Facial nerve tractography: A new tool to detect perineural invasion in parotid cancers

Abstract

Purpose To determine whether magnetic resonance (MR) tractography of facial nerve (VIIn) is useful to detect perineural invasion in case of parotid cancers. Materials and methods Thirty patients, aged 18 or over, were enrolled in this prospective study between December 2013 and March 2016 (IRB 5891), with a diagnosis of parotid tumors (15 malignant, 15 benign). They have undergone preoperative MR scans (3.0–T) before surgical management. The parameters of the diffusion–weighted (DW) sequence were b–value of 1000s/mm 2 ; 32 directions; voxel size: 2 mm isotropic; scan time: 9′ The post–processing generates whole–neck constrained spherical deconvolution (CSD)-based tractography data and super–resolution track–weighted imaging (TWI) technique. Two radiologists independently calculated the average path length value (APV) and fractional anisotropy (FA) for each VIIn. A Kruskal–Wallis test and a Bland–Altman graph were performed for the statistical analyses. Results APV of facial nerves associated with malignant tumors with neural invasion ( n = 7) were significantly higher (APV = 57.06, P n = 8; APV = 18.54), benign tumors ( n = 15; APV = 18,33) and healthy nerves ( n = 15; APV = 15.90). There was no significant difference concerning the FA. The inter–reader agreement between the two readers was excellent. Moreover, the new TWI maps provided a MRI contrast highlighting the VIIn as longer and enlarged in clinical and infra–clinical tumoral invasion than in other cases. Conclusion Whole–neck CSD-based facial nerve tractography helps to detect perineural invasion in case of parotid cancers.