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

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!

Hiromasa Takemura

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.

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