Kuhl Yannick, Mueller Florian, Thull Julian, Naunheim Stephan, Schug David, Schulz Volkmar
Department of Physics of Molecular Imaging Systems, Institute for Experimental Molecular Imaging, RWTH Aachen University, Aachen, Germany.
Institute of Imaging and Computer Vision, RWTH Aachen University, Aachen, Germany.
Med Phys. 2025 Jan;52(1):232-245. doi: 10.1002/mp.17475. Epub 2024 Nov 6.
Light-sharing detector designs for positron emission tomography (PET) systems have sparked interest in the scientific community. Particularly, (semi-)monoliths show generally good performance characteristics regarding 2D positioning, energy-, and timing resolution, as well as readout area. This is combined with intrinsic depth-of-interaction (DOI) capability to ensure a homogeneous spatial resolution across the entire field of view (FoV). However, complex positioning calibration processes limit their use in PET systems, especially in large-scale clinical systems.
This work proposes a new 3D positioning in-system calibration method for fast and convenient (re-)calibration and quality control of assembled PET scanners. The method targets all kinds of PET detectors that achieve the best performance with individual calibration, including complex segmented detector designs. The in-system calibration method is evaluated and empirically compared to a state-of-the-art fan-beam calibration for a small-diameter proof of concept (PoC) scanner. A simulation study evaluates the method's applicability to different scanner geometries.
A PoC scanner geometry of 120 mm inner diameter and 150 mm axial extent was set up consisting of five identical finely segmented slab detectors (one detector under test and four collimation detectors). A Na point source was moved in a circular path inside the FoV. Utilizing virtual collimation and by selecting gamma rays incident approximately perpendicular to the detector normal of the detector under test, training data was created for the training of a 2D positioning model with the machine-learning technique gradient tree boosting (GTB). Data with oblique ray angles was acquired in the same measurement for subsequent angular DOI calibration. For this, a 2D position estimate in the detector under test was calculated first. On this basis, the DOI label was calculated geometrically from the ray path within the detector to finally establish up to 3D training data.
With a mean absolute error (MAE) of 0.8 and 1.19 mm full-width at half maximum (FWHM) along the planar-monolithic slab dimension, the in-system methods performed similarly within 1% to the fan-beam collimator results. The DOI performance was at ∼90% with 1.13 mm MAE and 2.47 mm FWHM to the fan-beam collimator. Analytical calculations suggest an improved performance for larger scanner geometries.
The functionality of the 3D in-system positioning calibration method was successfully demonstrated with the measurements within a PoC scanner configuration with similar positioning performance as the bench-top fan-beam setup. The in-system calibration method can be used to calibrate and test fully assembled PET systems to enable more complex light-sharing detector architectures in, for example, large PET systems with many detectors. The acquired data can further be used for more complex energy and time calibrations.
用于正电子发射断层扫描（PET）系统的光共享探测器设计引起了科学界的关注。特别是，（半）整体式探测器在二维定位、能量和时间分辨率以及读出面积方面通常表现出良好的性能特征。这与固有的相互作用深度（DOI）能力相结合，可确保在整个视野（FoV）内具有均匀的空间分辨率。然而，复杂的定位校准过程限制了它们在PET系统中的应用，尤其是在大规模临床系统中。
本工作提出了一种新的三维定位系统内校准方法，用于对组装好的PET扫描仪进行快速便捷的（重新）校准和质量控制。该方法适用于各种通过单独校准可实现最佳性能的PET探测器，包括复杂的分段探测器设计。对该系统内校准方法进行了评估，并与用于小直径概念验证（PoC）扫描仪的最先进扇形束校准进行了实证比较。一项模拟研究评估了该方法对不同扫描仪几何形状的适用性。
建立了一个内径为120mm、轴向长度为150mm的PoC扫描仪几何模型，由五个相同的精细分段平板探测器组成（一个待测探测器和四个准直探测器）。一个钠点源在FoV内沿圆形路径移动。利用虚拟准直并通过选择近似垂直于待测探测器法线入射的伽马射线，创建训练数据以使用机器学习技术梯度树提升（GTB）训练二维定位模型。在同一测量中采集具有斜射线角度的数据用于后续的角度DOI校准。为此，首先计算待测探测器中的二维位置估计值。在此基础上，从探测器内的射线路径几何计算DOI标签，最终建立多达三维的训练数据。
沿平面整体平板尺寸，系统内方法的平均绝对误差（MAE）为0.8mm，半高宽（FWHM）为1.19mm，与扇形束准直器结果的差异在1%以内，表现相似。DOI性能方面，与扇形束准直器相比，MAE为1.13mm，FWHM为2.47mm，达到约90%。解析计算表明，对于更大的扫描仪几何形状，性能会有所提高。
在PoC扫描仪配置中通过测量成功证明了三维系统内定位校准方法的功能，其定位性能与台式扇形束设置相似。该系统内校准方法可用于校准和测试完全组装好的PET系统，以在例如具有许多探测器的大型PET系统中实现更复杂的光共享探测器架构。采集到的数据还可进一步用于更复杂的能量和时间校准。
