The valve of an active breathing control (ABC) device can be driven by the output of the navigator channel (or another fast-acquisition output) of the MRI scanner, rather than by inference from a measured breath flow rate. This has the advantage that the MRI output can yield a first-hand measurement of the diaphragm position or the tumour position. The computed lung volume obtained from a pneumotachograph is subject to drift over time for a variety of reasons, so a direct measurement of the anatomy will be more reliable. Where the MRI scanner is integrated with a radiotherapy device, the MRI data can be used to trigger the enforced breath-hold by the ABC, and the radiotherapy delivered while the ABC valve is shut. If the MRI data pertains to the actual position of the tumour, then the ABC device will (in effect) hold the tumour at a precise and reproducible point for treatment.