[Freesurfer] Coordinate Transformations

Hi,

I'm working on a project project comparing the cortical thickness in epileptic patients to the corresponding brain area of a control subject. To do this, I have been trying to transform the patient RAS coordinates to talairach coordinates and then transforming the talairach coordinates into the RAS coordinates of the control. In doing this, I have two problems. First, when I use the transformation matrix provided in the free surfer subject folder (multiply RAS coordinate by transformation matrix in /mri/transforms/talairach.xfm) the coordinates seem to map to similar although not exactly the same area. Second, what is the best way to transform a set a talairach coordinates to RAS coordinates?

Thank you for your help,

Jared