Intraparotid Facial Nerve Segmentation: A Reproducibility Study of Manual Segmentation on Neurographic Sequences

Preoperative delineation of intraparotid facial nerve using advanced MR neurography may aid surgical planning of parotid tumors, reducing complications. However, the reliability of manual segmentation for depicting the nerve pathway has not been extensively investigated. This study aimed to assess the reproducibility of manual segmentation of the intraparotid facial nerve on neurographic MRI sequences.

Neurographic MR datasets from 7 patients with unilateral benign parotid tumors were analyzed using 2 imaging sequences: double-echo steady-state (DESS) and T1 variable-flip-angle turbo spin-echo (VFA-TSE) with gadolinium. Two faculty head and neck radiologists and two radiology trainees independently segmented the intraparotid facial nerve in both tumorous and healthy glands using 3D Slicer. Given the absence of a gold standard, segmentation reliability was quantified through mean pairwise error, interobserver segmentation agreement for anatomical nerve branches, and reliability profiles along the nerve course. Comparisons by imaging sequence, gland status, and radiologist experience were performed using Wilcoxon signed-rank tests with Holm-Bonferroni correction.

The mean pairwise error was 1.58 [95% CI: 1.38-2.04] mm for the trunk, 3.20 [95% CI: 2.94-4.50] mm for the inferior division, and 3.58 [95% CI: 2.29-5.99] mm for the superior division. The VFA-TSE sequence showed lower pairwise segmentation errors than the DESS sequence, particularly for the trunk (1.26 [95% CI: 0.84-2.10] mm vs 1.85 [95% CI: 1.55-2.37] mm), although no comparisons reached statistical significance after correction for multiple testing. Nerve segments near tumors exhibited increased segmentation errors and greater interobserver variability. No significant differences were observed between radiologists of differing experience. The trunk showed high interobserver agreement, whereas peripheral branches displayed lower consistency, more frequent non-segmentation and increased subdivision instances.

Manual segmentation of the intraparotid facial nerve is reproducible for the trunk but remains challenging for peripheral branches. VFA-TSE may offer an advantage over DESS for trunk delineation, while radiologist experience level does not appear to limit segmentation reliability. Standardized segmentation protocols can mitigate the impact of prior experience, but imaging sequence choice may influence segmentation reproducibility. Further research should refine neurographic sequence and explore automated segmentation techniques to improve consistency in presurgical planning of parotid tumors.