During general anaesthesia, to ensure sufficient oxygen supply to the brain, the NIRS technology can be used. It consist of two sensors placed on the forehead, and measures the oxygen saturation of the tissue underneath the sensor. For patients undergoing back surgery placed in the prone position, the NIRS measurement will be affected, because the weight of the head will produce a pressure on the sensors. We wish to investigate how much the measurement is affected. Furthermore we plan to lift the head to remove the pressure, and record the time needed for the measurements to return to baseline.