NF - Add prolate white matter as defaults for multi-tensor signal sim.

[stefanv]

No description provided.

[Garyfallidis]

Correct, if you want a default value: 1500 and 500 are more common as Ariel suggests. It would be good to reference that information too. If you change that I am happy to merge your pull request.

[stefanv]

It's remarkably hard to find these numbers in literature--let me know if there are any obvious sources I should consult. I found these:

Pierpaoli, Basser, "Towards a Quantitative Assessment of Diffusion Anisotropy", Magnetic Resonance in Medicine

Corpus callosum: 1481 ± 93, 431 ± 54, 330 ± 87 (x 1e-6 mm^2/s)

Makki et al. "Characteristics of Abnormal Diffusivity in Normal-Appearing White Matter Investigated with Diffusion Tensor MR Imaging in Tuberous Sclerosis Complex", American Journal of Neuroradiology, October 2007

Corpus callosum (genu): 1344, 414, 275 with std 149, 77, 78 (x 1e-6 mm^2/s)
Corpus callosum (splenium): 1360, 375, 270 with std 134, 91, 104 (x 1e-6 mm^2/s)

[arokem]

Ah yes - I also have a bit of a confusion with regards to the units. I think that the 1.5/0.5 number I mentioned is with the units in um^2/s

The numbers you found seem good to me.

[arokem]

Other than that - this seems good to me. I vote for merging this in, with those references next to the values of the diffusivity. Please do also mention the units, so that its clear.