The location of significant rCBF changes identified on PET images is expressed as 3D coordinates (x, y, z) in reference to the standardized coordinate system of Talairach and Tournoux . This system uses the AC-PC plane (anterior and posterior commissures) as the z origin and the VCA (vertical perpendicular to AC) as the y origin. SPM95 uses Talairach and Tournoux  coordinates, SPM 96 and SPM 99 use a standard brain from the Montreal Neurological Institute (MNI). The problem introduced by the MNI standard brain is that its linear transformations do not completely match the brain into the Talairach space. The MNI brain is slightly larger than the Talairach brain. In order to make direct comparison across studies, we have chosen to translate the Talairach coordinates acquired before 1996 into MNI coordinates according to the following formula (given by Andreas Meyer-Lindenberg of Unit on PET, Clinical Brain Disorders Branch, National Institute of Mental Health on SPM mail list):
1.36 X_talairach + 0.91
1.03 Y_talairach + 3.42
-0.06 Y_talairch+1.14 Z_talairach + 0.31.
Even if this method has been used to reduce the variability between studies, it does not exclude methodological factor variability due to the analysis procedures—for example, intersubject averaging and spatial resolution.