A control system (54) for an X-ray scanner, the system comprising an input (512) arranged to receive an input signal identifying which of a plurality of electron emitters (25) is to be active, and to produce a plurality of outputs (510) each arranged to control operation of one of the emitters, wherein each of the outputs can be in a first state arranged to activate its respective emitter, a second state arranged to de-activate said emitter, or a third state arranged to put said emitter into a floating state.
