说明：  Portal images are acquired by electronic portal imaging devices (EPID) with megavoltage (MV) x-ray, but they are inherently poor in terms of contrast, due to Compton Effect. In comparison with diagnostic x-ray images, portal images usually lack sufficient detail information for normal human vision. Therefore, an effective method of enhancing these images would be very useful. This paper proposes a new approach that combines global and local enhancement technique