MR-Elastography of the Parotid Gland: Feasibility and First Experiences

Purpose: Dynamic steady-state MR-Elastography has the benefit to provide full 3D information to enable a correct reconstruction of the viscoelastic parameters. Purpose of this study was to investigate the feasibility of MR-Elastography of the parotid gland.

Methods: Twenty-seven healthy volunteers (17 women, ten men; mean age, 32yrs; age range 27 to 38) were examined in supine position with a 3T whole body scanner (3T Intera; Philips Medical Systems, Best, The Netherlands) using a 6-channel SENSE head coil. The mechanical transducer was placed on the left parotid gland pushing in left-right direction at a frequency of 100Hz. MR-Elastography data's were acquired using a motion sensitized spin echo sequence with EPI readout (TR 301ms, TE 40ms, flip angle 90°, EPI factor 3). Slice orientation was transverse with a FOV of 64mm, 64² pixels resolution and a slice-thickness of 2mm. Five adjacent slices were measured. Total scan time for all three spatial displacement directions was about 9 minutes. Five volunteers were additionally evaluated post stimulation of the parotid gland with commercially available lemon juice. Reconstruction of shear modulus (Maxwell model) and shear viscosity (Voigt model) is done utilizing the technique described by Sinkus et al.

Results: Very good penetration of the mechanical waves throughout the entire parotid gland was observed in 25 of 27 volunteers. Mode-conversion of the longitudinal wave at interfaces leads to shear waves generated everywhere inside the organ. The amplitude of the penetrated wave close to the transducer was 80 to 100µm. The mean shear modulus for the whole parotid gland of all 25 volunteers was 1.37±0.35 kPa. Shear viscosity was calculated with 0.45±0.14 Pa*s. Oral stimulation leads to an increase of shear modulus and shear viscosity.

Conclusion: This work demonstrates the technical feasibility of in-vivo parotid gland MR-Elastography. The shear modulus and shear viscosity in all volunteers proved to be very homogeneous. Small standard deviation of the shear modulus of healthy parotid glands indicates that a differentiation of tumorous lesions within the parotid gland should be possible.