Hi,
by NEX do you mean number of repeats or diffusion-weighted volumes? They are not the same thing.
It seems that you have acquired only 4 diffusion-weighted volumes anyway and possibly one of them is a b=0 volume (i.e. no diffusion contrast). The difference in the bvals/bvecs entries may be due to not including the b=0 entry in the bvecs, even if that should have been handled OK during dicom2nifti conversion.
In any case, you cannot fit the DTI model with 4 volumes, you need at least 7.
Cheers
Stam
On 20 Jun 2013, at 14:02, Nima Kasraie wrote:
> Hello experts.
>
> I have a question, any thoughts you have can help me out:
>
> Our physicians have been analyzing some DTI data in FSL and we've run into a little problem: On some patients we ran DTI with 4 NEX, and we're getting bvec/bval mismatches. For instance, a patient had DTI with 4 NEX. When we read the bvals we see 4 sets of numbers. When we read the bvecs, though, there are only three sets. This seems to happen when we generate gradient tables with MRIConvert, DTI Studio, and MRICron. I'm guessing that Siemens (B17) stores the bvecs in a dicom tag and perhaps buries it in a header. Maybe the string is too long and it gets truncated after the first three sets of numbers?
>
> Basically, we can't process the data with DTIFIT (in FSL) without matching bvec and bval tables.
>
> Any ideas you can share will immensely help us.
>
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