High-resolution mechanical imaging of glioblastoma by multifrequency magnetic resonance elastography

Abstract

Objective: To generate high-resolution maps of the viscoelastic properties of human brain parenchyma for presurgical quantitative assessment in glioblastoma (GB).

Methods: Twenty-two GB patients underwent routine presurgical work-up supplemented by additional multifrequency magnetic resonance elastography. Two three-dimensional viscoelastic parameter maps, magnitude |G*|, and phase angle φ of the complex shear modulus were reconstructed by inversion of full wave field data in 2-mm isotropic resolution at seven harmonic drive frequencies ranging from 30 to 60 Hz.

Results: Mechanical brain maps confirmed that GB are composed of stiff and soft compartments, resulting in high intratumor heterogeneity. GB could be easily differentiated from healthy reference tissue by their reduced viscous behavior quantified by φ (0.37±0.08 vs. 0.58±0.07). |G*|, which in solids more relates to the material's stiffness, was significantly reduced in GB with a mean value of 1.32±0.26 kPa compared to 1.54±0.27 kPa in healthy tissue (P = 0.001). However, some GB (5 of 22) showed increased stiffness.

Conclusion: GB are generally less viscous and softer than healthy brain parenchyma. Unrelated to the morphology-based contrast of standard magnetic resonance imaging, elastography provides an entirely new neuroradiological marker and contrast related to the biomechanical properties of tumors.

Figure 1. Example illustrating how the regions of interest (ROI) are defined one slice of the MRE magnitude image of patient #10: healthy tissue (yellow), tumor (black dotted line), perifocal margin (red dashed line), and edema (black solid line).

The green line demarcates the region chosen for homogeneous appearing matter (HAM).

Figure 2. Anatomical T2-weighted images (T2w), MRE magnitude images, and 3DMMRE parameter maps (|G*| and φ) of 2 GB patients (upper row: patient #15, bottom row: patient #14, corresponding to the tables).

The selected regions of tumor (dotted lines) and edema (solid line in #15) were used for the parameter evaluation as given in Table 2. The region of HAM is indicated by the dashed line. The yellow arrows indicate compartments of soft tissue properties (low |G*|) but different dissipative behavior (φ) in both tumors. |G*| was scaled from 0 to 3 kPa, φ was scaled from 0 to 2.5 rad.

Figure 3. Viscoelastic properties of GB based on the parameter ratios of |G*| and φ between tumor and healthy reference tissue (ref).

Numbers correspond to the list of tumors in tables 1 and 2. For illustration purposes, standard deviations (SD) indicating intratumoral heterogeneity are shown for the two tumors presented in figure 2 (solid lines in 14, and 15). All SD values are given in table 2.