We provide a radiotherapy apparatus including a source of therapeutic radiation, a source of imaging radiation having an energy level less than that of the therapeutic radiation, a detector for radiation lying within the field of both the therapeutic radiation and the imaging radiation and able to image both, a first imaging circuit supplied with the output of the detector, a second imaging circuit also supplied with the output of the detector, a first pulse trigger circuit adapted to trigger the source of therapeutic radiation to produce a pulse of therapeutic radiation and to trigger the first imaging circuit to capture an output of the detector; and a second pulse trigger circuit adapted to trigger the source of imaging radiation to produce a pulse of imaging radiation and to trigger the second imaging circuit to capture an output of the detector.