Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
Department of Biomedical Physics in Radiation Oncology, Deutsches Krebsforschungszentrum (DKFZ), Heidelberg, Germany.
Med Phys. 2023 Apr;50(4):2336-2353. doi: 10.1002/mp.16258. Epub 2023 Feb 18.
Particle imaging can increase precision in proton and ion therapy. Interactions with nuclei in the imaged object increase image noise and reduce image quality, especially for multinucleon ions that can fragment, such as helium.
This work proposes a particle imaging filter, referred to as the Prior Filter, based on using prior information in the form of an estimated relative stopping power (RSP) map and the principles of electromagnetic interaction, to identify particles that have undergone nuclear interaction. The particles identified as having undergone nuclear interactions are then excluded from the image reconstruction, reducing the image noise.
The Prior Filter uses Fermi-Eyges scattering and Tschalär straggling theories to determine the likelihood that a particle only interacts electromagnetically. A threshold is then set to reject those particles with a low likelihood. The filter was evaluated and compared with a filter that estimates this likelihood based on the measured distribution of energy and scattering angle within pixels, commonly implemented as the 3σ filter. Reconstructed radiographs from simulated data of a 20-cm water cylinder and an anthropomorphic chest phantom were generated with both protons and helium ions to assess the effect of the filters on noise reduction. The simulation also allowed assessment of secondary particle removal through the particle histories. Experimental data were acquired of the Catphan CTP 404 Sensitometry phantom using the U.S. proton CT (pCT) collaboration prototype scanner. The proton and helium images were filtered with both the prior filtering method and a state-of-the-art method including an implementation of the 3σ filter. For both cases, a dE-E telescope filter, designed for this type of detector, was also applied.
The proton radiographs showed a small reduction in noise (1 mm of water-equivalent thickness [WET]) but a larger reduction in helium radiographs (up to 5-6 mm of WET) due to better secondary filtering. The proton and helium CT images reflected this, with similar noise at the center of the phantom (0.02 RSP) for the proton images and an RSP noise of 0.03 for the proposed filter and 0.06 for the 3σ filter in the helium images. Images reconstructed from data with a dose reduction, up to a factor of 9, maintained a lower noise level using the Prior Filter over the state-of-the-art filtering method.
The proposed filter results in images with equal or reduced noise compared to those that have undergone a filtering method typical of current particle imaging studies. This work also demonstrates that the proposed filter maintains better performance against the state of the art with up to a nine-fold dose reduction.
粒子成像是提高质子和离子治疗精度的一种手段。与成像物体中的原子核相互作用会增加图像噪声并降低图像质量，尤其是对于可以发生碎裂的多核离子，例如氦。
本工作提出了一种粒子成像滤波器，称为先验滤波器（Prior Filter），它基于使用相对阻止本领（RSP）图形式的先验信息和电磁相互作用原理，来识别发生过核相互作用的粒子。然后将被识别为发生过核相互作用的粒子从图像重建中排除，从而降低图像噪声。
先验滤波器使用费米-埃杰斯散射和 Tschalär 展宽理论来确定粒子仅发生电磁相互作用的可能性。然后设置一个阈值来拒绝那些可能性较低的粒子。该滤波器经过评估，并与一种根据像素内能量和散射角的测量分布来估计这种可能性的滤波器（通常作为 3σ 滤波器实现）进行了比较。使用质子和氦离子对 20cm 水圆柱和人体胸部模拟体的模拟数据生成了射线照片，以评估滤波器对噪声降低的影响。该模拟还允许通过粒子历史记录评估去除次级粒子。使用美国质子 CT（pCT）合作原型扫描仪采集了 Catphan CTP404 感光体 phantom 的实验数据。使用先验滤波方法和包括实现 3σ 滤波器的最先进方法对质子和氦图像进行了滤波。对于这两种情况，还应用了专为这种类型的探测器设计的 dE-E 望远镜滤波器。
质子射线照片显示噪声略有降低（1mm 水等效厚度[WET]），但氦射线照片的噪声降低幅度更大（高达 5-6mm 的 WET），因为更好的二级过滤。质子和氦 CT 图像反映了这一点，对于质子图像，在 phantom 中心具有相似的噪声（0.02 RSP），而对于提出的滤波器为 0.03 RSP，对于 3σ 滤波器为 0.06 RSP。对于剂量减少高达 9 倍的数据进行重建，使用先验滤波器的图像保持比最先进的滤波方法更低的噪声水平。
与当前粒子成像研究中典型的滤波方法相比，所提出的滤波器可获得具有同等或更低噪声的图像。这项工作还表明，与最先进的方法相比，在剂量降低多达 9 倍的情况下，所提出的滤波器仍能保持更好的性能。
