1. A radiotherapy apparatus, comprising: a rotatable gantry, supporting a source of radiation, wherein the source of radiation is selectably configurable to produce a beam that is either a high-energy beam or a low-energy beam; at least one collimator associated with the source of radiation, and configured to limit a cross-sectional area of the beam produced by the source of radiation; a reconstruction device configured to: obtain images of a patient using the low-energy beam of the source of radiation; retain the obtained images at least temporarily; apply a threshold to the retained images to exclude images obtained before the threshold; and select at least three retained images meeting the threshold and reconstruct a CT volume using the selected images, wherein a first image of the selected images is weighted more heavily than a second image of the selected images when reconstructing the CT volume, the first image having been obtained after the second image; and a control device configured to conduct a treatment fraction using the high-energy beam of the source of radiation, and control the at least one collimator using information derived from the CT volume.