B-value did not correctly appear in DWI.mif file

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1

Dear MRTrix3 Community,

I tried to convert dwi data (in nifti format with FSL-like bval/bvec file) to .mif format and tried to run dwi2fod. The code worked, but left a following warning.

 [WARNING] The following image volumes were not successfully assigned to a b-value shell:
dwi2fod: [WARNING] 55 (462782.31946368003)

Then I check gradient vector in DWI.mif by a following command:

mrinfo -dwgrad DWI.mif

Then it turns out that b-value in one of gradient direction becomes erroneous number (b = 462782 4800 s/mm2.), even while all other gradient directions are correctly assigned with b = 4800 s/mm2.

Since original text file on bval only includes b = 0 or 4800 s/mm2, I don’t understand why b-value became an abnormal number only in one direction.

As a results of this incorrect b-value, I got abnormal number of FA value in the data.

Any thoughts will be greatly appreciated!

Sincerely,
Hiromasa Takemura

2

This will most likely be due to the automatic corrections done when we import the gradient table – full details on this page. The chances are one of your gradient vectors is not a unit vector, and this causes the code to adjust the b-value to compensate for what it assumes is a difference in gradient strength. You can disable this behaviour if you’re confident this correction shouldn’t be applied using the -bvalue_scaling option. But given how large the b-value gets scaled to in your case, I’d be concerned that there’s something wrong with your gradient directions – I’d encourage you to double-check these.

3

Thank you very much. -bvalue scaling option could solve this issue, but I will double-check with gradient directions.