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Mapping the facial nerve beyond the skull base: A comparative study of DESS and SPACE MRI sequences
May Yi Shan Lim1, Edward Zhang1, Constance Ee Hoon Teo1
1Hospital, Radiological Sciences Academic Clinical Program, SingHealth-Duke-NUS Academic Medical Centre, Singapore; Otorhinolaryngology - Head And Neck Surgery (E.Z., C.E.H.T, C.M.L., I.J.), Radiography (P.H.C.), Singapore General Hospital and Division of Neuroradiology (J.P.G.), Mass General Brigham, Boston, MA, USA.
Objectives:
Accurate mapping of the extra-cranial facial nerve is crucial in determining parotid tumor involvement and in pre-operative surgical planning. The study's goal is to evaluate the extent to which the DESS and SPACE MRI sequences can image the nerve.
Materials And Methods:
MRI parotid facial nerve scans of 11 patients (12 normal parotid glands) in a quaternary hospital were retrospectively analyzed. T1 SPACE pre and post contrast, T2 SPACE, and DESS MRI sequences were acquired. The visibility of the facial nerve trunk, temporofacial and cervicofacial divisions, temporal, zygomatic, buccal, marginal mandibular and cervical branches, as well as the facial nerve (FN) trunk to adjacent parotid parenchymal signal intensity ratio (SIR) and contrast-to-noise ratio (CNR) were evaluated. The data was analyzed with parametric and non-parametric tests.
Results:
The median SIR and CNR for the FN trunk to adjacent parotid parenchyma in ascending order are T2 SPACE, T1 SPACE post contrast, T1 SPACE non-contrast and DESS. Statistical significance was achieved for the CNR (p=0.007), in particular when comparing between T2 SPACE and DESS sequences which had the greatest difference in CNR (p=0.005). Statistical significance was not achieved for the SIR (p=0.32). T1 SPACE post contrast sequences had the highest recorded number of visible FN parts, with a median of 6 (75%) visible parts (IQR 6.0-7.0; 75.0-87.5%), followed by the DESS sequence, with median of 4 (50%) visible parts (IQR 3.0-7.0; 37.5-87.5%). Evaluating DESS and T1 SPACE sequences in tandem resulted in a median of 6 (75%) visible FN parts (IQR 6.0-8.0; 75.0-100.0%), with statistical improvement when compared to DESS alone (p=0.03) but not when compared to T1 SPACE (p>0.05).
Conclusion:
T1 SPACE post contrast is superior in delineating the extra-cranial facial nerve parts. Evaluating the two best MRI sequences, T1 SPACE and DESS, in tandem improves the performance of DESS alone but not when compared to T1 SPACE alone.
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