> Could anybody tell me the reason why we should do ACPC reorientation before
> normalization? Is it a necessary step?
It is not necessary to set the AC exactly, but it can help sometimes. Spatial
normalisation is a local optimisation procedure, which involves gradually
improving a set of starting estimates. If the starting estimates are too far
from the optimal solution, then the algorithm can become cought in a local
optimum.
For most images (with good brain coverage and few artifacts), it is enough to
have the AC within about 5cm from the origin, and the images oriented to
within about 30 degrees of the MNI averages.
Best regards,
-John
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