[Mrtrix-discussion] Response functions at lmax=6 vs lmax=8

[David Grayson]

Thanks for the responses here.


I should have mentioned, I have been using the freesurfer WM mask (co-registered to the B0) to exclude noisy voxels. Looks like there was an issue with co-registration. Now that this is fixed, indeed the lmax=8 response looks (very slightly) sharper than lmax=6.


However, I'm wondering if you can have a look at the following comparisons of lmax=6 vs lmax=8, using an eroded WM mask and using the full WM mask. The lmax=8 still produces the protruberance on the z-axis, which is exaggerated using the eroded WM mask. Does this suggest that I should perhaps stick with lmax=6, and avoid eroding any further the WM mask? Would you consider these RF's acceptable now?


http://imgur.com/42NCktt


Thank you very much,

David


________________________________
From: albnet at gmail.com <albnet at gmail.com> on behalf of Dorian P. <alb.net at gmail.com>
Sent: Tuesday, March 22, 2016 3:10 PM
To: J-Donald Tournier
Cc: David Grayson; mrtrix-discussion at www.nitrc.org
Subject: Re: [Mrtrix-discussion] Response functions at lmax=6 vs lmax=8

I always used to correct manually the single fiber mask. The automatic thresholding includes out of brain voxels in inferior slices. A simple threshold is not to be trusted, I think.

Dorian

On Tue, Mar 22, 2016 at 6:01 PM, J-Donald Tournier <jdtournier at gmail.com<mailto:jdtournier at gmail.com>> wrote:
Hi David,

I've never heard of this being referred to as a 'nubbin', but it's clear enough... ;)

Looking at your responses though, the 'nubbin' isn't all that worrying. What's more worrying is the fact that the lmax=6 response is sharper than the lmax=8 response, which seems wrong. Your lmax=8 response is certainly broader than I'd expect at b=3000.

So all this suggests that the response function estimation isn't working all that well, which is typically a symptom of a poor single-fibre mask. You used an FA threshold of 0.7 here, but bear in mind that this is by no means a 'default' - this is more of a guideline, as stated in the documentation<http://jdtournier.github.io/mrtrix-0.2/tractography/preprocess.html#csd>. Here's the relevant excerpt:

Note that this value is a guide only - feel free to use a different value if this does not produce satisfactory results. Ideally, you should now have a mask containing a few hundred voxels, all located within high FA white matter regions. It is very important to check that the single-fibre mask is suitable, as otherwise the response function produced in the following step may be totally inappropriate, which would seriously affect the quality of the CSD output. If needed, you can edit this mask image to remove unwanted voxels using the ROI analysis<http://jdtournier.github.io/mrtrix-0.2/general/mrview.html#roi> sidebar tool within MRview<http://jdtournier.github.io/mrtrix-0.2/general/mrview.html>.

I would have a good look at the single-fibre mask used in this step, and check against the above. The chances are you might find a lot of noisy high-FA edge voxels got included or something, despite the erosion step. This all depends on how good the initial brain mask was (I often found it difficult to exclude the nasal sinuses, for example). Unfortunately, this step can be a bit fiddly.

Alternatively, you could upgrade to MRtrix3<http://www.mrtrix.org> - we've put in quite a bit of work on estimating the response function<http://mrtrix.readthedocs.org/en/latest/concepts/response_function_estimation.html>...

All the best,
Donald.


On 22 March 2016 at 19:47, David Grayson <dgrayson at ucdavis.edu<mailto:dgrayson at ucdavis.edu>> wrote:
Hi MRtrixers,

I have HARDI datasets on a large sample of young adults (7-16yrs) taken with 72-dir and b0=3000. I am wondering whether I should proceed with lmax=6 or 8. I am using MRtrix 0.2.12.

I am concerned about using lmax=8 because in the response function there is often a ‘nubbin’ that appears right in the center of the Z-axis line that doesn’t appear at lmax=6. The below link shows an example of the RF’s generated for a single subject taken at lmax=6 (on the left) and lmax=8 (on the right). These are generated using default parameters (FA threshold of 0.7 for the single-fiber-orientation mask).
http://imgur.com/a/ODrcf

Hopefully it’s obvious what I’m talking about. Sometimes this nubbin is more pronounced than what I’m showing here. I’m not sure how concerned I should be about this, but based on my intuition about what the RF means, the Z-axis itself should represent the global minimum of the DW signal, which is obviously not the case with lmax=8. Should I avoid any appearance of this effect at all costs, or is there some rule of thumb for how big it can be before I should be concerned about it?

Thanks,

David

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--
Dr J-Donald Tournier (PhD)

Senior Lecturer, Biomedical Engineering
Division of Imaging Sciences & Biomedical Engineering
King's College London

A: Department of Perinatal Imaging & Health, 1st Floor South Wing, St Thomas' Hospital, London. SE1 7EH
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