Brain MR Elastography in the Clinic: Indications & Challenges

Katharina Schregel¹
¹Department of Neuroradiology, Heidelberg University Hospital, Heidelberg, Germany

Synopsis

Magnetic resonance elastography (MRE) is an innovative MRI technique that allows for the non-invasive quantification of biomechanical brain tissue properties. Neuro-oncologic, neuroinflammatory and neurodegenerative diseases affect the organization and composition of brain tissue and hence its biomechanical properties. Thus, MRE is a promising tool for the evaluation of brain pathologies rendering information complementary to other imaging methods. Even though research studies demonstrated the potential of MRE to improve neuroradiological diagnosis and follow-up of various brain pathologies, some challenges remain before this technique can be used in the clinical routine.

Magnetic resonance elastography (MRE) is an innovative MRI technique that allows for the non-invasive quantification of biomechanical tissue properties¹. For MRE, mechanical vibrations are externally applied to the tissue of interest and elicit compressional and shear waves. These waves propagate through the tissue and cause minimal displacement, which can be captured with a dedicated phase-contrast based MR sequence by applying motion-encoding gradients. Biomechanical properties can be calculated from the displacement field and depicted in so-called elastograms. MRE of the brain is feasible and safe in preclinical and clinical settings², but is not used as a diagnostic measure in the neuroradiological routine yet.
Neurological diseases affect the organization and composition of the brain tissue in different ways and hence influence the biomechanical properties³. MRE is therefore a unique tool for the evaluation of brain pathologies rendering information occult to other imaging methods. Biomechanical properties can complement information gained from established quantitative MRI techniques such as diffusion, perfusion or relaxometry and thus improve diagnosis and follow-up of various pathologies of the central nervous system.
A promising field of application for brain MRE is neuro-oncology. Initial studies demonstrated that tumor stiffness measured with MRE was in good agreement with the consistency evaluated manually by neurosurgeons intraoperatively^4–6. Hence, pre-operative MRE could be used for therapy planning, as the surgical approach may differ based on expected tumor stiffness. Moreover, MRE could contribute to diagnosis and characterization, as different tumor types differed in their biomechanical properties^7,8 and MRE parameters correlated with grade and molecular subtype of gliomas⁹ as well as characterized invasive behavior of tumors¹⁰. Furthermore, preclinical studies hint at a potential of MRE to capture effects of tumor treatment on the brain^11,12, which could prove useful in therapy monitoring of gliomas. Multiple sclerosis (MS), neuromyelitis optica spectrum disorders (NMOSD) and other neuroinflammatory diseases represent another interesting area for biomechanical brain evaluation. In MS, a global reduction of brain stiffness can be observed from early disease stages on^13–15. A similar accelerated softening of the brain reflecting global tissue degeneration occurs in NMOSD¹⁶. Even though separate MS lesions did not markedly differ in stiffness from surrounding brain parenchyma¹⁷, MRE could prove useful in gauging disease activity, as MRE parameters differentiated between local inflammatory processes with a strong immune cell infiltrate from disseminated inflammation and blood brain barrier leakage in a preclinical model of MS¹⁸. The neuroradiological work-up of patients with dementia could also benefit from MRE. Several studies found differences in global and regional biomechanical properties in patients with Alzheimer’s disease^19–22 and distinct patterns of softening differed between dementia subtypes²³. Interestingly, MRE can provide unique insights into structure-function relationships of the brain, as biomechanical properties seem to be associated with memory performance^24–27 or behavior²⁸. Diseases caused by disturbed cerebrospinal fluid flow are also a potential indication for MRE. Patients with normal pressure hydrocephalus present with softening of the brain²⁹ and alterations of microstructural connectivity³⁰, which improved after shunt treatment³¹. Therefore, biomechanical properties could render information on therapy effectiveness, expanding qualitative examination of ventricular size. Even though very promising, some challenges remain before MRE can be used clinically. Important aspects in this regard include a need for standardization of image acquisition as well as calculation and terminology of MRE parameters³², the necessity of diagnostic reference values or guidelines for interpretation of MRE results in different pathologies and an even broader understanding of the relation between pathophysiological mechanisms and biomechanical properties.

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)