Fast prospective registration of in vivo MR images of trabecular bone microstructure in longitudinal studies.

In micro-MRI studies of trabecular bone designed to evaluate structural changes in response to intervention, follow-up scan volumes do not typically align exactly with the baseline scan volumes due to the orientation and placement of the anatomic location, here the distal tibia, relative to the scanner coordinates. Failure of accurate registration of the follow-up to the baseline images introduces errors due to the inherent anisotropy in the trabecular network and anisotropic voxel size. In this work it is shown that these limitations can be overcome by incorporating on-line prospective registration into the data acquisition protocol. The technique is based on a short 3D localizer scan of 1 mm isotropic resolution prior to acquiring the high-resolution images. During the follow-up exam, localizer images are registered on-site with an algorithm relying on a fast Fourier transform for maximizing the correlation between baseline and follow-up localizers. Transformation parameters obtained in this manner are then fed into the scanner software so that the imaging slab for the high-resolution follow-up images is automatically positioned consistent with that of the baseline scan. Based on phantom and human subject studies it is shown that prospective registration yields very close matching between baseline and follow-up imaging volumes.