1. A method for retrospective internal gating comprising: acquiring a series of images at times t 1. .. tn including a moving object; extracting time-activity information for voxels of the images; generating a time varying object motion function based on the extracted time-activity information for the voxels of the images; determining, using the time varying object motion function, phase information for motion of the moving object; and generating an updated series of images correcting for the motion of the moving object using the determined phase information for motion of the moving object.