Abstract: An MRI guided surgical apparatus includes a heat source formed by a laser and an optical fiber carrying the heat energy into a part to be coagulated by hyperthermia with an end reflector to direct the energy in a beam to one side of the fiber end. The fiber includes a reinforcing sleeve along its length to prevent bending and twisting. The sleeve is mounted in a shielded, Piezo-electric motor which causes movement of the fiber longitudinally of its axis to move the end within the part and rotation of the fiber about its axis to cause the beam to rotate about the axis. A rigid elongate cannula is arranged for insertion to a position at the part of the patient having a bore for receiving a portion of the fiber adjacent the outlet end in sliding engagement therein such that the end can pass through the cannula into engagement with the part of the patient. A part of the sleeve projecting beyond the cannula is rigid and a further part connecting to the motor is stiff but less rigid.