Department of Radiation Oncology, Henry Ford Health System, Detroit, Michigan; Department of Radiation Oncology, Wayne State University School of Medicine, Detroit, Michigan.
Department of Radiation Oncology, University of Washington, Seattle, Washington.
Pract Radiat Oncol. 2018 Sep-Oct;8(5):342-350. doi: 10.1016/j.prro.2018.04.001. Epub 2018 Apr 6.
Recent advancements in synthetic computed tomography (synCT) from magnetic resonance (MR) imaging data have made MRI-only treatment planning feasible in the brain, although synCT performance for image guided radiation therapy (IGRT) is not well understood. This work compares geometric equivalence of digitally reconstructed radiographs (DRRs) from CTs and synCTs for brain cancer patients and quantifies performance for partial brain IGRT.
Ten brain cancer patients (12 lesions, 7 postsurgical) underwent MR-SIM and CT-SIM. SynCTs were generated by combining ultra-short echo time, T1, T2, and fluid attenuation inversion recovery datasets using voxel-based weighted summation. SynCT and CT DRRs were compared using patient-specific thresholding and assessed via overlap index, Dice similarity coefficient, and Jaccard index. Planar IGRT images for 22 fractions were evaluated to quantify differences between CT-generated DRRs and synCT-generated DRRs in 6 quadrants. Previously validated software was implemented to perform 2-dimensional (2D)-2D rigid registrations using normalized mutual information. Absolute (planar image/DRR registration) and relative (differences between synCT and CT DRR registrations) shifts were calculated for each axis and 3-dimensional vector difference. A total of 1490 rigid registrations were assessed.
DRR agreements in anteroposterior and lateral views for overlap index, Dice similarity coefficient, and Jaccard index were >0.95. Normalized mutual information results were equivalent in 75% of quadrants. Rotational registration results were negligible (<0.07°). Statistically significant differences between CT and synCT registrations were observed in 9/18 matched quadrants/axes (P < .05). The population average absolute shifts were 0.77 ± 0.58 and 0.76 ± 0.59 mm for CT and synCT, respectively, for all axes/quadrants. Three-dimensional vectors were <2 mm in 77.7 ± 10.8% and 76.5 ± 7.2% of CT and synCT registrations, respectively. SynCT DRRs were sensitive in postsurgical cases (vector displacements >2 mm in affected quadrants).
DRR synCT geometry was robust. Although statistically significant differences were observed between CT and synCT registrations, results were not clinically significant. Future work will address synCT generation in postsurgical settings.
磁共振（MR）成像数据的合成计算断层摄影术（synCT）的最新进展使得在脑部进行仅 MRI 治疗计划成为可能，尽管 synCT 用于图像引导放射治疗（IGRT）的性能尚不清楚。这项工作比较了脑癌患者 CT 和 synCT 的数字重建射线照片（DRR）的几何等效性，并量化了部分脑 IGRT 的性能。
10 名脑癌患者（12 个病灶，7 个术后）接受了 MR-SIM 和 CT-SIM。使用基于体素的加权求和方法组合超短回波时间、T1、T2 和液体衰减反转恢复数据集来生成 synCT。使用患者特定的阈值比较 synCT 和 CT DRR，并通过重叠指数、Dice 相似系数和 Jaccard 指数进行评估。评估了 22 个分次的平面 IGRT 图像，以量化 6 个象限中 CT 生成的 DRR 和 synCT 生成的 DRR 之间的差异。实施了经过验证的软件，使用归一化互信息执行 2 维（2D）-2D 刚性配准。为每个轴和 3 维向量差计算了绝对（平面图像/DRR 配准）和相对（synCT 和 CT DRR 配准之间的差异）位移。总共评估了 1490 次刚性配准。
在前后视图和侧视图中，重叠指数、Dice 相似系数和 Jaccard 指数的 DRR 一致性>0.95。75%的象限中归一化互信息结果是等效的。旋转配准结果可忽略不计（<0.07°）。在 9/18 个匹配的象限/轴中观察到 CT 和 synCT 配准之间存在统计学上的显著差异（P<0.05）。所有轴/象限的平均绝对位移分别为 CT 和 synCT 的 0.77±0.58 和 0.76±0.59mm。77.7±10.8%和 76.5±7.2%的 CT 和 synCT 配准的三维向量均<2mm。synCT DRR 对术后病例敏感（受影响象限中的向量位移>2mm）。
DRR synCT 几何形状很健壮。尽管在 CT 和 synCT 配准之间观察到统计学上的显著差异，但结果没有临床意义。未来的工作将解决术后环境中的 synCT 生成问题。
