Hierarchical Structure

Transforms along each branch are concatenated to form a composite transform applied to the leaf node.
The figure below displays an example MRML file and the tree-like graph it represents. The order of nodes in the file defines the tree structure; each transform is a parent node, and all the nodes following it (until the next transform) are its children. In the figure, the transform for the last volume in the file is the concatenation of all the transforms in the file. More complex trees, such as where multiple siblings have children, can be constructed.

In the figure above, pre-operative volumes named spgr and fmri were registered, and the fMRI transform was computed to align the fmri volume to the spgr. Then, the Pre-op transform was computed to align this fused, pre-operative data to the intra-operative volume called intra. Thus, the transform applied to the fmri volume is the concatenation of both transform nodes in the MRML file.