[Mristudio-users] update of gradient table in DtiStudio

[luca.binotto at unipd.it]

Hello Susumu,

I would like to know something about the way DtiStudio performs the update
of the gradient table after the realignment of the dti dataset with
respect to the mean_b0  volume.

The registration algorithm implemented in DtiStudio using mutual
information (MI):
1) does it uses normalized mutual information or which other formulation ?
2) how is the gradient table updated ? (rough simple theory)
After realignment of the DWI volumes, b-matrix (the gradient table) must
be updated.

Patient motion and eddy currents can cause errors in the calculation of
parameters that describe diffusion in each voxel of a DWI dataset.
In normal conditions eddy currents contribution should be
very small (< 1% ?).
But the b-matrix (gradient table) is calculated with respect to a fixed
coordinate system (at the acquisition time) so the b-matrix must be
rotated accordingly to the transformation applied for image registration
otherwise we should get poor fiber statistics and some fibers possibly
deviate.

 So the gradient table is affected  by rotations. Translations don't
change it.
  But what does it happen with resampling (applying a scaling factor: es.
affine transformation) to a dwi dataset?
 According to you is normally its effect small (as for eddy currents
effect) or can be more important?
For example a scaling factor such that a volume with a voxel size of
2x2x2 mm3  is resliced to a 1x1x1 mm3 voxel size can be important its
effect or "only" a lower image quality (eg. I read somewhere of a
"red background" superimposed to the correct color map)?


thanks in advance,
best regards,

Luca