Figure 4.

Groupwise registration method. Initially all images S_i are registered to an arbitrary DW image S^* taken as reference. Then, all images are resampled and averaged to obtain a first average S̄⁽¹⁾, which becomes the new reference. At iteration k, the images S_i are registered to the mean S̄^(k) to obtain the transforms $A_i^{(k)}$. After convergence, S̄^(k) is registered to $T_2^{\mathit{epi}}$, and the composition of transformations $T\circ A_i^{z(k)}$ is applied to map all images S_i into the coordinate system of $T_2^{\mathit{epi}}$.