DWI metrics for demyelination

mattvan83 · October 15, 2019, 2:18pm

Dear mrtrix3’s experts,

I would like to characterize demyelination based on DWI imaging. I know that RD is a DTI metric to assess demyelination but I wonder whether fixel analysis would be of interest in this case?

Best,
Matthieu

maxpietsch · October 17, 2019, 9:03am

Hi,

It may be sensitive but not specific. Your question is too big to answer here, I’d strongly recommend having a look at these papers and references therein:

https://www.sciencedirect.com/science/article/pii/S1053811917306857
https://onlinelibrary.wiley.com/doi/full/10.1002/mrm.21965
https://onlinelibrary.wiley.com/doi/full/10.1002/mrm.27101

I might be biased… but yes I’d recommend using FBA over tensor-based analysis but neither method is specific to specific myelin properties.

Cheers,
Max

mattvan83 · October 17, 2019, 1:35pm

Hi,

Thanks for this detailed answer !

Why do you say that it might be biased to use FBA over tensor-based analysis in this case?

Best,
Matthieu

maxpietsch · October 17, 2019, 4:34pm

was a somewhat tongue in cheek comment in that my pointing you towards our recommended pipeline could be influenced by some degree of brainwashing and/or emotional investment of working with people who developed these techniques. You’re coming to a Mercedes showroom and asking whether to get a Trabant or one of their 4x4s to cross the desert. That being said, if your data supports FBA and you want to investigate fibre-specific group-differences, FBA is currently your best way to get meaningful measures about fibre density and morphology.

mattvan83 · October 17, 2019, 6:25pm

Hi,

Thank you for this objective explanation, I understand.

I saw that for FBA it is recommended to have high b-values (>2000s/mm2). What would you advice me with multi-shell b-values of 1000 (32 dir) and 2000s/mm2 (32 dir)?

Best,
Matthieu

maxpietsch · October 18, 2019, 10:20am

32 directions in the outer shell are at the lower end but I think should be sufficient. I’d suggest you try it out (read on here).

The interpretation of fibre density in FBA hinges on a few assumptions, in particular, the ability to suppress extracellular signal, a maximum b-value of b=2000s/mm2 gets you pretty far into that regime.

More detailed discussions are in Dave’s papers:

ncbi.nlm.nih.gov

Apparent Fibre Density: a novel measure for the analysis of diffusion-weighted magnetic resonance images.

D Raffelt, JD Tournier, S Rose, GR Ridgway, R Henderson, S Crozier, O Salvado and A Connelly, NeuroImage, Feb 2012 15

This article proposes a new measure called Apparent Fibre Density (AFD) for the analysis of high angular resolution diffusion-weighted images using higher-order information provided by fibre orientation distributions (FODs) computed using spherical deconvolution. AFD has the potential to provide specific information regarding differences between populations by identifying not only the location, but also the orientations along which differences exist. In this work, analytical and numerical Monte-Carlo simulations are used to support the use of the FOD amplitude as a quantitative measure (i.e. AFD) for population and longitudinal analysis. To perform robust voxel-based analysis of AFD, we present and evaluate a novel method to modulate the FOD to account for changes in fibre bundle cross-sectional area that occur during spatial normalisation. We then describe a novel approach for statistical analysis of AFD that uses cluster-based inference of differences extended throughout space and orientation. Finally, we demonstrate the capability of the proposed method by performing voxel-based AFD comparisons between a group of Motor Neurone Disease patients and healthy control subjects. A significant decrease in AFD was detected along voxels and orientations corresponding to both the corticospinal tract and corpus callosal fibres that connect the primary motor cortices. In addition to corroborating previous findings in MND, this study demonstrates the clear advantage of using this type of analysis by identifying differences along single fibre bundles in regions containing multiple fibre populations.

mattvan83 · October 18, 2019, 12:57pm

Thank you for the references, I will read these papers.

So you advice me to use the 32 b=2000s/mm2 for FBA analysis. Wouldn’t it be possible to gain in using both shells b=1000s/mm2 and b=2000s/mm2 for CSD as suggested here?: [http://userdocs.mrtrix.org/en/latest/constrained_spherical_deconvolution/multi_shell_multi_tissue_csd.html]

maxpietsch · October 18, 2019, 1:10pm

Yes, use all shells and the MSMT CSD FBA pipeline.