Dear SPMers,
applying a *_sn.mat file to an image involves explicit resampling by means
of the "Normalise: Write" option or the "Deformations" utility. When using
nan(1,3) for the voxel size and nan(2,3) for the bounding box, the
dimensions of the template image are used, BUT with a slight tolerance, so
that in general, there is no exact one-to-one (voxel-by-voxel) mapping
between the template and the normalised image. Instead, I repeatedly
obtained a 1-voxel discrepancy in image size and/or a half-voxel offset
between the images in one or more direction(s).
Certainly, these differences are well accounted for by the information in
the header file, so that the images are correctly co-registered. Still, if I
want to have perfect spatial correspondence between the images, I need an
additional resampling step for the normalised image by means of, e.g., the
"ImCalc" feature. This requires about twice as much time and disk space, and
(what is worse) it introduces more smoothing due to the additional
interpolation step.
Is there a way to avoid the double resampling and directly tell the
"Normalise: Write" or "Deformation" (or any other) module(s) to use not only
the same voxel size and bounding box as in the template (or even better: in
any arbitrary) image, but even the very same voxel positions and image
dimensions?
Thank you very much in advance,
Andreas
|