National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, China.
Hematology & Oncology Department, Hefei First People's Hospital, Hefei, China.
J Appl Clin Med Phys. 2023 Nov;24(11):e14115. doi: 10.1002/acm2.14115. Epub 2023 Aug 13.
To evaluate the errors caused by metal implants and metal artifacts in the two-dimensional entrance fluences reconstructed using the back-projection algorithm based on electronic portal imaging device (EPID) images.
The EPID in the Varian VitalBeam accelerator was used to acquire portal dose images (PDIs), and then commercial EPID dosimetry software was employed to reconstruct the two-dimensional entrance fluences based on computed tomography (CT) images of the head phantoms containing interchangeable metal-free/titanium/aluminum round bars. The metal-induced errors in the two-dimensional entrance fluences were evaluated by comparing the γ results and the pixel value errors in the metal-affected regions. We obtained metal-artifact-free CT images by replacing the voxel values of non-metal inserts with those of metal inserts in metal-free CT images to evaluate the metal-artifact-induced errors.
The γ passing rates (versus PDIs obtained without a phantom in the beam field (PDI ), 2%/2 mm) for the back-projected two-dimensional entrance fluences of phantoms containing titanium or aluminum (BP /BP ) were reduced from 92.4% to 90.5% and 90.6%, respectively, relative to the metal-free phantom (BP ). Titanium causes more severe metal artifacts in CT images than aluminum, and its removal resulted in a 0.0022 CU (median) reduction in the pixel value of BP relative to BP in the metal-affected region. Moreover, the mean absolute error (MAE) and root mean square error (RMSE) decreased from 0.0050 CU and 0.0063 CU to 0.0034 CU and 0.0040 CU, respectively (vs. BP ).
Metal implants increase the errors in back-projected two-dimensional entrance fluences, and metals with higher electron densities cause more errors. For high-electron-density metal implants that produce severe metal artifacts (e.g., titanium), removing metal artifacts from the CT images can improve the accuracy of the two-dimensional entrance fluences reconstructed by back-projection algorithms.
评估基于电子射野影像装置（EPID）图像的反向投影算法重建二维入口剂量时金属植入物和金属伪影引起的误差。
使用瓦里安 VitalBeam 加速器上的 EPID 采集电子射野影像（PDI），然后使用商业 EPID 剂量学软件基于包含可互换无金属/钛/铝圆棒的头部体模的 CT 图像重建二维入口剂量。通过比较伽马结果和受金属影响区域的像素值误差，评估二维入口剂量中的金属诱导误差。通过将无金属插入物的体素值替换为无金属 CT 图像中金属插入物的体素值，获得无金属伪影的 CT 图像，以评估金属伪影引起的误差。
与无体模置于射束场中时的 PDI（PDI）相比，含钛或铝的体模的反向投影二维入口剂量（BP）的伽马通过率（2%/2mm）分别从 92.4%降至 90.5%和 90.6%，相对于无金属体模（BP）。钛在 CT 图像中引起的金属伪影比铝更严重，其去除导致受金属影响区域的 BP 相对于 BP 的像素值降低了 0.0022 CU（中位数）。此外，平均绝对误差（MAE）和均方根误差（RMSE）分别从 0.0050 CU 和 0.0063 CU 降至 0.0034 CU 和 0.0040 CU（相对于 BP）。
金属植入物会增加反向投影二维入口剂量中的误差，电子密度较高的金属会引起更大的误差。对于产生严重金属伪影（例如钛）的高电子密度金属植入物，从 CT 图像中去除金属伪影可以提高反向投影算法重建的二维入口剂量的准确性。
