Getting from SPM world coordinates (mm) to voxel coordinates
SPM likes its coordinates in mm. If you ever find the need to use voxel coordinates, e.g. for running analyses in other software packages, the below function gives a quick transform. Once added to your Matlab path, you can find your ROI in the SPM results interface, and then type mm_to_vox(hReg,xSPM) to get your current position in voxels. The relevant inputs (hReg, xSPM) must be in your workspace, but don't worry as this will always be the case if you used the standard SPM results viewer to bring up your data.
(updated 10 June to fix incompatibility with SPM8)
% Returns the voxel coordinates for the current position in the
% SPM results viewer.
% J Carlin 10/6/2011
%function xyz_vox = mm_to_vox(hReg,xSPM)
function xyz_vox = mm_to_vox(hReg,xSPM)
%hReg = findobj('Tag','hReg'); % get results figure handle
xyz_mm = spm_XYZreg('GetCoords',hReg); % get mm coordinates for current locationassignin('base','xyz_mm',xyz_mm); % puts xyz_mm in base workspace
xyz_vox = xSPM.iM*[xyz_mm;1]; %evalin('base','xSPM.iM*[xyz_mm;1]'); % avoid passing xSPM explicitly
xyz_vox = xyz_vox(1:3);