Exploring the relationship between larmor-frequency electrical conductivity, diffusivity, and tissue volume in the aging brain.

BACKGROUND

The aging brain undergoes various microstructural changes that influence its electrical properties. Conductivity, a measure of ion mobility, is particularly sensitive to these changes and can be assessed non-invasively using magnetic resonance electrical properties tomography (MREPT). Despite advancements in imaging techniques, the relationship between brain conductivity, diffusivity, and tissue volume in the context of aging and neurodegeneration remains incompletely understood. This study explores the relationships between electrical conductivity, diffusivity, and brain tissue volume in the aging brain, which is crucial for early diagnosis and monitoring of neurodegenerative diseases such as Alzheimer's, where these parameters could serve as potential biomarkers for disease progression.

METHODS

In this cross-sectional, prospective study, 77 patients were assessed brain MREPT and diffusion tensor imaging with multiple shells and gradient directions (b=0, 800, and 2,000 s/mm). High-frequency conductivity (HFC) was calculated and separated into extra-neurite (EC) and intra-neurite conductivities (IC). We analyzed correlations between these conductivity indices and other magnetic resonance imaging (MRI) metrics, controlling for age, and explored the relationship between conductivity, diffusion, and Mini-Mental State Examination (MMSE) scores using multiple regression analysis.

RESULTS

EC within the insular region negatively correlated with MMSE scores (r=-0.3027, P=0.0079). HFC in the hippocampus was positively associated with mean diffusivity (MD; β=192.4, P=0.008) and radial diffusivity (RD; β=207.6, P=0.004). HFC in the insula was positively associated with axial diffusivity (AxD; β=356.9, P=0.0004), MD (β=314.4, P=0.004), RD (β=275.5, P=0.012). EC in the hippocampus was positively associated with AxD (β=309.3, P=0.0001), MD (β=333.7, P<0.001), RD (β=341.8, P<0.001). EC in the insular was positively associated with AxD (β=324.1, P=0.0009) and MD (β=270.4, P=0.01). IC was positively correlated with intra-neurite diffusivity (ID) in the amygdala, thalamus, and insula.

CONCLUSIONS

These findings suggest that increased conductivity is associated with altered diffusivity and reduced cognitive performance, suggesting the use of MREPT to differentiate between conductivity changes due to ion mobility versus proton density, and how this approach contributes to understanding the aging brain and neurodegeneration. MREPT-derived measurements primarily reflect ion mobility and caution that clinical interpretations should consider the direct relationships between conductivity and diffusion changes.