Magnetic resonance imaging (MRI) is used for therapy planning. The motion or position of the treatment region is tracked over time for many cycles using MRI. For temporal resolution, the tracking is done in planes through the tumor at different orientations rather than using three-dimensional scanning. The tracking may be used for calculating a spatial probability density function for the target. Alternatively or additionally, spatiotemporal information derived from the surrogate is compared directly to that from the tracked object to determine the accuracy or robustness of the surrogate-to-target 3D correlation Gating or tracking based on this surrogate may be performed where the comparison indicates that the surrogate is sufficiently reliable (accurate).
