Calibration of Video Cameras to the Coordinate System of a Radiation Therapy Treatment Machine

There has been recent interest in using video cameras along with computer vision and photogrammetric techniques to aid in the daily positioning of patients for radiation therapy. We describe here a method to calibrate video cameras to the beam coordinate system of a radiation therapy treatment machine. Standard camera calibration relates the threedimensional coordinate system of a calibration phantom to the two-dimensional image coordinates. Using a calibration phantom designed for simultaneous X-ray and video imaging, both types of images can be calibrated to a single coordinate system. A series of X-ray images of the calibration phantom are taken using the motion of the treatment machine. Camera calibration parameters derived from these images are used to find a transformation from the coordinate system of the calibration phantom to the beam coordinate system of the treatment machine. This transformation was applied to the video camera calibration to provide a camera calibration directly to the beam coordinates of the machine.