gms | German Medical Science

Objective: Intraoperative identification of motor areas of the rhomboid fossa is routinely performed by direct electrical stimulation. This method, which became a standard for surgery in the floor of the 4^th ventricle, is not suitable for all cranial nerve nuclei at risk and especially problematic for not superficially located regions of interest. To expand known approaches to the brainstem and facilitate the intraoperative anatomical orientation, a cadaver study with 3D-reconstruction of relevant cranial nerve nuclei was performed.

Methods: 12-24 hours postmortem, 3D-MRI and CT datasets of two human heads were acquired without any preceding fixation. An in situ formalin fixation was performed and the brains were removed from the skull. After fixation in 10% formalin solution for 7-10 days 3D-MRI and CT datasets of the isolated brains were acquired. After further fixation and dehydration brain stem specimens were prepared and embedded in paraffin. A CT scan of the paraffin blocks was performed. The size of the specimens was measured with callipers at the different stages of fixation and preparation. The brainstem specimens were cut in slices of 50 µm and 100 µm thickness, which were then stained with cresylviolet. All slices were digitized with a high resolution scanner and imported into a dedicated graphics workstation. Volumes of interest were segmented slice by slice using the workstation. Regions of interest were three-dimensionally reconstructed as well as the brainstem volume itself.

Results: Shrinkage effects caused by fixation and embedding could be evaluated volumetrically by using MR/CT datasets and frameless stereotaxy software as well as calliper measurements. The nucleus ambiguus, dorsal motor nucleus of the vagus, nuclei of cranial nerves VI, VII and XII and the motor nucleus of the trigeminal could be identified and reconstructed three-dimensionally on both sides. Spatial relationships between these nuclei, brainstem surface and surrounding tissue could be depicted.

Conclusions: Three dimensional modelling of the brainstem and clinically relevant nuclei has the potential to become a useful tool in brainstem surgery by providing spatial information about electrophysiologically unidentifiable nuclei. For intraoperative use, the implementation of the data in frameless stereotaxy is planned.