I'm wondering about the relationship between FA and other scalars (AD, MD, RD) in DTI. To my understanding, FA is a summary measure of microstructural changes, while the other scalars are more specific measures of white matter microstructure. Is it possible that one can observe no differences between two groups in FA, but significant differences in the other scalars?
In my study, I compared FA values between two groups, and found no significant differences. Should I then analyze the other scalars as well, or should there not be any differences in these values as well?
All help appreciated!
Hi Lasse,
In my understanding, it is completely possible. The math to calculates the FA considers all three eigenvalues L1, L2 and L3, resulting in a 'dimensionless' value that weights how directional is your voxel. The absolute values (how big or how low) in this case, does not matter, only the relations among the Ls it is important.
The MD, AD and RD. directly quantifies the diffusion (mm²/s) by three simple, direct and distinct ways: MD the mean of all Ls; The AD the single value of L1 (higher L) and RD, the average of the 2 lowers Ls.
In this sense, we can have equals FA between groups, but in the case we have a bigger absolute L1 L2 and L3 for one of them, the MD may differ (but not necessarily). Similar to the RD and AD.
This is what I understand, I hope it helps a bit.
Hi Lasse,
In my understanding, it is completely possible. The math to calculates the FA considers all three eigenvalues L1, L2 and L3, resulting in a 'dimensionless' value that weights how directional is your voxel. The absolute values (how big or how low) in this case, does not matter, only the relations among the Ls it is important.
The MD, AD and RD. directly quantifies the diffusion (mm²/s) by three simple, direct and distinct ways: MD the mean of all Ls; The AD the single value of L1 (higher L) and RD, the average of the 2 lowers Ls.
In this sense, we can have equals FA between groups, but in the case we have a bigger absolute L1 L2 and L3 for one of them, the MD may differ (but not necessarily). Similar to the RD and AD.
This is what I understand, I hope it helps a bit.
The answer is Yes! To put it in a simple way, we could say that FA represents the disproportion between axial (L1) and radial diffusivity ((L2+L3)/2). So if axial and radial diffusivity increase or decrease in terms of absolute value but their ratio remains the same then FA will not change significantly. My advice is to always look at all four DTI metric (FA, MD, Axial and radial diffusivities), or at least at FA and MD. Because it can happen that FA does not change while MD does or the contrary. It all depends on the kind of microstructural changes you are dealing with,
As the other two posters noted it is entirely possible for FA to not change, while one of the three other values are different.
Dear Lasse,
Not only from a theoretical point of view provide RD/AD and FA different information. Especially neurodegenerative diseases have a strong effect on both RD and AD in areas with predominantly bundled (non-crossing) fibers. This leads to higher changes in RD and AD but only moderate change in FA. In contrast, demyelinating diseases like (early) remitting relapsing multiple sclerosis predominantly change RD but virtually not AD, resulting in a relatively higher effect on FA.
Indeed, I have found this in a number of studies. For example, please see my 2014 publication on this (attached).
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