# Effects of Parkinson’s disease on mechanical and microstructural properties of the brain

## TL;DR

Parkinson’s disease causes specific brain regions to soften, linked to changes in microstructure, while aging also affects stiffness across the brain.

## Contribution

Combines MRE and MD-dMRI to explore biomechanical and microstructural changes in PD patients, revealing distinct regional effects.

## Key findings

- Temporal and occipital lobes soften in PD patients, correlated with increased mean diffusivity.
- Mesencephalon shows neuronal atrophy but no softening in PD patients.
- Aging is strongly linked to decreased stiffness and altered microstructural parameters across the brain.

## Abstract

•The occipital and temporal lobes decrease most in stiffness due to PD.•The cerebrum softens due to aging effects coupled to neuronal atrophy.•The mesencephalon does not soften due to PD but show signs of neuronal atrophy.•The stiffness across the whole brain are most strongly correlated with MD and µFA.

The occipital and temporal lobes decrease most in stiffness due to PD.

The cerebrum softens due to aging effects coupled to neuronal atrophy.

The mesencephalon does not soften due to PD but show signs of neuronal atrophy.

The stiffness across the whole brain are most strongly correlated with MD and µFA.

Magnetic Resonance Elastography (MRE) is a novel technique to study the brain by measuring its mechanical properties, such as stiffness and viscosity. These properties may provide insights into how the microstructure of the brain changes due to a pathology, however the connection between these microstructural mechanisms and the measured biomechanical properties are still largely unknown. For this reason, the present exploratory study utilizes multidimensional diffusion magnetic resonance imaging (MD-dMRI), apart from MRE, to extract microstructural parameters of the whole brain tissue for a small cohort of 12 Parkinson disease (PD) patients and 17 healthy controls. A combination of these methods provides valuable insights into subtle changes due to PD as it probes variables such as microscopic fractional anisotropy (μFA) combined with measures of shear stiffness. MRE and MD-dMRI quantities across the brain are compared between the two groups and analyzed. It was found that there were significant softening effects in the temporal and occipital lobes due to PD, associated with an increase in the mean diffusivity in those regions, whereas other microstructural properties remained largely unchanged. The mesencephalon, on the other hand, displays changes in the MD-dMRI parameters consistent with neuronal atrophy, however no softening of this region was detected. In most regions, stiffness is significantly reduced due to age, which is correlated with a decrease in μFA and increase in MD. We hypothesize that age effects can mostly explain neuronal atrophy, whereas softening due to PD effects involve additional mechanisms.