Microscopic diffusion anisotropy in the human brain: Age-related changes

Area of Science:

• Neuroimaging
• Biophysics
• Gerontology

Background:

• Fractional anisotropy (FA) from diffusion tensor imaging (DTI) is limited by its dependence on both tissue microstructure and fiber orientation.
• Microscopic diffusion anisotropy measures, such as the microscopic anisotropy index (MA) from double diffusion encoding (DDE), are independent of orientation, providing direct cellular-level insights.

Purpose of the Study:

• To investigate age-related differences in brain microstructure using MA and FA.
• To compare the sensitivity of MA and FA in detecting these age-related changes, particularly in regions with complex fiber architecture.

Main Methods:

• Diffusion tensor imaging (DTI) and double diffusion encoding (DDE) were used to measure FA and MA, respectively.
• Measurements were performed on healthy young (<33 years) and aged (>60 years) volunteer groups.
• Analysis included 16 regions of interest (ROIs) to assess between-subject variations and age-related differences.

Main Results:

• Microscopic anisotropy index (MA) exhibited lower coefficients-of-variation (CV) than FA in the aged group, indicating reduced between-subject variability.
• Age-related decreases in MA were observed in more ROIs and with higher statistical significance compared to FA.
• MA detected microstructural changes in frontal regions with fiber crossings where FA showed no significant differences.

Conclusions:

• Microscopic anisotropy index (MA) offers improved sensitivity for detecting age-related microstructural changes in the brain compared to fractional anisotropy (FA).
• MA's independence from fiber orientation makes it valuable for analyzing complex white matter regions, such as those with fiber crossings.
• Combined analysis of MA and FA can enhance the identification and characterization of age-related tissue alterations.