  Acquisition-to-acquisition signal intensity variations (non-standardness) are
inherent in MR images. Standardization is a post processing method for
correcting inter-subject intensity variations through transforming all images
from the given image gray scale into a standard gray scale wherein similar
intensities achieve similar tissue meanings. The lack of a standard image
intensity scale in MRI leads to many difficulties in tissue characterizability,
image display, and analysis, including image segmentation. This phenomenon has
been documented well; however, effects of standardization on medical image
registration have not been studied yet. In this paper, we investigate the
influence of intensity standardization in registration tasks with systematic
and analytic evaluations involving clinical MR images. We conducted nearly
20,000 clinical MR image registration experiments and evaluated the quality of
registrations both quantitatively and qualitatively. The evaluations show that
intensity variations between images degrades the accuracy of registration
performance. The results imply that the accuracy of image registration not only
depends on spatial and geometric similarity but also on the similarity of the
intensity values for the same tissues in different images.
