What is the role of DWI in MS?
Acute or active plaques show diffusion restriction typically appear peripheral and incomplete ring which represent the growing margin of the plaque. Chronic plaques are gliotic and show facilitated diffusion (increase water diffusivity).
Fractional anisotropy map which combines water molecular diffusion with direction. White matter fibres appear bright as the water molecules diffuse relatively freely but only in certain directions (anisotrophy) while grey matter appears dark as their water molecules diffuse similarly, but not constrained to a specific direction (isotropy). At the site of the plaque the diffusion becomes isotrophic denoting white matter destruction at this point.
Anatomical color map identifies dominant white matter diffusion direction superimposed on T2 WI which delineates the anatomy. ROIs are put at the site of the plaque, near normal appearing white matter and abnormal appearing white matter with FA and ADC calculated. At the site of the plaque FA is markedly reduced and ADC is elevated compared to other ROIs. This indicates increase water molecular diffusivity at plaque site.
Tractography of the forceps minor fibres and fronto-occipital fasciculus reveals white matter disruption at the site of the plaque.