Longitudinal age-related changes in radial and axial diffusion using tract-based spatial statistics

Introduction: A range of neuropathological changes occur in normal ageing and an important aim of neuroimaging research is to detect these changes in vivo. Diffusion tensor imaging (DTI) measures water diffusion in tissue and is used to infer white matter structural integrity. Recent animal studies suggest that neuropathological change can be further elucidated, beyond simple tissue integrity, with evidence suggesting that increases in radial diffusion are associated with demyelination and increases in axial diffusion with axonal damage [1,2,3]. Histological studies in humans have further supported the association between radial diffusion and myelin, and several studies have identified increases in radial diffusion with older age [4,5,6]. Patterns of axial diffusion with increasing age are more complex with both increases and decreases observed in different brain regions, and radial diffusion has shown greater differences than axial diffusion when comparing older and young adults [4,5,6]. To date, no studies have investigated longitudinal changes in axial and radial diffusion. The aim of this study is to use tract based spatial statistics (TBSS) to investigate local age-related white matter structural change on a voxel-by-voxel basis over a 2-year period.