Abstract: An X-ray scanning apparatus comprises a number of multi-focus X-ray tubes (25) spaced around an axis X and arranged to emit X-rays through an object on the axis which are detected by sensors (52). Each tube (25) can emit X-rays from a plurality of source positions. In each scanning cycle, in which each of the source positions in each of the tubes is used once, the ordering of the positions used is arranged so as to minimize the thermal load on the tubes (25). This is achieved by ensuring that each source position is non-adjacent to the previously active one and the next active one.

Abstract: An X-ray tube includes an emitter wire (18) enclosed in a suppressor(14, 16). An extraction grid comprises a number of parallel wires (20) extending perpendicular to the emitter wire, and a focusing grid comprises a number of wires (22) parallel to the grid wires (20) and spaced apart at equal spacing to the grid wires (20). The grid wire are connected by means of switches to a positive extracting potential or a negative inhibiting potential, and the switches are controlled so that at any one time a pair of adjacent grid wires (22) are connected together to form an extracting pair, which produce an electron beam. The position of the beam is moved by switching different pairs of grid wires to the extracting potential.