In a combined MRI and radiation therapy system, a magnet structure and radiation therapy equipment are provided. The magnet structure comprises a single substantially cylindrical field coil structure comprising a number of superconducting coils joined by a support structure and extending axially of a central region. An outer vacuum chamber encloses the field coil structure in an evacuated volume. A cooling arrangement comprising cooling tubes is in thermal contact with the superconducting coils and receives a cryogen flowing through the cooling tubes. The radiation therapy equipment comprises a gamma radiation source rotatable about an axis of the field coil structure to direct a radiation beam substantially radially through the field coil structure. Parts of the field coil structure and the outer vacuum chamber are transparent to radiation emitted by the gamma radiation source whereby the radiation beam is directed through the field coil structure and the outer vacuum chamber without substantial interference.
