# Dwi2tensor wrong values

**URL:** https://community.mrtrix.org/t/dwi2tensor-wrong-values/1860
**Category:** Uncategorized
**Tags:** dwi2fod
**Created:** [August 24, 2018, 2:30pm UTC](https://community.mrtrix.org/t/dwi2tensor-wrong-values/1860 "2018-08-24T14:30:59Z")
**Posts on this page:** 1
**Showing post:** 3

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### Author: ![jdtournier](https://community.mrtrix.org/user_avatar/community.mrtrix.org/jdtournier/32/2594_2.png) [@jdtournier](https://community.mrtrix.org/u/jdtournier)
#### Post date: [August 25, 2018, 8:25am UTC](https://community.mrtrix.org/t/dwi2tensor-wrong-values/1860/3 "2018-08-25T08:25:55Z")

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It seems to me that the problem is essentially that the tensor estimated is not positive-definite – i.e. there are negative eigenvalues. This would explain the black glyphs being displayed in these locations. `dwi2tensor` uses a weighted least-squares fit, which doesn’t constrain the tensor parameters to be positive-definite. If `dtifit` imposes such a constraint, that would be enough to explain the discrepancy. You could verify with `tensor2metric dt.mif -num 1,2,3 -value ev.mif`, see if any of the eigenvalues at that location are negative.

As to what might be causing these issues: these typically boil down to the _b_=0 signal being _lower_ than the DW signal in certain directions. This can often happen due to Gibbs ringing for example: voxels near CSF can often have lower _b_=0 signal than anticipated because of this, and this is often enough to cause problems of this nature. If you’re not already, you could try using `mrdegibbs` before your motion and eddy-current correction (i.e. `dwipreproc`), see if that helps.

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