 Further modifications dating from 1961 included the use of a stereotactic frame and brain images to make it easier to navigate in the frontal lobes, and the use of radioactive seeds (up to 20) to destroy tissue in the subcaudate region.The operation was then called stereotactic subcaudate tractotomy and by the mid-1970s, although only used at two neurosurgical units (which by that time had replaced mental hospitals as the sites for psychosurgical operations), it accounted for 30 per cent of operations in the United Kingdom.Knight operated at the Brook Hospital in South London in collaboration with retired psychiatrist Strom-Olsen and then with psychiatrist Paul Bridges.