Patent Number: 
Section: claims

1. The investigative X-ray apparatus, comprising a source of X-rays emitting a cone beam centred on a beam axis, a collimator to limit the extent of the beam, and a two-dimensional detector, the apparatus being mounted on a support which is rotatable about a rotation axis, the collimator having a first state in which the collimated beam is directed towards the rotation axis and the second state in which the collimated beam is offset from the rotation axis, the two-dimensional detector being movable accordingly, the beam axis being offset from the rotation axis by a lesser amount than the collimated beam in the second state. 2. The investigative X-ray apparatus according to claim 1 in which the X-ray source is offset such that its natural axis is halfway between the two extremes called for by the collimator. 3. The investigative X-ray apparatus according to claim 2 in which the X-ray source is fixedly located in its offset position. 4. The investigative X-ray apparatus according to claim 3 of the preceding claims, having an isocentre, in which the X-ray source is not directed towards the isocentre. 5. The radiotherapeutic apparatus comprising:a therapeutic source of high energy radiation; andan investigative X-ray apparatus according to claim 3. 6. The investigative X-ray apparatus according to claim 2 in which the offset is between 3 and 4°. 7. The investigative X-ray apparatus according to claim 6 of the preceding claims, having an isocentre, in which the X-ray source is not directed towards the isocentre. 8. The radiotherapeutic apparatus comprising:a therapeutic source of high energy radiation; andan investigative X-ray apparatus according to claim 6. 9. The investigative X-ray apparatus according to claim 2 of the preceding claims, having an isocentre, in which the X-ray source is not directed towards the isocentre. 10. The radiotherapeutic apparatus comprising:a therapeutic source of high energy radiation; andan investigative X-ray apparatus according to claim 2. 11. The investigative X-ray apparatus according to claim 1 having an isocentre, in which the X-ray source is not directed towards the isocentre. 12. The radiotherapeutic apparatus comprising:a therapeutic source of high energy radiation; andan investigative X-ray apparatus according to claim 11. 13. The radiotherapeutic apparatus comprising:a therapeutic source of high energy radiation; andan investigative X-ray apparatus according to claim 1.