Delso Gaspar, Martinez Maria-Jose, Torres Irene, Ladebeck Ralf, Michel Christian, Nekolla Stephan, Ziegler Sibylle I
Klinikum Rechts der Isar der Technischen Universität München, 81675 München, Germany.
Med Phys. 2009 Sep;36(9):4126-35. doi: 10.1118/1.3193676.
The combination of MR and PET scanners can provide a powerful tool for clinical diagnosis and investigation. Among the existing approaches, the most challenging is that of complete hardware integration of both scanners. Such an integrated tomograph would allow simultaneous acquisition of both modalities, which could help solve issues such as cardiac and respiratory motion. Full integration imposes restrictions on the design of the PET part, such as detector configuration and maximum ring diameter. Furthermore, MR components surrounding the PET detector ring may cause gamma ray interactions, thus affecting PET performance. The purpose of this article is to assess the performance of a hypothetical whole-body integrated MR/PET scanner using Monte Carlo simulation techniques and compare it to state-of-the-art PET/CT devices used in clinical routine. The Monte Carlo simulation toolkit used for this study is the GEANT4 application for emission tomography. A hypothetical whole-body MR/PET tomograph fully integrated at hardware level and positioned between gradient and local coils of the MR scanner has been modeled. The NEMA 2-2001 protocol has been used to configure the simulations in order to measure sensitivity, scatter fraction, count losses, and random detections. Global sensitivity values as a function of the lower-level discriminator (LLD) energy are provided for time resolutions of 5 and 2.25 ns. In addition, the scatter fraction of the system is studied as a function of the LLD for energy resolution values of 10%, 15%, and 20%. Finally, true, scatter, random, and noise equivalent count rate curves as a function of activity concentration are given for dead-time values of 136, 432, and 1150 ns and for time resolution values of 2.25 and 5 ns. The influence on the count rate performance of the integrated PET scanner of the new geometry and interfering MR elements has been measured. The results show that the interference of the MR components has a much lower impact than the reduction in the detector ring diameter. Due to the larger solid angle coverage, the sensitivity is higher than that measured for a clinical PET/CT system (6200-10 900 cps/MBq at the center of the scanner) but not enough to compensate the degradation of the noise equivalent count rate due to increased scatter detection. The simulations prove the viability of an integrated MR/PET system and suggest that priority has to be given to either the improvement of the temporal resolution or the correction of triple coincidences if competitive performance is to be achieved.
磁共振成像（MR）和正电子发射断层扫描（PET）扫描仪的结合可为临床诊断和研究提供强大工具。在现有方法中，最具挑战性的是两种扫描仪的完全硬件集成。这样的集成断层扫描仪将允许同时采集两种模态的数据，这有助于解决诸如心脏和呼吸运动等问题。完全集成对PET部分的设计施加了限制，例如探测器配置和最大环直径。此外，PET探测器环周围的MR组件可能会引起伽马射线相互作用，从而影响PET性能。本文的目的是使用蒙特卡罗模拟技术评估一种假设的全身集成MR/PET扫描仪的性能，并将其与临床常规使用的先进PET/CT设备进行比较。用于本研究的蒙特卡罗模拟工具包是GEANT4发射断层扫描应用程序。已对在硬件层面完全集成且位于MR扫描仪梯度线圈和局部线圈之间的假设全身MR/PET断层扫描仪进行了建模。使用NEMA 2-2001协议配置模拟，以测量灵敏度、散射分数、计数损失和随机检测。针对5 ns和2.25 ns的时间分辨率，给出了作为较低电平鉴别器（LLD）能量函数的全局灵敏度值。此外，针对10%、15%和20%的能量分辨率值，研究了系统的散射分数作为LLD的函数。最后，针对136 ns、432 ns和1150 ns的死时间值以及2.25 ns和5 ns的时间分辨率值，给出了作为活度浓度函数的真、散射、随机和噪声等效计数率曲线。已测量了新几何结构和干扰性MR元件对集成PET扫描仪计数率性能的影响。结果表明，MR组件的干扰影响远低于探测器环直径减小带来的影响。由于更大的立体角覆盖，灵敏度高于临床PET/CT系统测量的灵敏度（扫描仪中心处为6200 - 10900 cps/MBq），但不足以补偿由于散射检测增加导致的噪声等效计数率下降。模拟证明了集成MR/PET系统的可行性，并表明如果要实现具有竞争力的性能，必须优先提高时间分辨率或校正三重符合。
