1. A method for retrospective internal gating comprising: a. Acquiring a series of images at times t 1. .. tn b. Extracting time-activity information for individual voxels; c. Prioritizing voxels for phase analysis, and assign weighting factors; d. Applying frequency filter to voxel time-activity curves; e. Using prioritization, combine voxel time-activity information into a time varying object motion function; f. Using the time varying object motion function for the mapping of image data to corresponding motion phases.