Varadhan Raj, Magome Taiki, Hui Susanta
Department of Radiation Oncology, University of Minnesota, Minneapolis, Minnesota 55455 and Minneapolis Radiation Oncology, Minneapolis, Minnesota 55432.
Department of Radiation Oncology, Masonic Cancer Center, University of Minnesota, Minneapolis, Minnesota 55455.
Med Phys. 2016 Jan;43(1):52. doi: 10.1118/1.4937935.
Little is known about the effect of force on organ deformation and consequently its impact on precision dose delivery. The purpose of this study was to evaluate the fundamental relationship between anatomic deformation and its causative physical force to ascertain if a threshold limit exists for deformable image registration (DIR) accuracy in uniform low contrast anatomy, beyond which its applicability may be clinically inappropriate.
To simulate a simplified model, a tissue equivalent deformable bladder phantom with 21 implanted fiducial markers was developed using a viscoelastic polymer. The bladder phantom was deformed by applying a force in increments from 10 to 70 N. DIR accuracy was studied using intensity based mim and Velocity B-spline algorithms by comparing the 3D vector of the 21 marker locations at the original target image with the synthetically derived marker positions from each target image obtained from DIR.
The relationship between applied force in 1D deformation along the axis of applied force and 3D deformation of the phantom showed a linear response. The maximum and average displacements of markers exhibited a nonlinear response to the applied force. In the absence of implanted markers, DIR performance was suboptimal with a threshold limit of only 20 N (5 mm deformation) beyond which the average marker error was ≥3 mm. DIR performance improved significantly with the addition of only one marker for the intensity based mim algorithm. In contrast, the Velocity B-spline algorithm showed reduced sensitivity to the number of markers introduced in both the source and target images.
The limits of applicability of DIR are strongly dependent on the magnitude of deformation. There is a threshold limit beyond which the accuracy of DIR fails in uniform low contrast anatomy. The sensitivity of the DIR performance to the number of fiducial markers present indicates that if DIR performance is solely assessed with the contrast rich features present in clinical anatomy, the results may not be reflective of the true DIR performance in uniform low contrast anatomy.
关于力对器官变形的影响以及由此对精确剂量输送的影响，目前了解甚少。本研究的目的是评估解剖变形与其成因物理力之间的基本关系，以确定在均匀低对比度解剖结构中，可变形图像配准（DIR）准确性是否存在阈值限制，超过该阈值其临床应用可能不合适。
为模拟简化模型，使用粘弹性聚合物制作了一个带有21个植入基准标记的组织等效可变形膀胱模型。通过以10至70 N的增量施加力使膀胱模型变形。使用基于强度的mim和Velocity B样条算法，通过比较原始目标图像上21个标记位置的三维向量与从DIR获得的每个目标图像中合成得出的标记位置，研究DIR准确性。
沿施加力轴的一维变形中施加的力与模型的三维变形之间的关系呈线性响应。标记的最大和平均位移对施加的力呈非线性响应。在没有植入标记的情况下，DIR性能次优，阈值限制仅为20 N（5 mm变形），超过该值平均标记误差≥3 mm。对于基于强度的mim算法，仅添加一个标记后DIR性能显著改善。相比之下，Velocity B样条算法对源图像和目标图像中引入的标记数量的敏感性降低。
DIR的适用范围强烈依赖于变形的大小。存在一个阈值限制，超过该限制，DIR在均匀低对比度解剖结构中的准确性会下降。DIR性能对基准标记数量的敏感性表明，如果仅根据临床解剖结构中存在的富含对比度的特征来评估DIR性能，结果可能无法反映DIR在均匀低对比度解剖结构中的真实性能。
