Radiotherapeutic apparatus

Radiotherapeutic apparatus comprising: a patient support 10; magnetic coils 16,18,20 disposed around the patient support 10 for creating a magnetic field; a radiation source 30 producing a beam of radiation directed toward the patient support 10 and mounted on a rotatable source 28 to rotate the radiation source 30 around the patient support 10; and a power supply for conveying electrical power to the radiation source comprising, in sequence, a step up transformer, a slip ring 40,42,44 and a step down transformer. The radiation source may be a linear accelerator. Preferably the electrical power provided to the radiation source 30 via the slip ring 40,42,44 is at a voltage of at least 10kv. In description, magnetic coils 16,18,20 are part of a MRI apparatus and the current required by the radiation source at standard supply voltages is high enough for slip ring generation of stray magnetic fields that interfere with those of magnetic coils 16,18,20; the advantage of the slip ring and transformer arrangement is that the step up transformer, by increasing the voltage supplied to the slip ring, also causes a reduction of the slip ring current, thereby reducing the stray interfering magnetic field level and improving image quality.