High-resolution MRI of the human parotid gland and duct at 7 Tesla

MR techniques have been reported as an alternative to conventional sialography. High-field systems (7 T) provide new contrasts coupled with increased signal-to-noise ratio, and hence higher spatial resolution. To our knowledge, no measurements of the parotid gland at 7 T have been reported. Therefore, our study aimed to optimize sequences for high-field MR imaging of the parotid gland and duct, as well as the facial nerve at 7 T and show the potential of high field imaging.

A 7 T whole-body scanner was used together with a 10-cm-diameter loop coil. Various GRE (MEDIC, DESS) and TSE (PD/T2, STIR) sequences were optimized and subsequently tested on 4 healthy volunteers and 4 patients. High-resolution images were compared with 1.5 T images both quantitatively (signal-to-noise ratio, contrast-to-noise) and qualitatively (visual rating of 2 independent readers).

The high 0.6 mm isotropic resolution of the 3D DESS sequence was very useful for defining an oblique orientation with most of the duct being in-plane for subsequent imaging. With the MEDIC sequence, very fine branches of the duct were visible; furthermore, MEDIC yielded a very good depiction of lymph nodes. Severe specific absorption rate problems were observed with the STIR sequence at 7 T. Gland tissue in tumor patients can be well characterized with the PD/T2 TSE. Highest contrast-to-noise between duct and gland was achieved with the 7 T DESS. At 1.5 T, only the STIR sequence showed comparable quality to the overall superiority of the 7 T sequences. The facial nerve could only be depicted close to the skull base.

MR imaging at 7 T provides excellent image contrast and resolution of the parotid gland and duct. The proposed protocol offers a noninvasive examination within about 30 minutes.