An ultrasound window to perform scanned, focused ultrasound hyperthermia treatments of brain tumors.

A series of calculations and tests were performed on polyethylene, polystyrene, acrylic, and a commercially available polymethyl methacrylate cranioplastic to determine which material would best serve as an acoustical window in the skull. The purpose of the window is to transmit focused ultrasound to treat brain tumors with hyperthermia. Each material was evaluated based on its ability to transmit power and to protect the brain. The results revealed that, of the four materials tested, polyethylene transmitted the largest percentage of incident power and is the toughest and best suited material to protect the brain. Further physical tests showed that a polyethylene plate does not significantly distort the sound field. Finally, acute and chronic dog tests (supported by theoretical calculations) using the polyethylene as an acoustical window through the skull showed that it will not reach temperatures greater than the target hyperthermia temperature (42 degrees C) at the required intensity levels unless the tumor is near the skull and the skin surface is not cooled. Since polyethylene effectively transmits power without distorting the sound field of overheating in dog tests, it may now be used in Phase 1 clinical ultrasound hyperthermia tests on human patients. However, the chronic animal studies indicated that the collagen which composes the dura thickens under the cranioplasty and absorbs increasingly greater amounts of power with time. A thick layer of organized fibrous tissue also formed on the external surface of the cranioplasty, filling in the cavity formed by the exterior surface of the cranioplasty and by the excision of the temporalis muscle.(ABSTRACT TRUNCATED AT 250 WORDS)