Diffusion 7T-MRI of the Facial-Vestibulocochlear Nerve Complex for Lateral Skull Base Surgery

Introduction: Vestibular schwannoma (VS) is the most common pathology in the lateral skull base, with a rising prevalence resulting in over 3,500 cases per 100,000 per year in the United States between 2004 and 2016. Management varies by tumor size; smaller tumors are typically managed with serial imaging, while larger, symptomatic tumors often require surgery. The primary goal of VS surgery is maximal tumor removal while preserving neurological function. Facial nerve palsy remains a significant concern, with large VS (> 30 mm in diameter) significantly more likely to result in facial paralysis compared to small tumors.

Recent interest in cranial nerve tractography has emerged to enhance diagnosis and inform surgical planning for complex skull base surgeries. However, current high-resolution T2-weighted MRI (hrT2) and diffusion-weighted imaging (DWI) struggle to reliably visualize the facial-vestibulocochlear complex (VII/VIII complex) in the presence of a tumor. Previous studies on posterior fossa tumors primarily used DWI with conventional single-shot echo planar imaging, which falls short in precisely locating small structures like the facial nerve preoperatively.

Our team has demonstrated the feasibility of visualizing the facial nerve by optimizing a multi-shell readout-segmented (rs) DWI sequence for 3T MRI. The use of 7T MRI offers significant advantages over 3T MRI, including the potential for higher spatial resolution, which may improve visualisation of intricate structures within the posterior fossa. However, higher magnetic field strengths can introduce imaging artefacts due to field inhomogeneity, especially near the inner ear, complicating visualization.

Our study aimed to optimize a rs-DWI protocol for 7T MRI of the posterior fossa to accurately delineate the facial–vestibulocochlear complex in healthy volunteers.

Methods: In this prospective feasibility study, rs-DWI was performed in healthy volunteers using 7T MRI. Color tissue maps (CTM) and probabilistic tractography of the cranial nerves were generated. Facial nerve segmentation was also carried out at hrT2 and verified by an attending neuroradiologist.

Results: Tractography and CTM were generated in all participants enabling the facial nerve to be accurately identified.

Conclusion: This is the first study to present an early assessment of ultility of CTM and tractography of the posterior fossa cranial nerves at 7T MRI. We optimized an rs-DWI 7T MRI protocol and developed a postprocessing pipeline to delineate the facial-vestibulocochlear complex in healthy volunteers. Leveraging the high spatial resolution of 7T MRI could enhance facial nerve identification before VS surgery, potentially reducing facial nerve injury rates and improving long-term outcomes. Subsequently, this imaging method will be validated in patients with VS and progress to prospective clinical evaluation with intraoperative validation and neuronavigation ([Fig. 1]).

Fig. 1 Initial scanning data from our research group (Shapey et al, unpublished). (a) (Left) structural (MP2RAGE) 7T and 3T scans, CISS scanning and ZOOMit diffusion-weighted MR imaging at 3T MRI. The facial nerve remains more visible on 7T than 3T; however, significant background noise is noted on 7TMP2RAGE. (b) (Right) demonstration of facial nerve tractography at 7T.

Acknowledgments

This work was supported by BrainLab.

Publication History

Article published online:
07 February 2025