Schmall Jeffrey P, Karp Joel S, Werner Matt, Surti Suleman
Department of Radiology, The University of Pennsylvania, Philadelphia, PA 19104, USA.
Phys Med Biol. 2016 Jul 21;61(14):5443-5455. doi: 10.1088/0031-9155/61/14/5443. Epub 2016 Jul 1.
There is a growing interest in the design and construction of a PET scanner with a very long axial extent. One critical design challenge is the impact of the long axial extent on the scanner spatial resolution properties. In this work, we characterize the effect of parallax error in PET system designs having an axial field-of-view (FOV) of 198 cm (total-body PET scanner) using fully-3D Monte Carlo simulations. Two different scintillation materials were studied: LSO and LaBr. The crystal size in both cases was 4 × 4 × 20 mm. Several different depth-of-interaction (DOI) encoding techniques were investigated to characterize the improvement in spatial resolution when using a DOI capable detector. To measure spatial resolution we simulated point sources in a warm background in the center of the imaging FOV, where the effects of axial parallax are largest, and at several positions radially offset from the center. Using a line-of-response based ordered-subset expectation maximization reconstruction algorithm we found that the axial resolution in an LSO scanner degrades from 4.8 mm to 5.7 mm (full width at half max) at the center of the imaging FOV when extending the axial acceptance angle (α) from ±12° (corresponding to an axial FOV of 18 cm) to the maximum of ±67°-a similar result was obtained with LaBr, in which the axial resolution degraded from 5.3 mm to 6.1 mm. For comparison we also measured the degradation due to radial parallax error in the transverse imaging FOV; the transverse resolution, averaging radial and tangential directions, of an LSO scanner was degraded from 4.9 mm to 7.7 mm, for a measurement at the center of the scanner compared to a measurement with a radial offset of 23 cm. Simulations of a DOI detector design improved the spatial resolution in all dimensions. The axial resolution in the LSO-based scanner, with α = ± 67°, was improved from 5.7 mm to 5.0 mm by incorporating a two-layer DOI detector. These results characterize the maximum axial blurring for a fully open 2 m long PET scanner and demonstrate that large sensitivity gains are possible with a modest reduction in resolution when using current clinical detector technology with no DOI capability.
人们对设计和构建具有很长轴向范围的正电子发射断层扫描（PET）扫描仪的兴趣与日俱增。一个关键的设计挑战是轴向范围过长对扫描仪空间分辨率特性的影响。在这项工作中，我们使用全三维蒙特卡罗模拟，对轴向视野（FOV）为198厘米（全身PET扫描仪）的PET系统设计中视差误差的影响进行了表征。研究了两种不同的闪烁材料：硅酸镥（LSO）和溴化镧（LaBr）。两种情况下晶体尺寸均为4×4×20毫米。研究了几种不同的相互作用深度（DOI）编码技术，以表征使用具有DOI功能的探测器时空间分辨率的提高。为了测量空间分辨率，我们在成像视野中心的温暖背景中模拟点源，此处轴向视差的影响最大，并且在几个偏离中心的径向位置进行模拟。使用基于响应线的有序子集期望最大化重建算法，我们发现，当将轴向接受角（α）从±12°（对应于18厘米的轴向视野）扩展到最大±67°时，LSO扫描仪在成像视野中心的轴向分辨率从4.8毫米降至5.7毫米（半高全宽）；使用LaBr也得到了类似结果，其轴向分辨率从5.3毫米降至6.1毫米。为作比较，我们还测量了横向成像视野中由于径向视差误差导致的分辨率下降；与在距扫描仪中心径向偏移23厘米处进行的测量相比，LSO扫描仪在扫描仪中心进行测量时，横向分辨率（径向和切向方向的平均值）从4.9毫米降至7.7毫米。DOI探测器设计的模拟在所有维度上提高了空间分辨率。通过采用两层DOI探测器，基于LSO的扫描仪在α = ±67°时的轴向分辨率从5.7毫米提高到了5.0毫米。这些结果表征了全开放式2米长PET扫描仪的最大轴向模糊，并表明在使用当前不具备DOI功能的临床探测器技术时，在分辨率适度降低的情况下仍有可能大幅提高灵敏度。
