Empty part of tensor image under the ODF display

Hi all experts,
I have done preprocessing procedure of MGH dataset in Mrtrix, and wanted to get diffusion tensor image through dwi2tensor, however, I get some empty parts of tensor image.

After reading relevant topics like this and this, I realized that it may be due to the negative eigenvalues. Then I use some kinds of constrained WLLS(CWLLS) method to get all non-negative eigenvalues below:

But there is still empty part in the resulting tensor image as shown in the following figure:

So I was wondering if any other problem may also cause this.