1. A method for determining patient motion in an image, the method executed by an imaging system including a computer with an electronic processing unit and a memory module storing a motion vector extraction module executable by the electronic processing unit, the method comprising: obtaining, at the computer, an image based on image data generated by a scanner during a scan, the image including at least three markers assumed to be in a rigid or semi-rigid configuration, each of the at least three markers having actual measured positions on a detector panel of the scanner in a first dimension and a second dimension; determining, with the electronic processing unit, a reference three-dimensional position for each of the at least three markers; defining, with the electronic processing unit, 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; and solving, with the electronic processing unit, 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.