Hi all,
I have a question about how FAST 4.1 differs from earlier versions,
esp 3.x. I get quite different results in two large samples, in terms
of grey matter to white matter ratio (ratio = 1.14 from FAST 4.1, vs
ratio = 1.67 from 3.x). This suggests big difference(s) in
classification between versions. there are many differences between
versions, obviously, but I did not expect these to impact the ratio
much if at all. I think a ratio of 1.6 is much more in line with the
literature.
any ideas what's going on, or how to track it down? I searched the
archives but did not see anything relevant, sorry if I missed it.
thanks in advance,
--Jeremy
gory details:
I ran T1 images from 114 subjects through bet (and checked that they
look reasonable after bet). I then used FAST 4.1 (on CentOS 5.3, 64-
bit) to segment them using defaults:
fast -t 1 -o <image> <image>
I looked at some of the resulting pve_* images and they look
reasonable in fslview. from the pve's I get CSF, grey, and white
matter volumes in mm^3 using fslstats and bc, as described on the
FAST web page (http://www.fmrib.ox.ac.uk/fsl/fast4/index.html)
my intuition is that the grey to white matter ratio should be pretty
robust against scanner and sample differences (but maybe that's
wrong). in the sample I'm using FAST 4.1 for, the ratio of GM : WM
is 1.14. This is markedly lower that the G/W ratio of 1.67 I got in
another large sample (different scanner & subjects, but I don't think
that should have a huge impact on the ratio). this was using FAST 3.x
(again on T1 images run through bet). so think that the version of
FAST is where the difference is, and that in my hands 4.1 is not
doing what it should.
also, there seems to be more CSF when using FAST 4.1 (~25% of intra-
skull volume, vs ~15% with 3.x).
any ideas on how to best track down what is going on (esp with 4.1)?
|