Multi-object segmentation of brain structures in 3D MRI using a computerized atlas

We present a hierarchical multi-object surface-based deformable atlas for the automatic localization and identification of brain structures in MR images. The atlas is a multi-object mesh of 3D fully connected surfaces built upon a face centered cubic grid. The registration of the atlas to a patient's MR image is done in two steps : a global registration followed by a multi-object active surface deformation. First, the cortical surface and the ventricular system are segmented using directional watersheds. The global registration is a second degree transformation whose coefficients minimize a distance measure between these surfaces and the equivalent surfaces in the atlas. As a refinement step, the globally registered atlas surfaces are locally deformed using multi-object active surfaces. The external force driving the surfaces towards the edges in the image is a decreasing function of the gradient, and includes prior image information. The active surface equations are then solved using the finite element method. The surfaces of the multi-object mesh are deformed in a hierarchical way, starting with objects exhibiting very well defined features in the image to objects showing less obvious features. Experiments involving several sub-cortical atlas objects are presented.