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Figure 4. Non-rigid registration applied to lesion detection in contrast-enhanced MR mammography. The subject is scanned at rest and then scanned repeatedly after introduction of a contrast agent. There is often rigid and non-rigid movement when the agent is given. The pre- and post-contrast images appear virtually identical but subtraction reveals many differences (top panel), most caused by motion. Rigid registration (middle panel) reduces the difference between scans significantly. Non-rigid registration using a hierarchical B-spline technique (bottom panel) removes virtually all the artefact associated with motion leaving clear evidence of an enhancing lesion.
