<p>Like <em>lea</em>'s comment indicated, FA is called "fractional" anisotropy simply because it's the degree of anisotropic diffusion, i.e., a ratio (Soares, Marques, Alves and Sousa, 2013).</p>

<blockquote>
  <p>Fractional Anisotropy is <strong>a normalized measure of the fraction of the tensor's magnitude due to anisotropic diffusion</strong>, corresponding to the degree of anisotropic diffusion or directionality and ranges from 0 (isotropic diffusion) to 1 (anisotropic diffusion).</p>
</blockquote>

<p><strong>References</strong></p>

<ul>
<li><a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594764/" rel="nofollow">Soares, J. M., Marques, P., Alves, V., &amp; Sousa, N. (2013). A hitchhiker’s guide to diffusion tensor imaging. Frontiers in Neuroscience, 7, 31. doi:10.3389/fnins.2013.00031</a></li>
</ul>

CC BY-SA 3.0
2015-05-05T18:10:45.867Z
2015-05-05T18:10:45.867Z
Christian Hummeluhr
<p>Why is <a href="https://en.wikipedia.org/wiki/Fractional_anisotropy" rel="nofollow">Fractional Anisotropy</a> (FA), a measure in diffusion tensor imaging, called "fractional"? I'd have thought "local", i.e. voxel-wise, would have been the correct way to qualify the anisotropy that is calculated from the tensor model, so why fractional?</p>

CC BY-SA 3.0
2014-08-05T10:01:26.310Z
I am not completely fresh on the subject, but from what I recall when studying the subject, FA measurements are meant to represent not an absolute signal, but a radio between axial and radial diffusion.
lea
CC BY-SA 3.0
2014-08-05T12:47:07.080Z
Makes sense! :)
z8080
<measurement><neuroimaging><fmri>
Why "fractional" anisotropy?
z8080
# Why "fractional" anisotropy?

(Asked by: username_0 on 2014-08-01)

Why is [Fractional Anisotropy](https://en.wikipedia.org/wiki/Fractional_anisotropy) (FA), a measure in diffusion tensor imaging, called "fractional"? I'd have thought "local", i.e. voxel-wise, would have been the correct way to qualify the anisotropy that is calculated from the tensor model, so why fractional?

**Tags:** measurement, neuroimaging, fmri

> **Comments:**
> **username_1**: I am not completely fresh on the subject, but from what I recall when studying the subject, FA measurements are meant to represent not an absolute signal, but a radio between axial and radial diffusion.  
> **username_0**: Makes sense! :)  

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## 1 answers

### Answer by username_2 on 2015-05-05. Score: 3

Like *username_1*'s comment indicated, FA is called "fractional" anisotropy simply because it's the degree of anisotropic diffusion, i.e., a ratio (Soares, Marques, Alves and Sousa, 2013).

> Fractional Anisotropy is **a normalized measure of the fraction of the tensor's magnitude due to anisotropic diffusion**, corresponding to the degree of anisotropic diffusion or directionality and ranges from 0 (isotropic diffusion) to 1 (anisotropic diffusion).

**References**

* [Soares, J. M., Marques, P., Alves, V., & Sousa, N. (2013). A hitchhiker’s guide to diffusion tensor imaging. Frontiers in Neuroscience, 7, 31. doi:10.3389/fnins.2013.00031](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594764/)


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