  Being a task of establishing spatial correspondences, medical image
registration is often formalized as finding the optimal transformation that
best aligns two images. Since the transformation is such an essential component
of registration, most existing researches conventionally quantify the
registration uncertainty, which is the confidence in the estimated spatial
correspondences, by the transformation uncertainty. In this paper, we give
concrete examples and reveal that using the transformation uncertainty to
quantify the registration uncertainty is inappropriate and sometimes
misleading. Based on this finding, we also raise attention to an important yet
subtle aspect of probabilistic image registration, that is whether it is
reasonable to determine the correspondence of a registered voxel solely by the
mode of its transformation distribution.
