Dear Andrew and list,
many thanks for the previous reply which has been very helpful and enabled
us to
proceed with our project. Unfortunately we'll need some further guidance as
we
have not been able to validate our basic assumptions.
We have used an affine transform (only) to map gray and white segmentation
maps
into Talairach space, resampling from 0.9375x0.9375x1.5mm voxel size (voxel
volume 1.32 mm3) to 1mm isotropic. Sinc interpolation was used. The
Transformation matrix Q was
X1 = 1.201*X - 0.137*Y - 0.073*Z + 1.270
Y1 = 0.165*X + 1.051*Y + 0.444*Z + 1.284
Z1 = 0.004*X - 0.514*Y + 1.113*Z - 2.366
(from mm3 to mm3) with an absolute determinant of 1.71, which should be
equal to
the ratio of voxel volumes before and after the transformation, as we
discussed
previously. However, we get (counting voxels greater than 0):
gray: 797705 voxels = 1051662 mm^3
ngray: 2094698 voxels = 2094698 mm^3
Volume ratio: 1.992
white: 476199 voxels = 627801 mm^3
nwhite: 1607171 voxels = 1607171 mm^3
Volume ratio 2.559
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.
We are quite puzzled by this result and would greatly appreciate your help.
Best
regards and a nice start of the week,
Andreas
Andreas Meyer-Lindenberg, MD PhD (Dr. med. Dr. med. habil.)
Research Fellow
Unit on Integrative Neuroimaging, Clinical Brain Disorders Branch,
National Institute of Mental Health, NIH
10-4C101, 9000 Rockville Pike
Bethesda, MD 20892-1365
(W) (301) 496 9672 (F) (301) 496 7437 email [log in to unmask]
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