Department of Physics, University of Zurich, Zurich, Switzerland.
Department of Physics, University of Zurich, Zurich, Switzerland; Radiotherapy Hirslanden AG, Rain 34, Aarau, Switzerland.
Z Med Phys. 2022 Feb;32(1):120-128. doi: 10.1016/j.zemedi.2020.03.002. Epub 2020 Jun 3.
Proton computed (transmission) tomography (pCT) refers to the process of imaging an object by letting protons pass through it, while measuring their energy after, and their position and (optionally) direction both before and after their traversal through that object. The so far experimental technique has potential to improve treatment planning of proton therapy by enabling the direct acquisition of a proton stopping power map of tissue, thus removing the need to obtain it by converting X-ray CT attenuation data and thereby eliminating uncertainties which arise in the mentioned conversion process. The image reconstruction in pCT requires accurate estimates of the proton trajectories. In experimental pCT detector setups where the direction of the protons is not measured, the air gap between the detector planes and the imaged object worsens the spatial resolution of the image obtained. In this work we determined the mean proton paths and the corresponding spatial uncertainty, taking into account the presence of the air gap.
We used Monte Carlo simulations of radiation transport to systematically investigate the effect of the air gap size between detector and patient on the spatial resolution of proton (ion) computed tomography for protons with an energy of 200MeV and 250MeV as well as for helium ions (He-4) with an energy of 798MeV. For the simulations we used TOPAS which itself is based on Geant4.
For all particles, which are detected at the same entrance and exit coordinate, the average ion path and the corresponding standard deviation was computed. From this information, the dependence of the spatial resolution on the air gap size and the angular confusion of the particle beam was inferred.
The presence of the airgap does not pose a problem for perfect fan beams. In realistic scenarios, where the initial angular confusion is around 5mrad and for typical air gap sizes up to 10cm, using an energy of 200MeV a spatial resolution of about 1.6mm can be achieved. Using protons with E=250MeV a spatial resolution of about 1.1mm and using helium ions (He-4) with E=798MeV even a spatial resolution below 0.7mm respectively is attainable.
质子计算机（透射）断层摄影术（pCT）是指通过让质子穿过物体来对物体进行成像的过程，同时测量它们在穿过物体后的能量，以及它们在穿过物体前后的位置和（可选）方向。到目前为止，这种实验技术有可能通过直接获取组织的质子阻止本领图来改善质子治疗的计划制定，从而无需通过转换 X 射线 CT 衰减数据来获得该图，从而消除了在上述转换过程中出现的不确定性。pCT 中的图像重建需要准确估计质子轨迹。在质子方向未被测量的实验 pCT 探测器设置中，探测器平面与被成像物体之间的气隙会降低所获得图像的空间分辨率。在这项工作中，我们考虑到气隙的存在，确定了质子路径的平均值和相应的空间不确定性。
我们使用辐射传输的蒙特卡罗模拟系统地研究了探测器与患者之间的气隙大小对质子（离子）计算机断层摄影术的空间分辨率的影响，质子能量为 200MeV 和 250MeV，氦离子（He-4）能量为 798MeV。对于模拟，我们使用了基于 Geant4 的 TOPAS。
对于在相同的进入和退出坐标处被检测到的所有粒子，计算了平均离子路径和相应的标准偏差。根据这些信息，推断了空间分辨率与气隙尺寸和粒子束的初始角混淆的关系。
气隙的存在对于完美的扇形束没有问题。在现实情况下，初始角混淆约为 5mrad，并且对于典型的气隙尺寸高达 10cm，使用能量为 200MeV，可以实现约 1.6mm 的空间分辨率。使用能量为 250MeV 的质子，可以实现约 1.1mm 的空间分辨率，使用能量为 798MeV 的氦离子（He-4）甚至可以实现低于 0.7mm 的空间分辨率。
