Question on Tensor Fitting with Single vs. Multishell Data in MRtrix3

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Dear MRtrix3 experts,

I have a question regarding the tensor fitting process for analyzing diffusion maps, specifically about how it differs between single shell and multishell data.

According to the mrtrix3 manual, I understand that tensor fitting is achieved by first applying Weighted Linear Least Squares (WLS) followed by Iterated Weighted Linear Least Squares (IWLS).

DWI data I am working with consists of images acquired at eight b-zero, b=800 and b=2800 (with 30 directions each), and I intend to calculate FA, ADC, AD, and RD using these images.

In this case, does the fitting process primarily utilize the lower b=800 data only, or if both shells are used for tensor fitting, could you explain how this fitting is conducted?

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dwi2tensor uses all available images (i.e. across all shells).

If you wish to only use the images corresponding to specific b-values you have to select them yourself in a separate command, typically dwiextract.

E.g. to fit the diffusion tensor to the b<=800 data only, discarding the b=2800 data you would run:

dwiextract -shells 0,800 dwi.mif dwi_lowb.mif
dwi2tensor -mask mask.mif dwi_lowb.mif dt.mif

or you could string these commands together using pipes as:

dwiextract -shells 0,800 dwi.mif - | dwi2tensor -mask mask.mif - dt.mif

To get a list of the b-value shells available in your dataset, you could use:

mrinfo dwi.mif -shell_bvalues
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Thank you! :smiley: