing resolution measurements as a function of system singles rate;

Objectives: . phantom experiments of using a NEMA IEC Body phantom filled with six ¹⁸F hot spheres with varying contrast ratios of 2, 4, 8, 16, 32 and 64 were imaged (6-paired scans on both systems); and C. investigational whole-body oncologic FDG PET/CT scans of 40 clinical patients were acquired on the dPET before or after the cPET-based clinical standard of care (485 MBq / 75 min p.i) PET imaging. Target lesion SUV and SNR of including normal tissues were assessed and compared via blinded image reviews.

Results: An average timing resolution of 322ps FWHM was obtained on dPET compared to 580ps FWHM on cPET. Timing resolution as a function of singles rate (0-93Mcps) demonstrated significantly improved and robust distribution on the dPET (320-350ps) compared to the cPET (580-900ps). Substantial enhancement of hot spheres with better delineation, improved contrast and accurate shapes without deformation or artifacts adapting to different contrast ratios (2-64) were achieved on dPET than cPET. Compared to the cPET, SUV and recovery coefficients were improved about 40%-5% across spheres from 10 to 37mm and contrast ratio from 2 to 64 on dPET. For clinical PET images, 120 assessable lesions were analyzed, with the 322ps dPET presented SNR of ~2x improvement and 20%±14% higher SUV than non TOF, versus for the 580ps cPET an SNR of ~1.6x and SUV of -10% ±18% vs non TOF. No significant SUV differences in normal tissues were found between nonTOF and TOF using the 325ps dPET (p>0.05), while significantly higher SUVs were found on non-TOF compared to TOF of 580ps cPET (p<0.01). The dPET consistently ranked as better image quality with improved lesion contrast and small lesion detectability than the cPET. ~10% of the lesions identified on 325ps TOF were not confidently assessable on 580ps TOF, while 25% were non-assessable on nonTOF.

Conclusions: The solid state digital PET technology enables time-of-flight PET of the 3^rd generation (300-400ps) compared to the prior PMT technology based 500-600ps. The measurable improvement in system performance highlights the benefit of the 322ps time-of-flight timing resolution leading to improvements in SNR, quantitative accuracy, precise localization, lesion detectability and clinical diagnostic confidence.