Department of Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging, RWTH Aachen University, Aachen, Germany.
Hyperion Hybrid Imaging Systems GmbH, Aachen, Germany.
Med Phys. 2022 Dec;49(12):7469-7488. doi: 10.1002/mp.16015. Epub 2022 Nov 5.
Current clinical positron emission tomography (PET) systems utilize detectors where the scintillator typically contains single elements of 3-6-mm width and about 20-mm height. While providing good time-of-flight performance, this design limits the spatial resolution and causes radial astigmatism as the depth-of-interaction (DOI) remains unknown.
We propose an alternative, aiming to combine the advantages of current detectors with the DOI capabilities shown for monolithic concepts, based on semi-monolithic scintillators (slabs). Here, the optical photons spread along one dimension enabling DOI-encoding with a still small readout area beneficial for timing performance.
An array of eight monolithic LYSO slabs of dimensions 3.9 × 32 × 19 mm was read out by a 64-channel photosensor containing digital SiPMs (DPC3200-22-44, Philips Digital Photon Counting). The position estimation in the detector's monolithic and DOI direction was based on a calibration with a fan beam collimator and the machine learning technique gradient tree boosting (GTB).
We achieved a positioning performance in terms of mean absolute error (MAE) of 1.44 mm for the monolithic direction and 2.12 mm for DOI considering a wide energy window of 300-700 keV. The energy resolution was determined to be 11.3%, applying a positional-dependent energy calibration. We established both an analytical and machine-learning-based timing calibration approach and applied them for a first-photon trigger. The analytical timing calibration corrects for electronic and optical time skews leading to 240 ps coincidence resolving time (CRT) for a pair of slab-detectors. The CRT was significantly improved by utilizing GTB to predict the time difference based on specific training data and applied on top of the analytical calibration. We achieved 209 ps for the wide energy window and 198 ps for a narrow selection around the photopeak (411-561 keV). To maintain the detector's sensitivity, no filters were applied to the data during processing.
Overall, the semi-monolithic detector provides attractive performance characteristics. Especially, a good CRT can be achieved while introducing DOI capabilities to the detector, making the concept suitable for clinical PET scanners.
目前的临床正电子发射断层扫描（PET）系统使用探测器，其中闪烁体通常包含宽度为 3-6 毫米、高度约 20 毫米的单个元件。虽然提供了良好的飞行时间性能，但这种设计限制了空间分辨率，并导致径向像差，因为深度-交互（DOI）仍然未知。
我们提出了一种替代方案，旨在结合当前探测器的优势和单片概念显示的 DOI 功能，基于半单片闪烁体（平板）。在这里，光光子沿着一个维度扩散，从而可以使用仍然很小的读出面积进行 DOI 编码，这有利于定时性能。
一组尺寸为 3.9×32×19 毫米的 8 个单片 LYSO 平板由一个包含数字硅光电倍增管（DPC3200-22-44，Philips Digital Photon Counting）的 64 通道光电传感器读出。探测器的单片和 DOI 方向的位置估计基于扇形束准直器的校准和机器学习技术梯度提升树（GTB）。
我们在考虑 300-700keV 的宽能窗的情况下，在单片方向上实现了平均绝对误差（MAE）为 1.44 毫米的定位性能，在 DOI 方向上实现了 2.12 毫米的定位性能。通过应用位置相关的能量校准，确定了能量分辨率为 11.3%。我们建立了基于分析和基于机器学习的定时校准方法，并将其应用于第一光子触发。分析定时校准校正了电子和光学时间偏移，导致一对平板探测器的 240ps 符合分辨时间（CRT）。通过利用 GTB 根据特定训练数据预测时间差，并在分析校准的基础上应用，大大提高了 CRT。对于宽能窗，我们实现了 209ps，对于光峰（411-561keV）周围的窄选择，我们实现了 198ps。为了保持探测器的灵敏度，在处理数据时没有应用任何滤波器。
总体而言，半单片探测器提供了有吸引力的性能特征。特别是，在向探测器引入 DOI 功能的同时，可以实现良好的 CRT，使该概念适用于临床 PET 扫描仪。
