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

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?

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 

Thank you! :smiley: