Hi Jason,
I think perhaps my answer wasn't clear. What I was saying was the small
asymmetries such as the ones you described are *maintained* by FLIRT
because the registration is dominated by the global cost function -- not by
local factors of this sort. As a result, VBM approaches to looking for
asymmetries are fine (but typically use a symmetric template, particularly
if they allow non-linear warping which FLIRT does not). See, for instance,
Watkins KE, Paus T, Lerch JP, Zijdenbos A, Collins DL, Neelin P, Taylor J,
Worsley KJ, Evans AC. (2001) Structural asymmetries in the human brain: a
voxel-based statistical analysis of 142 MRI scans. Cereb Cortex. 2001
Sep;11(9):868-77.
Having said that, if you are making a morphometric comparison of PT lengths
or HG volumes, then you should almost certainly be measuring them on the
raw structural at it's initial resolution after doing a 6DOF transformation
into a standard position/orientation. You don't want any scaling or
sheering to alter the raw numbers you measure. See:
Foundas AL, Eure KF, Luevano LF, Weinberger DR. (1998) MRI asymmetries of
Broca's area: the pars triangularis and pars opercularis. Brain Lang. 1998
Oct 1;64(3):282-96.
Joe
Joseph Devlin, Ph. D.
FMRIB, Dept. of Clinical Neurology
University of Oxford
John Radcliffe Hospital
Headley Way, Headington
Oxford OX3 9DU, U.K.
Phone: +44 (0)1865 222 494
Fax: +44 (0)1865 222 717
Email: [log in to unmask]
|