A method and a system for determining patient motion in an image. The method includes obtaining an image based on image data generated by a scanner during a scan. The image includes at least three markers assumed to be in a rigid or semi-rigid configuration. Each of the at least three markers has actual measured positions on a detector panel of the scanner in a first dimension and a second dimension. The method further includes determining a reference three-dimensional position for each of the at least three markers and defining equations describing the relationship between the reference three-dimensional position and the actual measured positions of each of the at least three markers, geometric parameters of the scanner, and patient motion. The method finally includes solving numerically the equations to derive a six-component motion vector describing patient motion for the image that accounts for differences between the reference three-dimensional position of each of the at least three markers and the actual measured positions for each of the at least three markers.
