Establishment of multifrequency magnetic resonance elastography for adrenal gland imaging: feasibility and reproducibility for assessing stiffness and fluidity in the healthy adrenal gland

Abstract

Background: The adrenal glands are small endocrine organs located in the abdominal cavity, playing a critical role in hormone production. However, they have not been studied using magnetic resonance elastography (MRE), a technique known for its sensitivity to viscoelastic differences in various abdominal organs. While MRE has been successfully applied to assess tissue stiffness in larger organs, its application to the adrenal glands remains unexplored. The purpose of this study is to establish multifrequency MRE for the non-invasive quantification of healthy adrenal viscoelasticity, with the goal of providing reference values that could be used to assess adrenal masses in future research.

Methods: In this prospective cross-sectional study, conducted from December 2020 to November 2021 at a tertiary care academic center, we analyzed the adrenal glands of 15 healthy outpatient participants, who were enrolled through convenience sampling, using multifrequency MRE at 3 T. Tomoelastography post-processing was applied at frequencies of 30, 40, 50 and 60 Hz using shear wave speed (SWS) in m/s as a surrogate for stiffness and loss angle of the complex shear modulus in rad for fluidity. Statistical analysis included the coefficient of repeatability (CR) to assess repeatability in a subgroup of twelve participants, intraclass correlation coefficient (ICC) to evaluate interobserver variability between analyses from two independent observers, and the two-tailed Wilcoxon test to evaluate lateral and sex differences as well as the relative increase in sharpness of stiffness maps after applying motion correction. Additionally, correlation analyses were performed to examine the relationship between loss angle and body mass index (BMI).

Results: Multifrequency MRE was 100% feasible. Total median SWS and loss angle with their interquartile range (IQR) were 1.31 (1.24-1.33) m/s and 0.84 (0.79-0.91) rad with very good repeatability (CR =0.05/ICC =0.89) and (CR =0.12/ICC =0.73), respectively. Motion correction resulted in a relative increase in SWS sharpness of 12%±12% (P=0.04). Loss angle was negatively correlated with BMI (r=-0.55; P=0.04). No lateral or sex differences were observed.

Conclusions: Multifrequency MRE provides direct analysis of viscoelasticity of the adrenal glands on high-resolution elastograms with an excellent repeatability, providing direct analysis of small organs such as the adrenal glands directly on the elastograms. Our study provides first reference values of viscoelasticity of the adrenal gland.

Keywords: Magnetic resonance elastography (MRE); adrenal gland; shear wave speed (SWS); stiffness; viscoelasticity.

Figure 1

Representative MRI magnitude image and shear wave speed map (elastogram) of a healthy volunteer. The left adrenal gland is magnified in the yellow boxes and marked with a yellow arrow in both images. MRI, magnetic resonance imaging; SWS, shear wave speed.

Figure 2

Bland-Altman plots of repeatability. Presenting differences of two measures: (A) shear wave speed (stiffness) and (B) loss angle of the complex shear modulus (fluidity). n=15. Dotted line (blue) = mean, upper dashed line (23) =+1.96 SD, lower dashed line (23) =−1.96 SD. SD, standard deviation.

Figure 3

Bland-Altman plot of interobserver agreement. Presenting differences of two observers: (A) shear wave speed (stiffness) and (B) loss angle of the complex shear modulus (fluidity). n=15. Dotted line (blue) = mean, upper dashed line (23) =+1.96 SD, lower dashed line (23) =−1.96 SD. SD, standard deviation.

Figure 4

Correlation of the variables age and BMI with stiffness and fluidity. (A) Stiffness, with SWS as a surrogate, does not correlate with age. (B) Stiffness does not correlate with BMI. (C) Fluidity does not correlate with age. (D) Fluidity is negatively correlated with BMI. BMI, body mass index; SWS, shear wave speed.

Figure 5

Visualisations demonstrating the impact of 2D motion correction on MRI of the adrenal gland. (A) Representative MRI magnitude and elastogram without motion correction. (B) Representative MRI magnitude and elastogram with 2D motion correction. The healthy left adrenal gland is marked with a yellow arrow in each image. (C) A relative increase in sharpness of 12%±12% was determined after motion correction (P=0.04). 2D, 2-dimensional; MRI, magnetic resonance imaging; SWS, shear wave speed.