Department of Computer Science, Electrical Engineering and Mathematical Sciences, Western Norway University of Applied Sciences, 5020 Bergen, Norway. Department of Physics and Technology, University of Bergen, 5020 Bergen, Norway. These authors contributed equally to this work.
Phys Med Biol. 2020 Jul 9;65(13):135012. doi: 10.1088/1361-6560/ab8ddb.
List mode proton imaging relies on accurate reconstruction of the proton most likely path (MLP) through the patient. This typically requires two sets of position sensitive detector systems, one upstream (front) and one downstream (rear) of the patient. However, for a clinical implementation it can be preferable to omit the front trackers (single-sided proton imaging). For such a system, the MLP can be computed from information available through the beam delivery system and the remaining rear tracker set. In this work, we use Monte Carlo simulations to compare a conventional double-sided (using both front and rear detector systems) with a single-sided system (only rear detector system) by evaluating the spatial resolution of proton radiographs (pRad) and proton CT images (pCT) acquired with these set-ups. Both the pencil beam spot size, as well as the spacing between spots was also adjusted to identify the impact of these beam parameters on the image quality. Relying only on the pencil beam central position for computing the MLP resulted in severe image artifacts both in pRad and pCT. Using the recently extended-MLP formalism that incorporate pencil beam uncertainty removed these image artifacts. However, using a more focused pencil beam with this algorithm induced image artifacts when the spot spacing was the same as the beam spot size. The spatial resolution tested with a sharp edge gradient technique was reduced by 40% for single-sided (MTF = 3.0 lp/cm) compared to double-sided (MTF = 4.9 lp/cm) pRad with ideal tracking detectors. Using realistic trackers the difference decreased to 30%, with MTF of 4.0 lp/cm for the realistic double-sided and 2.7 lp/cm for the realistic single-sided setup. When studying an anthropomorphic paediatric head phantom both single- and double-sided set-ups performed similarly where the difference in water equivalent thickness (WET) between the two set-ups were less than 0.01 mm in homogeneous areas of the head. Larger discrepancies between the two set-ups were visible in high density gradients like the facial structures. A complete CT reconstruction of a Catphan[Formula: see text] module was performed. Assuming ideal detectors, the obtained spatial resolution was 5.1 lp/cm for double-sided and 3.8 lp/cm for the single-sided setup. Double- and single-sided pRad with realistic tracker properties returned a spatial resolution of 3.8 lp/cm and 3.2 lp/cm, respectively. Future studies should investigate the development of dedicated reconstruction algorithms targeted for single-sided particle imaging.
列表模式质子成像是基于对穿过患者的质子最可能路径（MLP）的精确重建。这通常需要两套位置灵敏探测器系统，一套在患者的上游（前），另一套在下游（后）。然而，对于临床实施，省略前探测器（单边质子成像）可能更为理想。对于这样的系统，可以从束流输送系统和剩余的后探测器组提供的信息中计算 MLP。在这项工作中，我们使用蒙特卡罗模拟通过评估使用这些设置获取的质子射线照片（pRad）和质子 CT 图像（pCT）的空间分辨率来比较传统的双边系统（同时使用前和后探测器系统）和单边系统（仅使用后探测器系统）。调整铅笔束光斑大小和光斑之间的间距，以确定这些束参数对图像质量的影响。仅依靠铅笔束中心点来计算 MLP 会导致 pRad 和 pCT 中的严重图像伪影。使用最近扩展的 MLP 形式，该形式包含铅笔束不确定性，可以消除这些图像伪影。然而，当使用此算法且光斑间距与光斑大小相同时，使用更聚焦的铅笔束会导致图像伪影。使用锐利边缘梯度技术进行测试的空间分辨率与具有理想跟踪探测器的双边（MTF = 3.0 lp/cm）相比，单边（MTF = 4.0 lp/cm）pRad 降低了 40%。使用实际的跟踪器，差异减小到 30%，具有实际双边的 MTF 为 4.0 lp/cm，实际单边的 MTF 为 2.7 lp/cm。当研究儿科头部人体模型时，单边和双边设置的性能相似，两个设置之间的水等效厚度（WET）差异在头部的均匀区域小于 0.01 毫米。在面部结构等高密度梯度区域，两个设置之间的差异更为明显。对 Catphan[Formula: see text]模块进行了完整的 CT 重建。假设理想的探测器，双边获得的空间分辨率为 5.1 lp/cm，单边为 3.8 lp/cm。具有实际跟踪器特性的双边和单边 pRad 的空间分辨率分别为 3.8 lp/cm 和 3.2 lp/cm。未来的研究应该调查针对单边粒子成像的专用重建算法的开发。
