Ultrasound therapy: a comparative study of different coupling media

Four different materials, water, oil, gel, and silicon, were considered for their relative efficiency as coupling media for ultrasonic therapy. The power loss (attenuation) as a function of distance in each medium was experimentally studied. The degree of impedance match (power reflection into the transmitter) between the medium and transducer head was also determined by measuring the standing wave ratio. The results of the study indicated that oil and water have a large degree of attenuation as well as poor impedance match with the sound head. Gel offered a smaller degree of attenuation and better impedance match. Silicon appeared to offer little attenuation but the encasing material has so much impedance mismatch with the sound head that very little power is transmitted into the silicon medium. For future automated clinical applications, the use of encased silicon such as is used for breast implant is proposed, if mismatch is reduced.