Inversion-recovery MR elastography of the human brain for improved stiffness quantification near fluid-solid boundaries

Purpose In vivo MR elastography (MRE) holds promise as a neuroimaging marker. In cerebral MRE, shear waves are introduced into the brain, which also stimulate vibrations in adjacent CSF, resulting in blurring and biased stiffness values near brain surfaces. We here propose inversion-recovery MRE (IR-MRE) to suppress CSF signal and improve stiffness quantification in brain surface areas. Methods Inversion-recovery MRE was demonstrated in agar-based phantoms with solid-fluid interfaces and 11 healthy volunteers using 31.25-Hz harmonic vibrations. It was performed by standard single-shot, spin-echo EPI MRE following 2800-ms IR preparation. Wave fields were acquired in 10 axial slices and analyzed for shear wave speed (SWS) as a surrogate marker of tissue stiffness by wavenumber-based multicomponent inversion. Results Phantom SWS values near fluid interfaces were 7.5 +/- 3.0% higher in IR-MRE than MRE (P = .01). In the brain, IR-MRE SNR was 17% lower than in MRE, without influencing parenchymal SWS (MRE: 1.38 +/- 0.02 m/s; IR-MRE: 1.39 +/- 0.03 m/s; P = .18). The IR-MRE tissue-CSF interfaces appeared sharper, showing 10% higher SWS near brain surfaces (MRE: 1.01 +/- 0.03 m/s; IR-MRE: 1.11 +/- 0.01 m/s; P < .001) and 39% smaller ventricle sizes than MRE (P < .001). Conclusions Our results show that brain MRE is affected by fluid oscillations that can be suppressed by IR-MRE, which improves the depiction of anatomy in stiffness maps and the quantification of stiffness values in brain surface areas. Moreover, we measured similar stiffness values in brain parenchyma with and without fluid suppression, which indicates that shear wavelengths in solid and fluid compartments are identical, consistent with the theory of biphasic poroelastic media.

Automated summary

In this study, a new method IR-MRE was proposed to improve the quantification of stiffness in brain surface areas. The results showed that IR-MRE can effectively suppress CSF signal, improve the accuracy of stiffness values, and provide clearer depiction of tissue anatomy near brain surfaces.