Ultrasound coupling media: their relative transmissivity

The relative transmissivities of several commonly used ultrasonic coupling agents were measured to determine if there were significant differences in their effectiveness. Coupling media were evaluated in thin films to simulate clinical use. In order to minimize experimental error in this determination, transmitted energy was measured in the far field of the sonating transducer; the receiving transducer was angled slightly to minimize standing waves; and the sonating chamber was made as anechoic as possible. It was found that there is no practical difference in the transmissivities of the common coupling agents, except that hydrocortisone ointments and creams had lower transmissivities, probably due to microscopically entrapped air. When used as a thin film, absorption of ultrasound in the media was insignificant, and variation in transducer pressure was found to cause differences in energy transmitted greater than those found between the common coupling media. Consequently, it was concluded that coupling media can be chosen primarily on the basis of cost and convenience.