Figure 3.

An illustration of intersection based slice motion estimation: Any intersecting pairs of slices through the moving fetal head must match along their intersection when mapped correctly together into the 3D anatomical space. The square difference in intensity along an intersection is related to corresponding changes in the spatial transformation applied to each slice. By considering all slice pairs acquired in a multislice study consisting of many stacks of slices, a numerical optimization scheme can be formulated to minimize discrepancies, using the covariance matrix of all matches with respect to the slice transformation parameters. This collectively adjusts the 3 translations and 3 rotations of each slice to bring them into mutual alignment.