REVIEW: MR elastography of brain tumors

Abstract

MR elastography allows non-invasive quantification of the shear modulus of tissue, i.e. tissue stiffness and viscosity, information that offers the potential to guide presurgical planning for brain tumor resection. Here, we review brain tumor MRE studies with particular attention to clinical applications. Studies that investigated MRE in patients with intracranial tumors, both malignant and benign as well as primary and metastatic, were queried from the Pubmed/Medline database in August 2018. Reported tumor and normal appearing white matter stiffness values were extracted and compared as a function of tumor histopathological diagnosis and MRE vibration frequencies. Because different studies used different elastography hardware, pulse sequences, reconstruction inversion algorithms, and different symmetry assumptions about the mechanical properties of tissue, effort was directed to ensure that similar quantities were used when making inter-study comparisons. In addition, because different methodologies and processing pipelines will necessarily bias the results, when pooling data from different studies, whenever possible, tumor values were compared with the same subject's contralateral normal appearing white matter to minimize any study-dependent bias. The literature search yielded 10 studies with a total of 184 primary and metastatic brain tumor patients. The group mean tumor stiffness, as measured with MRE, correlated with intra-operatively assessed stiffness of meningiomas and pituitary adenomas. Pooled data analysis showed significant overlap between shear modulus values across brain tumor types. When adjusting for the same patient normal appearing white matter shear modulus values, meningiomas were the stiffest tumor-type. MRE is increasingly being examined for potential in brain tumor imaging and might have value for surgical planning. However, significant overlap of shear modulus values between a number of different tumor types limits applicability of MRE for diagnostic purposes. Thus, further rigorous studies are needed to determine specific clinical applications of MRE for surgical planning, disease monitoring and molecular stratification of brain tumors.

Keywords: Brain tumor; Glioma; MR elatography; Meningioma; Pituitary adenoma; Surgical planning.

Fig. 1

Flow-chart of study selection.

Fig. 2

Studies were selected from the following search terms: Pubmed query: (“mr elastography” or “elastography”) AND (“brain tumor” or “glioma” or “glioblastoma” or “meningioma” or “pituitary adenoma” or “vestibular schwannoma”).

Fig. 3

Plots of (A) G’, (B) G”, (C) |G*| and (D) phase angle φ from studies from five different groups. The black solid and red dotted horizontal lines correspond to the mean and mean ± 2 standard deviations (SDs) respectively after pooling all data for a particular tumor together. Note that ±2 SD's equal the 95% confidence interval. Abbreviations are: GBM=glioblastoma; AA=anaplastic astrocytoma; LGG=low grade glioma; Mening.=meningioma; Met=metastatic tumor; NAWM=normal appearing white matter.

Fig. 4

Percent differences of tumor shear modulus parameters with that of NAWM where the percentage difference is calculated for each individual subject thereby using the subject as its own internal control.

Fig. 5

Scatter plot of tumor to normal appearing white matter percent difference of phase angle and IG*I.