The contributions of relative brain viscosity to brain function and health.
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Magnetic resonance elastography has emerged over the last two decades as a non-invasive method for quantitatively measuring the mechanical properties of the brain. Since the inception of the technology, brain stiffness has been the primary metric used to describe brain microstructural mechanics. However, more recently, a secondary measure has emerged as both theoretical and experimental significance, which is the ratio of tissue viscosity relative to tissue elasticity. This viscous-to-elastic ratio describes different but complementary aspects of brain microstructural health and is theorized to relate to microstructural organization, as opposed to stiffness, which is related to tissue composition. The relative viscosity of brain tissue changes regionally during maturation, aging and neurodegenerative disease. It also exhibits unique characteristics in brain tumours and hydrocephalus, and is of interest for characterizing traumatic head impacts. Most notably, regional measures of relative brain tissue viscosity appear to hold a unique role in describing cognitive function. For instance, in young adults, relatively lower hippocampal viscosity compared to elasticity repeatedly and sensitively relates to spatial, declarative and verbal memory performance. Importantly, these same trends are not found with hippocampal stiffness, or hippocampal volume, highlighting a potential sensitivity of relative viscosity to underlying cellularity that contributions to normal healthy brain function. Likewise in young adults, in the orbitofrontal cortex, lower relative viscosity relates to better performance on fluid intelligence tasks, and in the Broca's area of children ages 5-7, lower relative viscosity is indicative of better language performance. In these instances, this ratio shows heightened sensitivity over other structural MRI metrics, and importantly, provides a quantitative and intrinsic alternative to measuring structure-function relationships with task-based fMRI. There are ongoing efforts to improve the accuracy and repeatability of the relative viscosity measurement, and much work is needed to reveal the cellular underpinning of changes to tissue viscosity. But it appears clear that regionally measuring the viscous-to-elastic ratio holds the potential to noninvasively reveal an aspect of tissue microstructure that is clinically, cognitively and functionally relevant to our understanding of brain function and health.