IXI dataset diffusion data - number of directions


The IXI dataset openly provides scans of diffusion MRI from a large number of subjects. There is however a strange discrepancy in the data. The b-vectors and b-values that are provided have 16 entries each. This includes a zero entry in the b-values, which corresponds to the first volume of the acquisition. The first b-vector is [0, 0, 1], but there is also a [0, 0, -1] in the table further down, so I suspect that this is also supposed to be zero. However, there are 17 volumes of diffusion data provided. They are provided as 17 different nifti files, named “XXX-DTI-00.nii.gz”, “XXX-DTI-01.nii.gz”… “XXX-DTI-16.nii.gz”, where “XXX” is the individual subject identifier – something like “IXI640-Guys-1106”.

In the past when I analyzed this data, I scratched my head, tried a couple of things and then came to the conclusion that dropping the last volume makes sense. This was done based on looking at FA, MD and color FA maps for a few individuals and based on the fact that dropping that volume results in the best model fit for the DTI model. But now I am wondering whether anyone out there knows what is actually going on here.

Why are there 17 volumes in this data, but only 16 b-values/b-vectors? If the last volume really shouldn’t be used for fitting the data, what is that last volume for?