3D double-echo steady-state with water excitation MR imaging of the intraparotid facial nerve at 1.5T: a pilot study

Abstract

Background and purpose: The intraparotid facial nerve is difficult to delineate using conventional MR sequence. Our aim was to assess the value of 3D DESSWE MR imaging in depicting the normal anatomy of the intraparotid facial nerve.

Materials and methods: A 3D-DESSWE sequence was performed with optimum parameters in 18 healthy volunteers on a 1.5T MR imaging unit. The data obtained were reconstructed in relation to the course of the facial nerve by using the MPR and thin-section MIP programs. Images acquired were analyzed by 2 neuroradiologists. They made the initial evaluations independently but resolved inconsistencies by collaborative review and consensus agreement. The certainty of identifying the intraparotid facial nerve was scored and recorded on an arbitrary scale of 0-2. The lengths of the facial nerves were measured; CNRs of the facial nerves and parotid ducts were calculated. Statistical evaluation of the results was achieved by the 2-tailed Wilcoxon test, ANOVA, and a paired t test.

Results: In all subjects, 3D DESSWE images provided a reliable definition of the normal facial nerve anatomy bilaterally. There were no significant differences between scores, lengths, and CNRs of bilateral facial nerves (P > .05). The intraparotid facial nerve, parotid ducts, and retromandibular vein showed high signal intensity while the surrounding soft tissue showed relatively low signal intensity. The signal intensity between the facial nerves and parotid ducts was significantly different (P < .01); but no differences could be obtained for the CNRs between the 2 structures (P > .05).

Conclusions: The 3D DESSWE sequence can display the intraparotid course of the normal facial nerve, including the relationship between the facial nerve and the parotid duct.

Figures

Fig 1.

A, DESS sequence timing diagram. The DESS sequence samples 2 types of signals: One FID (FISP) echo is sampled immediately after each radio-frequency pulse, while the remaining transverse magnetization is refocused just before the next excitation pulse to form a spin-echo (PSIF) signal intensity. B, The signal intensity of the DESS sequence is simulated as a function of an FA from 20° to 90° for the assumed area (T1 = 0–1500 ms and T2 = 0–1500 ms, with an interval of 10 ms, 150 × 150 points). C, The factor of contrast between gray and white matter is simulated as a function of FAs from 10° to 90°. D, With the average of gray/white matter (T1 = 800 ms, T2 = 90 ms) as the reference substance, the factors of several fluid contrasts are simulated as a function of FAs from 10° to 90° for DESS (blue lines) and conventional SSFP (red lines) sequences.

Fig 2.

Note that both SI and SD of the parotid ducts are higher than those of the facial nerves, but there is no obvious difference between both structures for SNR and CNR.

Fig 3.

Oblique-sagittal coronal MPR (A and B) and MIP (C and D) images of the intraparotid facial nerve demonstrate its cervicofacial division (long arrows) and small branches (short arrows). The cervicofacial division separates the deep and superficial lobes of the parotid gland.

Fig 4.

A, The axial source image shows a curved reconstruction plane (long curved line), which is behind the retromandibular vein, following the main course of the facial nerve and parotid duct. B, The curved MPR reconstructed image shows the main trunk of the facial nerve (thick arrow) and the bifurcation of the temporofacial (crossed arrow) and the cervicofacial (thin arrow) divisions, which is posterior and inferior to the retromandibular vein (curved arrow). The course of the parotid duct (arrowheads) from the hilus of the gland to the mouth is shown. C, Diagram illustrates 1 pattern of the intraparotid facial nerve branching.

Fig 5.

A and B, Axial MIP image (A) and source image (B) show the facial nerve (long arrow) and retromandibular vein (curved arrow). In the MIP image, more information concerning the branches of the parotid duct (short arrow) and lymph node (crossed arrow) is demonstrated, and the length of the facial nerve is shown to be longer than that in the source image. C and D, Sagittal MIP image (C) and axial source image (D) show that the parotid duct (long arrows) is in the anterior and inferior portion of the gland. In the sagittal MIP image, the branches of the parotid duct (short arrow) and lymph nodes (crossed arrow) are shown.