What's the minimal diffusion directions for FBA, and how to judge the peak delineation of FODs?

Dear editors,

I have a cohort with 40 diffusion weighted directions (several patients were 32 diffusion weighted directions) and a b-value =2000s/mm 2. However, some researchers consider with low angular resolution the derived FODs lack the sharp peak delineation in crossing fiber regions that is necessary to provide the enhanced tracking capability offered by CSD. Is the DWI parameter (40 diffusion weighted directions, b-value =2000s/mm 2) not appropriate for publishing paper of FBA?

Here are FODs from the wmfod of my cohort, how to judge the peak delineation of FODs is sharp or not?

screenshots_2021-02-18 21.16.26

This a hard question to answer with a straight yes or no. Personally, I feel 40 directions are already enough to get good fODFs, especially with a moderate b-value of 2,000s/mm² – that’s already sufficient for spherical harmonic order lmax=6. What will determine the quality of the fODFs more at this stage is the SNR of the images. It’s hard to assess from your screenshots, but it looks reasonable from what I can see.

More to the point: sharp fODFs would be more important for tractography than for fixel-based analysis. There are steps in the FBA pipeline to minimise the influence of ‘broader’ fODFs or slight misalignment across subjects, so I think your data is sufficient for that. Besides, in the context of FBA, if your data are insufficient, that would be expected to reduce your statistical power to detect differences, but it wouldn’t be expected to introduce artificial differences in your results.

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Thank you for your helpful reply! It is surprising to me that

in the context of FBA, if your data are insufficient, that would be expected to reduce your statistical power to detect differences, but it wouldn’t be expected to introduce artificial differences in your results

It improves my confidence of my positive FBA results that they are reliable!