In radiotherapy apparatus comprising an imager for the MV beam and a separate kV imaging system, it is common for there to be significant artifacts in the kV image arising from the MV pulse. We disclose such a radiotherapy apparatus which is adapted to (i) cause the therapeutic source to emit radiation while keeping the diagnostic source inactive, (ii) read an image from the detector associated with the therapeutic source, (iii) cause the diagnostic source to emit radiation and while keeping the therapeutic source inactive, (iv) read an image from the detector associated with the diagnostic source, and (v) repeat as necessary. The control unit is preferably adapted to, at steps (ii) and (iv), read an image from both of the detectors, or to clear the “inactive” detector. The detectors are ideally flat-panel detectors, capturing a two-dimensional image.