Dear all, That warning is likely to rise due to some background voxel. However, if this warning appears consistently, please rise an issue in Dipy including some example that triggers that warning. Regarding the model parameters (Ajay’s question - sorry for the late reply), fwdti returns a matrix which last dimensions corresponds to: 1) Three diffusion tensor's eigenvalues 2) Three lines of the eigenvector matrix each containing the first, second and third coordinates of the eigenvector 3) The volume fraction of the free water compartment (See functions documentation). So if you want to save tensor parameters in lower triangule format you have to convert them first. Something like this: from dipy.reconst.dti import lower_triangular from dipy.reconst.vec_val_sum import vec_val_vect evals = fwdtifit.evals evecs = fwdtifit.evecs dt = lower_triangular(vec_val_vect(evecs, evals)) Best Regards, Rafael