Hello,
I have recently posted a question about img2talcoord (thanks for fixing
that bug) and this is, sort of, a follow up on that.
Let's say that I have an image of a brain that I want to use as a
standard image for registration. And let's say that this image is
91x109x91 with resolution of -2x2x2. Furthermore, the origin (that
corresponds to AC) is at voxel (46,64,37) and that is specified in the
header. So basically, the dataset is the same at the mni brain (except
for the datatype; header files are almost identical) and the brain
itself is very similar to the mni brain, but smaller (it is based on T1
images). If I used that brain as reference brain for registration, will
the "talairach" cluster coordinates that Feat outputs be in the space
that is defined by the parameters above (resolution, origin...)?
If the above is correct, and if I say further that this brain fits the
Talairach atlas (it fits the orientation and extent of the Talairach
atlas), then the cluster coordinates would be in talairach space (as
opposed to mni space, when mni standard brain is used). Right?
Well, I tried a little test. I have run the same analysis twice, the
difference being only in the reference brain used. If I use the above
assumption, the coordinates calculated with mni brain as reference
(mni) and then corrected (using correction described by Matthew Brett)
to fit talairach atlas (mni->tal), will be approximately the same as
those calculated using the Talairach brain (tal). Of course, under the
assumption that Brett's transformation maps mni to the "same" brain as
the one I described above.
My assumption above has proved wrong. The difference between corrected
mni coordinates and tal coordinates is much bigger than expected: x
values are the closest - the biggest difference is under 2mm; y
coordinates differ by as much as 10mm and z even 17mm (and that's with
a sample of 38 points; these are the same points in the original space
for both analysis, I doublechacked). That is much too big of a
difference if mni and tal brains differ up to 10mm. I have used slicer
to see how well the brains match (in their original space, no
transformation between the two), and they are not that different.. (I
have that picture and will try to attach it). Here are a few points:
mni coordinates:
6.95 -92.4 19.1
-9.38 -85.7 2.32
6.96 -91.3 8.14
16.8 -93.7 29.8
mni -> tal corrected:
6.8805 -88.638 22.03
-9.2862 -82.918 6.2865
6.8904 -88.076 11.906
16.632 -89.405 31.924
tal coordinates:
6.6 -96 5.3
-9.59 -86.8 -10.1
6.62 -93.3 -5.23
16.4 -99 15.5
I even tried calculating the transformation between mni and tal brains
- I wanted to see whether applying this transformation to mni
coordinates would give me numbers close to the tal coordinates but no.
When I applied that transformation to mni coordinates I got numbers
close to the mni -> tal corrected values (Brett) but still far from tal
coordinates.
Any thoughts on this would be helpful... thanks!
Zrinka Bilusic-Vezmar
UCLA Brain Mapping
660 Charles Young Drive South
Los Angeles, CA 90095
310-794-5060
[log in to unmask]
www.brainmapping.org
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