This surgical device uses a laser beam to coagulate tissue. The elongate device carries the beam through an optical fiber that has a chamfered end for deflecting the beam out of a distal end of the device at an angle. Tissue coagulation is monitored using a magnetic resonance imaging system that provides periodic tissue temperature data. A small fluid supply duct provides cooling fluid that expands to a gas near the distal end of the device, thereby cooling the tip.