A method for defining the shape of a radiation beam that is directed toward a subject and to a free-standing imaging detector detects the position and orientation of the imaging detector relative to a radiation source, then adjusts an aperture that lies in the path of the radiation beam to shape the beam for incidence on a predetermined area of the detector according to the detected imaging detector position. The radiation source is energized to emit the shaped radiation beam and the image data about the subject is acquired from the imaging detector.
