Dwi2tensor using only b=1000 by extracting from a multi-shell data

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Hi,
My dwi data is multi-shell (b=0,1000,2000), but I want to reconstruct the DTI model only using the b=1000 shell. Could anyone kindly give me some suggestions for how to deal with it?

I notice dwiextract may extract the b=1000 nii volume, but how about the bvec files for the DTI model?

Thanks