[Mrtrix-discussion] Diffusivity along the peak FOD directions

[Robert Smith]

Hi Dan

Regarding the interpretation of Fibre Orientation Distribution amplitudes,
I would suggest looking at the diffusion simulations in:
Raffelt, D., Tournier, J-D., Rose, S., Ridgway, G.R., Henderson, R.,
Crozier, S., Salvado, O., Connelly, A.. Apparent Fibre Density: A novel
measure for the analysis of diffusion-weighted magnetic resonance images.
NeuroImage 2012:59;3976-3994.

For high *b*-value acquisitions, and assuming that you do *not* normalise
to the *b=0* signal (which we recommend, and occurs by default), the
amplitude of the FOD is approximately proportional to the *intra-cellular
volume* of axons aligned in that particular orientation. The FOD peak
amplitudes are then a reasonable sparse representation of this, so you can
estimate the relative volume of axons in a voxel in different orientations.
Note however that I said *volume* of axons rather then *number*; a single
high *b*-value acquisition is more or less insensitive to axon diameter, so
we can't distinguish between many small-diameter axons and fewer
large-diameter axons, because the total intra-cellular volume is more or
less the same.

The FOD peak amplitudes are however not the ideal marker, as orientation
dispersion can introduce 'spread' into the FOD lobe and reduce the peak
amplitude. I have developed a method that instead computes the integral of
individual FOD lobes, which is more robust against these effects; the
details should be included in a publication later this year.

Best regards
Rob

--

Robert Smith
PhD Candidate

The Florey Institute of Neuroscience and Mental Health
Melbourne Brain Centre - Austin Campus
245 Burgundy Street
Heidelberg Vic 3084
Ph: +61 3 9035 7128
Fax: +61 3 9035 7301
www.florey.edu.au



On Tue, Sep 11, 2012 at 3:20 AM, Dan Wu <dwu18 at jhu.edu> wrote:

> Dear expert,
>
> I used the find_SH_peaks to get the peak directions of the FODs. I am
> wondering what the magnitude of the peak FOD represents? Does it relates to
> the diffusion signal along that direction or it is just a relative
> magnitude?
>
> Thanks a lot!
>
> Dan
>
>
> --
> Dan Wu
>
> PhD Candidate
> Department of Biomedical Engineering
> Johns Hopkins University
> email: dwu18 at jhu.edu
> phone: 443-564-9191
>
>
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