12th Annual Symposium
Physics of Cancer
Leipzig, Germany
Aug 30 - Sept 1, 2021
Invited Talk
MR Elastography of Brain Tumors
John Huston, III.
Mayo Clinic, 200 First St. SW Rochester, MN 55905, USA
Contact:  | Website
Magnetic resonance elastography (MRE) is an imaging technique for noninvasively and quantitatively assessing tissue stiffness, akin to palpation. MRE is further able to assess the mechanical properties of tissues that cannot be reached by hand including the brain. The technique is a three-step process beginning with the introduction of shear waves into the tissue of interest by applying an external vibration. Next, the resulting motion is imaged using a phase-contrast MR pulse sequence with motion encoding gradients that are synchronized to the vibration. Finally, the measured displacement images are mathematically inverted to compute a map of the estimated stiffness. In the brain, the technique has demonstrated strong test-retest repeatability with typical errors of 1% for measuring global stiffness, 2% for measuring stiffness in the lobes of the brain, and 3–7% for measuring stiffness in subcortical gray matter. In healthy volunteers, multiple studies have confirmed that stiffness decreases with age, while more recent studies have demonstrated a strong relationship between viscoelasticity and behavioral performance. Furthermore, several studies have demonstrated the sensitivity of brain stiffness to neurodegeneration, as stiffness has been shown to decrease in multiple sclerosis and in several forms of dementia.

MRE has become a very useful tool to reliably characterize brain tumor consistency and adherence, offering key information for neurosurgery planning. Surgical strategies vary depending on tumor consistency such that soft tumors generally require less invasive endoscopic, keyhole surgeries, while firm fibrotic tumors may quire more open surgical strategies. Conventional MRI has shown some success in pre-operatively identifying extreme cases of extra-axial tumor stiffness, with very soft (high T2 signal) as opposed to very stiff (low T2 signal). But grading stiffness quantitatively with standard MRI sequences is not currently possible.

Overall, MRE stiffness has been shown to preoperatively agree with a surgeon’s intraoperative assessment of intracranial tumor stiffness. The impact of brain MRE for pre-surgical assessment is anticipated to be quite high. However, as with any new diagnostic technology, care needs to be taken in understanding what artifacts and pitfalls are present in MRE stiffness maps. Image signal-to-noise, tissue inhomogeneity, edge effects, and large wavelengths in stiff tissues can impact the accuracy of stiffness estimates in tumors, making small tumors and/or heterogeneous tumors with high vascularity challenging to assess.

Tumor adhesion to normal brain tissue also has a direct impact on the difficulty of tumor resection. A MRE displacement field can also be used to generate a qualitative measure of tumor adhesion, referred to as slip interface imaging (SII). SII uses discontinuities in the displacement field to probe tissue/tumor boundaries that are slipping relative to one another as wave energy is transferred between two mediums. These slips result in relatively high spatial gradients in displacement that can be detected as high strain values in normalized octahedral strain images. Some non-slip factors such as quick changes in wavelength due to large modulus contrasts, as well as wave scattering at interfaces, can impact these measurements and need to be considered during pre-surgical planning.
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