For what I know, FA is at significant in presence of water molecules, given that their orientation could be modified by a not uniform magnetic field.I had done my PhD thesis about DTI

Best regrads

Donatella Giuliani

thanks for your answers specially Patrycja

in the mentioned paper it has related the FA to gray matter compression. I want to know is there any other conclusion about increased or decreased FA in the gray matter?

FA is the parameter that shows the earliest changes. The problem comes if you want to interpret it as many factors can influence FA. Did you use the exact same scanner, same hardware and sequence for your set up? Increased FA could mean an increase in myelinsation of the fibers that reach into the GM ... but that is only one of many possible explainations. Did you have a look at RD, MD, AD?

 Dear Benjamin

The scanner and protocol for data acquisition and analysis were the same. I wanna know is there any explanation for FA change in Gray matter or not? or whether the changes are meaningful or it is due to noise artifact or wrong analysis method etc.

I don't know your complete set up (and what the subjects did inbetween). But if you used the same scanner and get significant results in part of the GM I would consider the data meaningful.

Noise could be a problem, as in general the signal intensity with higher b-values in the GM is lower than in the WM, but if you have enough signal in your raw data in the GM than it is probably neglectable.

Even in the GM you have fibers with a myelin sheet (just have a look at publications regarding MS plaques in GM --> demylination). Just the percentage of myelin is much less, therefore a change in FA could be meaningful.