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.