Patent Number: 
Section: claims

1. A radiation source, comprising:an electron gun,a pair of targets each locatable in the path of a beam produced by the electron gun, the targets having different emission characteristics, wherein a first target of the pair is an electron window that emits x-radiation used for the production of images, andan electron absorber insertable into and withdrawable from the path of the beam for removing electrons from the beam but allowing x-radiation to pass through. 2. A radiation source according to claim 1, wherein the electron window is of Nickel. 3. A radiation source according to claim 1, wherein a second target of the pair is of Copper, Tungsten, or a composite including Copper and Tungsten. 4. A radiation source according to claim 1, wherein the electron absorber comprises a material of a lower atomic number than the electron window. 5. A radiation source according to claim 1, wherein the electron absorber is of Graphite, Carbon, Beryllium or Aluminium. 6. A radiation source according to claim 1, wherein the electron gun is within a vacuum chamber, and at least one of the targets is located at a boundary of the vacuum chamber. 7. A radiation source according to claim 1 further comprising:a primary collimator located in the beam subsequent to the targets. 8. A radiation source according to claim 7, wherein the electron absorber is located in the primary collimator. 9. A radiation source according to claim 7, wherein there are a plurality of primary collimators interchangeably locatable in the path of the beam, at least one of which primary collimators contains the electron absorber. 10. A radiation source according claim 1, wherein the electron window is substantially transparent to the beam. 11. A radiation source according to claim 1 further comprising:at least one of a bow-tie filter and a diagnostic filter, such filter being selectably locatable in the path of the beam. 12. A radiation source according to claim 1 further comprising:at least one adjustable collimator extendable into the path of the beam thereby to delimit it, the adjustable collimator being sized to substantially attenuate a megavoltage x-ray beam. 13. A radiotherapy apparatus including a radiation source according to claim 1. 14. A radiotherapy apparatus according to claim 13, wherein the radiation source is rotatable around a horizontal axis that lies in the path of the beam. 15. A radiotherapy apparatus according to claim 14, wherein the horizontal axis is perpendicular to the beam. 16. A radiotherapy apparatus according to claim 13, further comprising a flat panel imaging device in the path of the beam. 17. A radiotherapy apparatus according to claim 16, wherein the flat panel imaging device includes a scintillator layer. 18. A radiotherapy apparatus according to claim 17, wherein the scintillator layer includes of at least one of Caesium Iodide, Gadolinium Oxisulphide and Cadmium Tungstate. 19. A radiotherapy apparatus according to claim 13, further comprising a patient support between the source and the flat panel imaging device. 20. A radiotherapy apparatus according to claim 13 further comprising:a control means programmed to obtain a plurality of two-dimensional images using x-radiation produced by the radiation source, anda reconstruction means adapted to reconstruct those images to form a three-dimensional CT dataset.