Correction of geometrical distortions in video based electronic portal imaging systems

In clinical routine electronic portal imaging systems are most commonly used for simple therapy verification and documentation. However this method has a much greater potential. Since most video based portal imaging systems deliver images, which are spoiled by geometric distortions [2,4], additional effort is necessary for image corrections. These can be done to a degree that allows absolute measurements of clinically relevant information as for example leaf positions of a multileaf collimator. Both the effort necessary for the corrections and the inaccuracies remaining afterwards strongly depend on the mechanical stability of the setup and image quality. While the mechanical aspects are the responsibility of the manufacturer the main influence on the image quality - noise - can be reduced by averaging of several images.