Fully digital PET is unaffected by any deterioration in TOF resolution and TOF image quality in the wide range of routine PET count rates

Julien Salvadori,Freddy Odille,Laetitia Imbert,Gilles Karcher,Pierre-Yves Marie

doi:10.1186/s40658-020-00344-5

Abstract

PurposeDigital PET involving silicon photomultipliers (SiPM) provides an enhanced time-of-flight (TOF) resolution as compared with photomultiplier (PMT)-based PET, but also a better prevention of the count-related rises in dead time and pile-up effects mainly due to smaller trigger domains (i.e., the detection surfaces associated with each trigger circuit). This study aimed to determine whether this latter property could help prevent against deteriorations in TOF resolution and TOF image quality in the wide range of PET count rates documented in clinical routine.MethodsVariations, according to count rates, in timing resolution and in TOF-related enhancement of the quality of phantom images were compared between the first fully digital PET (Vereos) and a PMT-based PET (Ingenuity). Single-count rate values were additionally extracted from the list-mode data of routine analog- and digital-PET exams at each 500-ms interval, in order to determine the ranges of routine PET count rates.ResultsRoutine PET count rates were lower for the Vereos than for the Ingenuity. For Ingenuity, the upper limits were estimated at approximately 21.7 and 33.2 Mcps after injection of respectively 3 and 5 MBq.kg-1 of current 18F-labeled tracers. At 5.8 Mcps, corresponding to the lower limit of the routine count rates documented with the Ingenuity, timing resolutions provided by the scatter phantom were 326 and 621 ps for Vereos and Ingenuity, respectively. At higher count rates, timing resolution was remarkably stable for Vereos but exhibited a progressive deterioration for Ingenuity, respectively reaching 732 and 847 ps at the upper limits of 21.7 and 33.2 Mcps. The averaged TOF-related gain in signal/noise ratio was stable at approximately 2 for Vereos but decreased from 1.36 at 5.8 Mcps to 1.14 and 1.00 at respectively 21.7 and 33.2 Mcps for Ingenuity.ConclusionContrary to the Ingenuity PMT-based PET, the Vereos fully digital PET is unaffected by any deterioration in TOF resolution and consequently, in the quality of TOF images, in the wide range of routine PET count rates. This advantage is even more striking with higher count-rates for which the preferential use of digital PET should be further recommended (i.e., dynamic PET recording, higher injected activities).

Highlights

Three PET/CT systems using silicon photomultipliers (SiPM) are currently commercialized, namely the Philips VereosTM, the Siemens Biograph VisionTM, and the GE DiscoveryTM MI
Contrary to the Ingenuity PMT-based PET, the Vereos fully digital PET is unaffected by any deterioration in TOF resolution and in the quality of TOF images, in the wide range of routine PET count rates
The TOF resolution is the key parameter in this setting and is currently measured at rather low counting rates, around 2 Mcps, as part of the daily procedures of quality control of PET cameras [19], whereas higher counting rates are typically reached in clinical routine [2]

Summary

Introduction

Three PET/CT systems using silicon photomultipliers (SiPM) are currently commercialized, namely the Philips VereosTM, the Siemens Biograph VisionTM, and the GE DiscoveryTM MI. Their performances according to the National Electrical Manufacturers Association (NEMA) NU standard have already been described [1,2,3,4]. When compared with PMT-based PET, they provide better quality time-of-flight (TOF) images [5,6,7,8,9,10], leading to enhanced diagnostic confidence and accuracy for oncologic diseases [11,12,13,14]. The TOF resolution is the key parameter in this setting and is currently measured at rather low counting rates, around 2 Mcps, as part of the daily procedures of quality control of PET cameras [19], whereas higher counting rates are typically reached in clinical routine [2]

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