Hello Bri
Just to point out that, out of all white matter pathways, the fornix is probably the most susceptible to partial volume artefacts with CSF, and so any meaningful comparison of microstructural indices within the tract should really incorporate some measure to ameliorate this. This is particularly true in an ageing population - such as you are studying. Several studies have shown the virtues of CSF-suppression at acquisition time for the fornix (Luis Concha and Christian Beaulieu immediately spring to mind). Alternatively, incorporating an extra term in the fitting of the signal can ameliorate the problem. Ofer Pasternak's method seems to work very well. For the impact of different post-acquisition approaches to handling the partial volume problem, see Claudia Metzler-Baddeley's recent paper in NeuroImage on 'How and how not to correct for CSF-contamination in diffusion MRI' - where there are even some TBSS results in the fornix. One word of caution, it isn't safe to add a covariate for volume in the GLM you use for the analysis, as Claudia's paper points out.
Hope this helps
Derek
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