Water diffusion changes in Wallerian degeneration and their dependence on white matter architecture

This study investigates water diffusion changes in Wallerian degeneration. We measured indices derived from the diffusion tenser (DT) and TB-weighted signal intensities in the descending motor pathways of patients with small chronic lacunar infarcts of the posterior limb of the internal capsule on o ne side. We compared these measurements in the healthy and lesioned sides a t different levels in the brainstem caudal to the primary lesion. We found that secondary white matter degeneration is revealed by a large reduction i n diffusion anisotropy only in regions where fibers are arranged in isolate d bundles of parallel fibers, such as in the cerebral peduncle. In regions where the degenerated pathway crosses other tracts, such as in the rostral pens, paradoxically there is almost no change in diffusion anisotropy, but a significant change in the measured orientation of fibers. The trace of th e diffusion tenser is moderately increased in all affected regions. This al lows one to differentiate secondary and primary fiber loss where the increa se in trace is considerably higher. We show that DT-MRI is more sensitive t han TS-weighted MRI in detecting Wallerian degeneration. Significant diffus ion abnormalities are observed over the entire trajectory of the affected p athway in each patient. This finding suggests that mapping degenerated path ways noninvasively with DT-MRI is feasible. However, the interpretation of water diffusion data is complex and requires a priori information about ana tomy and architecture of the pathway under investigation. In particular, ou r study shows that in regions where fibers cross, existing DT-MRI-based fib er tractography algorithms may lead to erroneous conclusion about brain con nectivity.