A new ultrasonic device that removes tiny fat droplets from blood should help prevent brain damage after heart surgery.

Two thirds of patients undergoing major heart operations suffer some form of mental impairment afterwards, such as a reduced ability to perform mental arithmetic or remember phone numbers. In half of these patients the problems are permanent.

The cause is still controversial, but most researchers think that minute fat droplets lodging in the blood vessels of the brain are responsible. It is thought these block the supply of oxygen to tiny clusters of nerve cells.