Tensor2metric works not well to give negative RD, AD and RD inside the brain



Hi, mrtrix experts:

I am using dwi2tensor and tensor2metric to generate the DTI maps (FA, MD, RD, AD) with my diffusion data with quite low resolution in one direction: 1.3672 x 1.3672 x 2.7.

So after i fit it with tensor2metric, and I visualized it in freeview and just to show the negative value,

we can see that several voxels inside the brain gave negative values, this is worse after I register the individual DTI maps onto a template with interpolation, which makes the negative regions bigger.

Do you have encountered the similar issues???
Thanks in advance

Good night


Hi @Junhao_Wen,

This is not so surprising, as the dwi2tensor implementation isn’t constrained to produce tensors with positive eigenvalues. Once one or more eigenvalues are negative, the FA, MD, RD and AD metrics can go beyond their intended physically sensible range, e.g. MR, RD and AD values below 0 or FA values beyond 1. The most straightforward (yet naive) way to deal with this, if these values cause you trouble, is to cap them to a sensible range; i.e. make the negative values zero.

Interpolation will indeed “spread” the problem slightly (potentially). Depending on the type of interpolation, even an image with only positive values (though some zeros) may introduce negative values. To avoid this, stick to linear interpolation (not cubic and beyond).