Dear John, Bas and Marko,
Thank you all so much for your insightful responses. (Let's not forget Dr. Mclaren whose response started the discussion.) But I still have some questions with which I would appreciate further discussion.
For Marko and John,
If MNI coordinates are in fact in units of mm, does SPM account for voxel sizes and shapes when displaying voxel coordinates?
More specifically, does a common MNI coordinate (5 -2 3) correspond to the same geometric position for a common reference original point when different voxel sizes are used, e.g. [2 2 2] and [3 3 3.75]. If not, how do we deal with this issue.
This has puzzled me for quite some time...
For Bas,
1) We might have diverged understanding of the concepts of "voxel space" and "mm space ( which I assume is some sort of stereotactic spaces such as MNI) ".
For example, let's say that a particular image has dimension of [79 95 68].
To what I know, its "voxel space" ranges from 1 to 79*95*68 or alternatively, from (0, 0, 0) to (79, 95, 68). In other words, the coordinates of a certain voxel in "voxel space" are in fact the matrix indices of that voxel.
If I am correct, then coordinates in "voxel space" must be integers, since it is impossible to have floating point numbers as matrix indices. But I found floating point numbers, e.g. 29.6 19.1 33.3 when displaying such images. So where did I go wrong?
2) One more thing, I assume that V.mat is the affine transformation matrix used in normalising the raw images to MNI space. So I don't see why and how it can be applied for the purpose of voxel to mm conversion...
Many thanks and best regards,
Ce
