Hi Elijah - I suggest looking at the axial diffusivity (AD) and radial diffusivity (RD) to help you interpret these results. These are the diffusivity parallel and perpendicular to the main budnle going through a voxel, respectively. Essentially, FA tells you how different the AD and RD are from each other, and MD tells you how big AD and RD are in aggregate.

Here are just a couple of all the possible scenarios that can lead to opposite FA/MD relationships:

AD increases, RD stays the same => MD increases, FA increases AD stays the same, RD increases => MD increases, FA decreases

Hope this helps,

a.y

On Thu, 24 Mar 2016, Elijah Mak wrote:

Hi Freesurfer Community, I recently used mri_segstats to extract FA and MD as suggested here: http://freesurfer.net/fswiki/FsTutorial/Diffusion

Using the aparc+aseg and wmparc (resampled to diffusion space), I looked at some of the correlations between FA and MD in the same regions, and came across a few interesting ones.

wm_lh_entorhinal FA & wm_lh_entorhinal MD: r = 0.6, p <0.001 ctx_lh_entorhinal FA & ctx_lh_entorhinal MD: r = -0.3, p < 0.020

wm_lh_precuneus FA & wm_lh_precuneus MD: r = -0.6, p <0.001

As I understand, a clear-cut interpretation especially with regards to FA changes is not so easy!  But could someone discuss / speculate as to why FA and MD could be positively correlated within the entorhinal WM region? That was found separately within a group of a elderly healthy controls and a group of dementia subjects.

Thank you for your time!

Best Wishes, Elijah