An MRI guided surgical apparatus includes a heat source formed by a laser (21) and an optical (20) 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. A reinforcing sleeve (38) for the fiber is mounted in a shielded, Piezo-electric motor (22) which causes movement of the fiber longitudinally and angularly within a rigid elongate cannula. A magnetic resonance imaging system is arranged to generate a series of output signals over a period of time representative of temperature in the part as the temperature of the part changes during that time. The heat source is controlled in heat energy applied and location and orientation of the beam to stop heating when the temperature at the boundary of a tumor reaches the required hyperthermic temperature. Cooling of the tip portion of the probe can be effected by encasing the fiber in a tubing material defining a duct for the fiber and supply and return parallel ducts for cooling fluid, the tubing being encased in a titanium stiffening sleeve which is driven in the longitudinal and rotational directions to move the fiber tip.
