9.1 Introduction

Recently developed magnetic resonance imaging (MRI) techniques with potential applications for the evaluation of vestibular schwannoma (VS) include MR elastography (MRE) and diffusion tensor tractography. Although these techniques remain investigational, recently published work using both techniques in the preoperative evaluation of VS appears promising.

9.2 Magnetic Resonance Elastography

MRE is an emerging MR-based imaging sequence first described by Muthupillai et als. Literatur in 1995 that utilizes mechanical waves to quantify the shear modulus, or stiffness, of a substance in kilopascal (kPa). The three elements are (1) an external vibrating driver that transmits shear waves in adjacent tissue, (2) imaging the tissue of interest with a phase contrast MRI sequence, and (3) performing mathematical inversions of the wave images to create quantitative maps of tissue stiffness (Fig. 9‑1 ).s. Literatur Essentially, MRE is a means of “palpating” a tissue with imaging and has been developed for liver, breast, skeletal muscle, heart, and brain.s. Literatur ^,​ s. Literatur The technology is most mature in hepatic applications and is replacing biopsy in many centers.s. Literatur

In the brain, MRE is being used to evaluate stiffness in both global disease such as Alzheimer’s,s. Literatur ^,​ s. Literatur multiple sclerosis,s. Literatur and normal pressure hydrocephaluss. Literatur and focal lesions such as meningioma,s. Literatur ^,​ s. Literatur ^,​ s. Literatur glioblastoma,s. Literatur ^,​ s. Literatur pituitary macroadenoma,s. Literatur ^,​ s. Literatur and VS.s. Literatur More recently, a form of MRE has been developed called slip-interface imaging (SII) to evaluate the surgical plane between extra-axial brain tumors and adjacent normal neural structures.s. Literatur SII uses the wave images of MRE to create two images, shear line and octahedral shear strain (OSS) images (Fig. 9‑2 ). Shear-line imaging works by intravoxel phase dispersion and a low signal line indicates tissue separation, while lack of this line indicates adhesion. OSS works by the maximum change in shear displacement across all possible planes and is a measurable value, though on imaging it is typically indicated by an increasing orange-to-yellow color scale. Thus, MRE has the potential to provide preoperative evaluation of both tumor stiffness and tumor-brain adhesion. These are factors previously unreliably measured that may influence surgical difficulty, extent of resection, and complications such as facial nerve outcomes in VS resection.

Lee et als. Literatur speculated that VS consistency might impact facial nerve outcome after microsurgical resection, with firm, avascular tumors being more easily removed leading to less injury. In a prospective study, Esquia-Medina et als. Literatur have shown that tumor adhesion affects facial nerve outcomes; tumors with strong adhesion had poorer House-Brackmann scores.

However, no study has evaluated MRI characteristics for adhesion and only one study has evaluated for consistency in VS. Copeland et als. Literatur found that soft tumors tended to be T2 hyperintense, while firm tumors tended to be T2 hypointense. There was no correlation with tumor consistency and extent of resection or facial nerve outcomes. However, they preselected tumors that were very soft or hard at surgery; thus, the MRI findings may not apply for tumors that fall between those extremes.

Since MRE investigations have only recently been available to evaluate focal intracranial lesions, experience for evaluating consistency in VS is limited. In a mixed series including pituitary macroadenoma, meningioma, and glioblastoma, Sakai et als. Literatur reported on MRE stiffness in six patients with VS finding a mean shear stiffness range of 1.7 to 2.5 kPa and a maximum shear stiffness range of 1.7 to 3.7 kPa. They speculate MRE measurements might correlate with Antoni patterns on pathologic analysis.

At our institution, we have evaluated 11 VS and correlated MRE measurements for both consistency and adhesions. Literatur with surgical findings, extent of resection, and facial nerve outcome. Regarding consistency (Fig. 9‑3 ), surgeons categorized five VSs as soft, four as intermediate, and two as firm. For each category, mean MRE measurements increased (3.09 ± 0.58 [2.39–3.95], 3.17 ± 0.48 [2.58–3.74], and 3.5 ± 0.39 [3.26, 3.80] kPa, respectively), but the increases were not statistical significant (ANOVA, p = 0.633). Two soft tumors had MRE values greater than 3.0 kPa, but were vascular, a characteristic that may generate misleading MRE values in meningiomas. Literatur and liver.s. Literatur Regarding tumor adherence, surgeons categorized five tumors as having complete, three as partial, and three as no separation. SII, using OSS as it correlates better than shear-line imaging,s. Literatur categorized seven tumors as complete, three as partial, and one as no separation (κ = 0.70; 95% CI: 0.35–1.00; Fig. 9‑4 ). Both tumors with discrepancy between SII and surgical findings had a cyst in the posterior portion of the tumor, a feature that may lead to poor correlation due to intratumoral motion in a heterogeneous lesion.s. Literatur There was no correlation with MRE values or SII analysis with regard to extent of resection or facial nerve outcomes.

Currently, MRE is the only noninvasive imaging technique consistently able to evaluate tumor stiffness and tumor-brain adhesion. While more study is needed to determine the clinical utility of MRE for VS, it has demonstrated potential to allow better preoperative assessment of surgical difficulty, risk, and outcome.