A radiotherapy system comprises a patient support, moveable along a translation axis, an imaging apparatus, comprising a first magnetic coil and a second magnetic coil, the first and second magnetic coils having a common central axis parallel to the translation axis, and being displaced from one another along the central axis to form a gap therebetween, the imaging apparatus being configured to obtain an image of a patient on the patient support, a source of radiation mounted on a chassis, the chassis being rotatable about the central axis and the source being adapted to emit a beam of radiation through the gap along a beam axis that intersects with the central axis, a multi-leaf collimator comprising a plurality of elongate leaves movable between at least a withdrawn position in which the leaf lies outside the beam, and an extended position in which the leaf projects across the beam, and a radiation detector mounted to the chassis opposite the source, the radiation detector having a plurality of detector elements aligned with the elongate leaves when projected onto an isocentric plane.