Diffusion tensor imaging and axonal tracking in the human brainstem.

Diffusion tensor MRI was used to demonstrate in vivo anatomical mapping of brainstem axonal connections. It was possible to identify the corticospinal tract (CST), medial lemniscus, and the superior, medial, and inferior cerebellar peduncles. In addition, the cerebral peduncle could be subparcellated into component tracts, namely, the frontopontine tract, the CST, and the temporo-/parieto-/occipitopontine tract. Anatomical landmarks and tracking thresholds were established for each fiber and, using these standards, reproducibility of automated tracking as assessed by intra- and interrater reliability was found to be high (kappa > 0.82). Reconstructed fibers corresponded well to existing anatomical knowledge, validating the tracking. Information on the location of individual tracts was coregistered with quantitative MRI maps to automatically measure MRI parameters on a tract-by-tract basis. The results reveal that each tract has a unique spatial signature in terms of water relaxation and diffusion anisotropy.