Tracking and targeting the tumors are simultaneous processes in the image-guided radiotherapy (IGRT); this is expected to boost the efficiency, the reliability, and the safety in the treatment. Varian Medical Systems (VMS) has already produced and installed the first IGRT machine; the device comprises the VMS Clinac equipped with the On-Board Imager (OBI) component. Cone-beam CT (CBCT) imaging, one of the options of the OBI machine, aims at high-quality volumetric reconstruction. A number of calibrations are needed in order to operate our CT-imaging machines properly; they ensure that the machine components are properly aligned, the mechanical distortions are small, and yield important output that is used in the reconstruction of the actual scan data. The geometrical calibration is achieved by using a needle phantom. In order to increase the dynamic range of our imager (hence, to obtain reliable information simultaneously in the high- and the low-attenuation areas of the irradiated object), VMS has developed a dual-gain mode. Next on our agenda is the suppression of (ring, streak, and beam-hardening) artefacts in our reconstructed images and the further development of our detectors in order to remove patterns relating to lag and ghosting effects.