Australian e-Health Research Center, CSIRO ICT Commonwealth Scientific and Industrial Research Organisation Information and Communication Technologies Centre, Queensland, Australia.
Int J Radiat Oncol Biol Phys. 2012 May 1;83(1):e5-11. doi: 10.1016/j.ijrobp.2011.11.056. Epub 2012 Feb 11.
Prostate radiation therapy dose planning directly on magnetic resonance imaging (MRI) scans would reduce costs and uncertainties due to multimodality image registration. Adaptive planning using a combined MRI-linear accelerator approach will also require dose calculations to be performed using MRI data. The aim of this work was to develop an atlas-based method to map realistic electron densities to MRI scans for dose calculations and digitally reconstructed radiograph (DRR) generation.
Whole-pelvis MRI and CT scan data were collected from 39 prostate patients. Scans from 2 patients showed significantly different anatomy from that of the remaining patient population, and these patients were excluded. A whole-pelvis MRI atlas was generated based on the manually delineated MRI scans. In addition, a conjugate electron-density atlas was generated from the coregistered computed tomography (CT)-MRI scans. Pseudo-CT scans for each patient were automatically generated by global and nonrigid registration of the MRI atlas to the patient MRI scan, followed by application of the same transformations to the electron-density atlas. Comparisons were made between organ segmentations by using the Dice similarity coefficient (DSC) and point dose calculations for 26 patients on planning CT and pseudo-CT scans.
The agreement between pseudo-CT and planning CT was quantified by differences in the point dose at isocenter and distance to agreement in corresponding voxels. Dose differences were found to be less than 2%. Chi-squared values indicated that the planning CT and pseudo-CT dose distributions were equivalent. No significant differences (p > 0.9) were found between CT and pseudo-CT Hounsfield units for organs of interest. Mean ± standard deviation DSC scores for the atlas-based segmentation of the pelvic bones were 0.79 ± 0.12, 0.70 ± 0.14 for the prostate, 0.64 ± 0.16 for the bladder, and 0.63 ± 0.16 for the rectum.
The electron-density atlas method provides the ability to automatically define organs and map realistic electron densities to MRI scans for radiotherapy dose planning and DRR generation. This method provides the necessary tools for MRI-alone treatment planning and adaptive MRI-based prostate radiation therapy.
在磁共振成像（MRI）扫描上直接进行前列腺放射治疗剂量规划，可减少由于多模态图像配准带来的成本和不确定性。使用结合 MRI-线性加速器的自适应规划也将需要使用 MRI 数据进行剂量计算。本研究的目的是开发一种基于图谱的方法，将真实的电子密度映射到 MRI 扫描上，以进行剂量计算和数字重建射线照相（DRR）生成。
从 39 例前列腺患者中收集了全骨盆 MRI 和 CT 扫描数据。来自 2 名患者的扫描显示出与其余患者群体明显不同的解剖结构，因此排除了这 2 名患者。基于手动勾画的 MRI 扫描生成了全骨盆 MRI 图谱。此外，还从配准的 CT-MRI 扫描中生成了共轭电子密度图谱。通过将 MRI 图谱全局和非刚性配准到患者的 MRI 扫描，自动生成每个患者的伪 CT 扫描，然后将相同的变换应用于电子密度图谱。在计划 CT 和伪 CT 扫描上对 26 名患者的器官分割进行了 Dice 相似系数（DSC）比较和点剂量计算。
通过等中心点处的点剂量差异和相应体素的一致距离来量化伪 CT 和计划 CT 之间的一致性。发现剂量差异小于 2%。卡方值表明计划 CT 和伪 CT 剂量分布等效。对感兴趣的器官，在 CT 和伪 CT 体素的 Hounsfield 单位之间没有发现显著差异（p>0.9）。基于图谱的骨盆骨骼分割的平均±标准偏差 DSC 评分为 0.79±0.12，前列腺为 0.70±0.14，膀胱为 0.64±0.16，直肠为 0.63±0.16。
电子密度图谱方法能够自动定义器官，并将真实的电子密度映射到 MRI 扫描上，用于放射治疗剂量规划和 DRR 生成。该方法为 MRI 单治疗计划和基于 MRI 的自适应前列腺放射治疗提供了必要的工具。
