Medial temporal lobe viscoelasticity and relational memory performance

Abstract

Structural and functional imaging studies have been among converging lines of evidence demonstrating the importance of the hippocampus in successful memory performance. The advent of a novel neuroimaging technique – magnetic resonance elastography (MRE) – now makes it possible for us to investigate the relationship between the microstructural integrity of hippocampal tissue and successful memory processing. Mechanical properties of brain tissue estimated with MRE provide a measure of the integrity of the underlying tissue microstructure and have proven to be sensitive measures of tissue health in neurodegeneration. However, until recently, MRE methods lacked sufficient resolution necessary to accurately examine specific neuroanatomical structures in the brain, and thus could not contribute to examination of specific structure-function relationships. In this study, we took advantage of recent developments in MRE spatial resolution and mechanical inversion techniques to measure the viscoelastic properties of the human hippocampus in vivo, and investigated how these properties reflect hippocampal function. Our data reveal a strong relationship between relative elastic/viscous behavior of the hippocampus and relational memory performance (N = 20). This is the first report linking the mechanical properties of brain tissue with functional performance.

Viscoelasticity of subcortical gray matter structures

“…This concept appears to be much less strong in ξ where the only significant relationship exists between the Am and Hc – both located in the medial temporal lobe – and is unchanged by the consideration of global properties. This may support the sensitivity of ξ that we reported previously for the Hc [Schwarb et al, 2016]. …”

“…This is especially critical for potential studies of tissue structure and its role in cognitive performance, as the integrity of specific SGM structures is critical in dissociable cognitive processes such as memory [Cohen and Squire, 1980], executive function [Buckner, 2004], and skill learning [Laforce and Doyon, 2001]. In fact, we have previously reported that hippocampal viscoelasticity is strongly associated with behavioral performance on a relational memory task [Schwarb et al, 2016]. …”

Phantom evaluations of nonlinear inversion MR elastography

“…One example in the brain is the hippocampus, which has a volume of around 4 cm 3 for each hemisphere, and plays a crucial role in memory function. Schwarb et al used NLI-MRE to show that lower damping ratio in the hippocampus is strongly correlated with increased performance in memory-related tasks [34], and our study is the first phantom demonstration that damping ratio imaging at these scales is accurate.…”

“…Recently, Schwarb et al reported a strong correlation between damping ratio of the hippocampus and memory task performance in a cohort of 20 volunteers which showed, for the first time, a relationship between viscoelastic properties and functional outcomes, as well as changes in hippocampal damping ratio after an exercise intervention in a group of multiple sclerosis patients. [34–36] Despite the potential clinical significance of attenuation in these studies, independent measurements are difficult to acquire in vivo. Phantom studies are a vital step in validation of a spatially resolved imaging modality; however, to the best of our knowledge, no images with known attenuation parameter contrasts have been published to date, which undermines confidence in the various MRE processing pipelines to accurately recover spatially distributed parameters.…”

“…In this study, we present four phantoms that were imaged using standardized nonlinear inversion (NLI) parameters applied in our brain MRE work to date. [34, 35] Recovered elasticity values are compared to independent mechanical testing. Image contrast to noise ratio was assessed as a function of inclusion diameter, and edge and line spread functions were calculated as measures of imaging performance for phantoms with stiffness and attenuation contrast.…”