1. A method of estimating target motion for image guided radiotherapy (IGRT) systems, comprising: a. acquiring by a kV imaging system sequential images of a target motion; b. computing by said kV imaging system from said sequential images an image-based estimation of said target motion expressed in a patient coordinate system; c. transforming by said kV imaging system said image-based estimation in said patient coordinate system to an estimate in a projection coordinate system; d. reformulating by said kV imaging system said projection coordinate system in a converging iterative form to force a convergence of said projection coordinate system to output a resolved estimation of said target motion; and e. displaying by said kV imaging system said resolved estimation of said target motion.