| So both ratios are quite a lot higher than the expected 1.71, but even
more
| disconcertingly, the ratios are not the same for the two maps (as they
should be
| from my understanding of affine geometry). The images look fine. We have
also
| counted voxels above a certain threshold > 0 (0.05, 0.1) with the same
result.
| The options for masking of brain and target volume also have no effect on
this.
I see you tried with voxels above a threshold of 0, 0.05 and 0.1. Try
integrating
the voxels in the volume rather than counting voxels above a certain
threshold.
When you resample the data during the spatial normalisation, I would expect
to see
more nonzero voxels appear, simply through the effects of the interpolation.
Note
also that if you count voxels above a threshold of 0.8 or 0.9, I would
expect to see
a decrease in volume in the spatially normalised images.
There are two explanations for the discrepency between the volume ratios of
grey and
white matter. The first is related to the number of voxels below the
threshold
that are within the size of the sinc kernal from voxels above it. After
interpolation, more superthreshold voxels will be created. The second
reason
is to do with the bounding box that is used, resulting in some of the
original voxels
being lost (as they lie outside the bounding box after spatial
normalisation).
All the best,
-John
|