Negative-contrast neurography: Imaging the extracranial facial nerve and its branches using contrast-enhanced variable flip angle turbo spin echo MRI
Background and Purpose: Various 'positive-contrast' neurographic methods have been investigated for imaging the extracranial course of the facial nerve. However, nerve visibility can be inconsistent with these sequences and may depend on the composition of the parotid gland, limiting consistent identification. To address this, we describe and evaluate a 'negative-contrast' method for imaging of the extracranial facial nerve using three-dimensional variable flip angle turbo spin echo (VFA-TSE) imaging. We investigate strategies for further optimization, including parotid-specific VFA-TSE optimization and the use of gadolinium-based contrast agent (GBCA). Materials and Methods: 6 healthy volunteers and 10 patients with parotid tumors underwent VFA-TSE and double echo steady state (DESS) imaging of the extracranial facial nerve at 3T. The main trunk, divisions and branches of the extracranial facial nerve were manually segmented by three radiologists, enabling CNR and Hausdorff distance computation and confidence scoring. CNR, Hausdorff distance and confidence scores were compared between sequences and between pre- and post-contrast imaging to evaluate the effect of GBCA. Results: CNR, Hausdorff distances and confidence scores were superior for VFA-TSE compared to DESS imaging. GBCA administration produced a further increase in CNR of nerve against parotid and improved differentiation of nerve from tumor. Conclusion: Imaging of the extracranial facial nerve with VFA-TSE depicts the nerve as a low signal structure ('black nerve') against the high signal parotid parenchyma ('white parotid') and outperforms positive-contrast DESS imaging in terms of CNR, segmentation consistency and confidence. GBCA further increases negative contrast and improves differentiation of nerve from tumor.