Patent Number: 
Section: claims

1. A method of cone beam CT scanning, the method comprising:performing cone beam CT scanning to acquire two-dimensional projection images of a patient;for each of a plurality of projection images, using a feature within the projection image to determine breathing phase for that projection image; andapplying respiration correlation techniques directly to the projection images based on the determined breathing phase. 2. A method of cone beam CT scanning according to claim 1 in which projection images that have comparable breathing phases are selected from the complete data set on completion of the acquisition and are used to reconstruct the volume data. 3. A method of cone beam CT scanning according to claim 1 in which the feature is the position of the patient's diaphragm. 4. A method of cone beam CT scanning according to claim 1 in which visual and/or audible prompts are provided for the patient's breathing. 5. A method of cone beam CT scanning according to claim 1 in which therapeutic radiation is delivered during the scan at times correlated with the patient's breathing cycle. 6. A cone beam CT scanner comprising:means for performing cone beam CT scanning to acquire two-dimensional projection images of a patient;means for using a feature within the projection image for each of a plurality of projection images to determine breathing phase for that projection image; andmeans for applying respiration correlation techniques directly to the projection images based on the determined breathing phase. 7. A cone beam CT scanner according to claim 6 arranged to select projection images that have comparable breathing phases from the complete data set on completion of the acquisition and to use these to reconstruct the volume data. 8. A cone beam CT scanner according to claim 6 in which the feature is the position of the patient's diaphragm. 9. A cone beam CT scanner according to claim 6 including means to provide visual and/or audible prompts for the patient's breathing. 10. A radiotherapy device comprising a cone beam CT scanner and a source of therapeutic radiation, wherein the CT scanner:performs cone beam CT scanning to acquire two-dimensional projection images of a patient;uses a feature within the projection image for each of a plurality of projection images to determine breathing phase for that projection image; andapplies respiration correlation techniques directly to the projection images based on the determined breathing phase to deliver therapeutic radiation during the scan.