In a method for stabilizing the size of a focal spot of an X-ray tube and contrast modulation function of a X-ray tube, the tube has a cathode comprising a concentrator and a heating filament. It has an anode is placed so as to be facing the cathode. With the filament, the cathode produces an electron beam bombarding the focal spot of the anode. A bias voltage is applied between a terminal of the filament and a terminal of the concentrator. The tube has means to measure the size of the focal spot and the function of contrast modulation. It also has means to regulate the size of the focal spot and the contrast modulation function by means of a bias voltage.