Perineural Metastasis

This patient had known left parotid adenoid cystic carcinoma. The axial T1-weighted fat-saturated gadolinium-enhanced MR image shows enlargement and enhancement of the left facial nerve at the stylomastoid foramen (click image for an annotated version).
Adenoid cystic carcinoma is notorious for its ability to metastasise by perineural spread.
Facial nerve perineural enhancement may be a normal finding within the bony facial canal. Enhancement outside of the canal (below the skull base, within the internal auditory meatus, or in the cerebellopontine angle cistern) is always pathological. Other signs of perineural invasion are nerve thickening and widening of the bony canal. MR is more accurate when detecting perineural invasion than CT.
1. Gebarski SS, Telian SA, Niparko JK. Enhancement along the normal facial nerve in the facial canal: MR imaging and anatomic correlation. Radiology 1992;183:391-394
2. Hanna E, et al. The sensitivity and specificity of high-resolution imaging in evaluating perineural spread of adenoid cystic carcinoma to the skull base. Arch Otolaryngol Head Neck Surg 2007;133(6):541-5

Credit: Dr Laughlin Dawes