A metric for evaluation of deformable image registration

We propose a new metric, local uncertainty (LU), for the evaluation of deformable image registration (DIR) for dose accumulation in radiotherapy. LU measures the uncertainty of placement of each voxel in an image set after a DIR. The underlying concept of LU is that the distance between a focused voxel and a surrounding voxel on an image feature such as an edge is unchanged locally when the organ that includes these voxels is deformed. A candidate for the focused voxel after DIR can be calculated from three surrounding voxels and their distances. The positions of the candidates of the focused voxel calculated from several groups of any three surrounding voxels would vary. The variation of candidate positions indicates uncertainty of the focused voxel position. Thus, the standard deviation of candidate positions is treated as an LU value. The LU can be calculated in uniform signal regions. Assessment of DIR results in such regions is important for dose accumulation. The LU calculation was applied to a pair of computed tomography (CT) head and neck examinations after DIR. These CT examinations were for the initial radiotherapy planning and re-planning for a treatment course where the tumor underwent shrinkage during treatment. We generated an LU image showing high LU values in the shrinking tumor region and low LU values in undeformable bone. We have proposed the LU as a new metric for DIR