[Mrtrix-discussion] Re: color coding of FODs

[Donald Tournier]

Hi Zhuang,

The colour-coding is done strictly according to the X,Y,Z components at
that point on the surface, with no regard to which FOD lobe that point
might visually appear to belong to. You'll find the full width at half-max
of the FOD peaks at lmax 8 are around ~20°, so for an off-axis peak like
the one you're showing, the actual peak might be along Z-Y (giving a
greenish-blue colour - cyan), but one side of the lobe might be closer to Z
(blue) and the other closer to Y (green).

Colour-coding according to the closest peak direction could be done, and
might be OK when the peaks are well-defined, but there will definitely be
cases when two peaks are close to each other and aren't resolved properly,
in which case this approach would give misleading results. The current
approach is unambiguous and easy to implement, and as far as I know,
similar to what other packages might do.

Cheers,

Donald.


On 13 March 2013 10:47, Zhuang Song <zhuang.song at gmail.com> wrote:

>
> I am having some problem to understand the color coding of the FOD profile
> constructed by the CSD method of MRtrix. For my understanding, the color
> should be the same along the same axis of the FOD. However quite often I
> see the color changes a lot along the same orientation of FODs.
>
> I attached one example picture of FOD here but not sure if you can see the
> attachment in this maillist. In this example, the FOD color changes from
> green to blue in the first primary orientation. What should I interpret it?
> Does it mean that the primary direction is a mix of diffusion in both green
> and blue directions?
>
> Thanks,
> Zhuang
>



-- 
*Dr Jacques-Donald Tournier
*
Research Fellow

The Florey Institute of Neuroscience and Mental Health
Melbourne Brain Centre - Austin Campus
245 Burgundy Street
Heidelberg  Vic  3084
Ph:  +61 3 9035 7033
Fax:  +61 3 9035 7307
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