| We've noticed this as well regarding the gray matter and I'm not sure that
| larger ventricles is an adequate explanation. First of all other than
| caudate there should be very little gray matter near the lateral
| ventricles. However, when comparing normals and patients with various
| degenerative illnesses we see huge differences in the ventricular regions
| (particularly lateral ventricles) and the changes are not just limited to
| the caudate regions. The reason for this anomalous? segmentation is not
| entirely clear. Consider that it is usually the case that the ventricular
| areas are showing more "gray matter" for normals than for patients with
| disease. This means the segmentation algorithm is assigning as gray matter
| these periventricular regions in the normal group. Why should this be?
Without spending a while looking closely at the data, I couldn't say for
sure. Voxels are most likely to be classified as grey if their intensities
are similar to other voxels that have been classified as grey matter and
they occur in regions where there is a higher a priori probability of
finding grey matter (according to the prior probability images). Also,
if a brain contains more grey matter, then there is a higher a priori
probability that a voxel is grey.
Around the ventricles, partial volume between CSF and WM is often classified
as grey. Take a close look at these regions to see if there are any obvious
patterns. Taking a close look at the con_ images is also very useful when
it comes to interpreting the results.
|
| Possible solutions include masking out the ventricles, but the fact that
| they show up does make other true areas of difference suspect. What are
| your thoughts?
If you see a difference using VBM, then it means that there probably is
a difference (providing the assumptions about rates of false positives
hold). VBM is tuned to identifying structural differences due to
increases or decreases in grey matter, but it also produces blobs that
may be due to some other real structural differences, such as the example
relating to ventricle sizes from the previous email.
All the best,
-John
|