Salvadori Julien, Odille Freddy, Karcher Gilles, Marie Pierre-Yves, Imbert Laetitia
Department of Nuclear Medicine and Nancyclotep Molecular Imaging Platform, CHRU-Nancy, Université de Lorraine, F54000, Nancy, France.
Université de Lorraine, INSERM, UMR 1254, F54000, Nancy, France.
EJNMMI Phys. 2021 Jan 6;8(1):1. doi: 10.1186/s40658-020-00344-5.
Digital 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.
Variations, 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.
Routine 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 of current F-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.
Contrary 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).
与基于光电倍增管（PMT）的正电子发射断层显像（PET）相比，采用硅光电倍增管（SiPM）的数字PET具有更高的飞行时间（TOF）分辨率，而且能更好地防止主要由于触发域较小（即与每个触发电路相关的探测表面）导致的与计数相关的死时间增加和堆积效应。本研究旨在确定后一种特性是否有助于防止临床常规中记录的广泛PET计数率范围内TOF分辨率和TOF图像质量的恶化。
比较了首款全数字PET（Vereos）和基于PMT的PET（Ingenuity）在定时分辨率以及与TOF相关的体模图像质量增强方面随计数率的变化。另外，从常规模拟和数字PET检查的列表模式数据中，以每500毫秒的间隔提取单计数率值，以确定常规PET计数率的范围。
Vereos的常规PET计数率低于Ingenuity。对于Ingenuity，在分别注射3和5 MBq.kg的当前F标记示踪剂后，上限估计约为21.7和33.2 Mcps。在5.8 Mcps时，对应于Ingenuity记录的常规计数率下限，散射体模提供的定时分辨率对于Vereos和Ingenuity分别为326和621 ps。在更高的计数率下，Vereos的定时分辨率非常稳定，而Ingenuity则逐渐恶化，在21.7和33.2 Mcps的上限分别达到732和847 ps。Vereos的平均与TOF相关的信噪比增益稳定在约2，而Ingenuity从5.8 Mcps时的1.36降至21.7和33.2 Mcps时的1.14和1.00。
与基于Ingenuity PMT的PET不同，Vereos全数字PET在广泛的常规PET计数率范围内不受TOF分辨率恶化的影响，因此也不受TOF图像质量恶化的影响。对于更高的计数率，这一优势更为明显，为此应进一步推荐优先使用数字PET（即动态PET记录、更高的注射活度）。
