An example of the approximated Hessian matrix, calculated for the registration of three orthogonally planned stacks. Axial, sagittal and coronal stacks are denoted by ax, sc and cr, respectively. The translation of a slice in one stack is correlated with the translation of the slices from other stacks. The correlation between a translation and a rotation can be both (+) and (−) within one stack, depending on which side in the stack a slice is located with respect to the center slice of the stack. The rotation-rotation correlation can be identified likewise. Grid lines are added for reading.