Fig. 3.

SimpleITK Python code for multi-modality rigid registration. Initialization aligns the geometric centers of the two volumes. The optimized metric is mutual information using 1% of the image voxels and a gradient descent optimizer. A novel feature of the ITKv4 registration framework is the option for automatic scaling of the optimized parameters so that a unit change in each parameter will have similar effects in physical space, the

SetOptimizerScalesFromPhysicalShift

function. This is of particular importance with rigid registration where our parameter space consists of rotation angles in radians and translation in millimeters