A method for controlling emission of a beam of electrons in an X-ray imaging tube. The X-ray imaging tube has an anode and a cathode. The method comprises a step in which at least one emission device included in the cathode emits an accelerated incident beam of electrons onto an impact focal spot on the anode to generate X-rays. An emission device is associated firstly with an assembly of polarizing plates and secondly with a focusing cup. An electric generator simultaneously applies a beam focusing voltage to the assembly and/or to the cup to control a characteristic dimension of the focal spot, and a cut-off voltage to the cup to control the emission of the beam by the device. The invention also concerns a corresponding cathode, tube and imaging system.