The intracranial anatomical distribution of white fibers has long been described from post-mortem brain dissection in latest centuries. Whereas MRI recently brought new possibilities to show the in vivo architecture of the central nervous system, diffusion tensor imaging achieved tracking of white fiber bundles connecting brain areas or projecting sensorimotor pathways. Nonetheless, tracking of small anatomical fibers like cranial nerves remains challenging. Even if they are often seen on T2 steady state sequence along their cisternal segments, their course inside the brainstem or through skull base foramina is difficult to feature. Furthermore, cranial nerves cannot be followed in case of stretching or encasement by skull base tumors. Today, we present a study of cranial nerves tractography and its potential applications in neuroanatomy and skull base surgery.

Ten patients, who presented complex skull base tumors, were evaluated by brain MRI with a 32 directions diffusion sequence and a T2 steady state sequence. Distortions due to skull base bone were corrected. Then, fiber tracking was attempted thanks to the Mtrtix3 software. The healthy side was used as control. Post-processing parameters were adapted to each cranial nerve in normal and pathologic conditions.

Tractography of cranial nerves I to XI (except IV) was performed for various complex skull base tumors: vestibular schwannomes (4), epidermoid cysts (3), meningiomas (3). Small cranial nerves as VI or VII/VIII were difficult to track when stretching around tumors or diffusion signal similar to the tumor. Fiber tracking included cisternal part of nerves but also their trajectory within the brainstem and through the skull base.

Tractography was strongly dependent of tracking parameters: design of ROI, fractional anisotropy threshold, curvature angle, number of fibers and minimal length. These have to be refined to track very small white fibers around skull base tumors or along their brainstem course.