Recent work has produced high-resolution Magnetic Resonance Elastography (MRE) viscoelastic parameter maps, including brain MRE at different field strengths, vibration frequencies and image resolutions [1-5]. This variability in methods caused considerable variation of stiffness estimates ranging from 1.08 kPa (at 40,50,60 Hz) [4] to 3.7 (at 45 Hz) [3] for in vivo white matter. Since brain tissue features spatial heterogeneities at multiple scales, an improved image resolution, as is attainable at ultrahigh fields, would add detail within broader shear wavelengths. We hypothesize that fine features increase wave curvature which is expected to lower wavelength-based stiffness estimates. Here we investigated the impact of field strength and image resolution on brain MRE stiffness results obtained by multifrequency dual elasto visco (MDEV) inversion [4]. A cohort of 18 healthy volunteer subjects was scanned at 1.5T, 3T and 7T using the protocol described in [4]. We used three drive frequencies of 30, 40 and 50 Hz. The 1.5T and 3T scans had isotropic voxel resolution of 2mm (20 contiguous transversal slices). The 7T scan had isotropic voxel resolution of 1mm (42 contiguous transversal slices). To investigate the impact of feature fineness apart from field strength, a fourth data set was evaluated in which the 7T set was downsampled to 2mm isotropic voxels using the Matlab function 'imresize' (MathWorks, Natick, MA, USA). Images were phase unwrapped using the Laplacian based technique in [7] and denoised using divergence-free wavelets [8] which were thresholded with SureShrink using median absolute deviation noise estimates, and complex dualtree wavelets [9] thresholded with Overlapping Group Sparsity [10] using a Principal Component Analysis-based blind noise estimation technique [11]. For measurements, the cortex was thresholded by measuring the mean and standard deviation (SD) of a pure noise region in each image, then masking all values below two SD's above the mean noise level. Mean and SD of each masked volume were measured. Example slices and histograms of these images were also examined. Each of the four cohorts was tested for significance of field strength effect using the Wilcoxon signed rank test.

Plots of mean and SD for each subject at each field strength are shown in Figure 1. Means across the cohort for 1.5T, 3T and 7T_2mm resampling were 1634 Pa (+/-613), 1743 Pa (+/-811) and 1786 (+/-634) Pa respectively, while mean for the original 7T 1mm resolution was 927 (+/-364) Pa. In the paired sign-rank tests, there were no significant effects for field strength between any of the three 2mm acquisitions (p values of 0.13 for 1.5T vs 3T, 0.84 for 1.5T vs. 7T_2mm, and 0.38 for 3T vs. 7T_2mm) while the impact of 1mm vs. 2mm voxel edge lengths was significant with p < 0.001 in all cases.

An example slice from each field strength for a single subject chosen at random is in Figure 2. Below it are the histograms for the masked brain region in each case. It can be seen that while the three 2mm voxel edge length histograms are roughly symmetrical, the 7T 1mm histogram is qualitatively different with heavy right skew.

In agreement to previous work, our study shows no significant impact of field strength on stiffness estimation at identical voxel size [6]. The impact of a 1mm isotropic resolution on stiffness values, however, is highly significant. Examination of histograms of example slices suggests that a different distribution of features is being captured at the higher resolution, supporting the hypothesis that the additional fine viscoelastic features detected at the higher field strength create more tortuosity in the wave, lowering a wavelength-based stiffness estimate. In conclusion, the MDEV inversion protocol appears robust to field strength, with lower elastogram values in higher-resolution elastograms likely a function of finer detail in the image creating more curvature in the shear waves.