relationship between FA and MD

[Manuel Blesa Cábez]

Dear Do,

I'm new working on DWI-MRI and I find this blog very interesting. I hope you can help me. I run TBSS and after obtain all the results for FA and MD, I would like to know if there are any relationship between them since a biological point of view, I mean if for example FA is high and MD is low, means that the axons are bigger and there are less axons than if the FA is low and MD is high, something like this.

Best regards

Manuel Blesa

[Do Tromp]

Hi Manuel,

That is indeed one of the key questions in diffusion imaging - how do diffusion measures relate to neural microstructure - but is unfortunately not entirely figured out. I would suggest you read this blog post:

http://www.diffusion-imaging.com/2013/01/relation-between-neural-microstructure.html

And I will attach a couple of articles that might be interesting.

Best of luck,

~Do

--

Do Tromp, MSc
Graduate student
Neuroscience Training Program
Kalin lab, Department of Psychiatry
University of Wisconsin, Madison, USA

www.bi.wisc.edu/~tromp

www.diffusion-imaging.com

[Rodrigo Perea]

Hey Manuel,

This is a very interesting question and so far I haven't found a very convincing answer, which may be because DTI reflects changes in white matter (WM) indirectly. 

From my experience, FA is an overall assessment of white matter (WM) tract integrity, ranging from normalized values 0 (very isotropic, no direction dependent) to 1 (very anisotropic, or very directional dependent).  In relation with WM, it could mean that higher FA is related to higher axon density, better myelination, etc. 

MD, which is mean diffusivity is the average value of three (let's say x-y-z) directions of diffusion. It is more difficult to interpret this measurement but generally the higher the number, the more diffuse the tract will be, which can tell you similar information as lower FA values. I like to use radial diffusivity measures (RD), which is more related to a transverse direction of diffusion and could be more related to axonal diameter or myelination/demyelination. 

I hope this helps, and I would like to know what other researchers think as well :). For more information I would recommend you to read this:

Beaulieu, C. (2002). "The basis of anisotropic water diffusion in the nervous system - a technical review." NMR Biomed 15(7-8): 435-455. 

Rodrigo

Rodrigo D. Perea

Graduate Research Assistant

Bioengineering Program

The University of Kansas

[Manuel Blesa Cábez]

Thanks Do and Rodrigo, I'll take a look to this information and try to find the best solution to my problem.

Best regards,

Manuel Blesa