DIAGNOSTIC ULTRASONOGRAPHY *

Diagnostic ultrasonography-the use of sound waves in the examination of patients-is a relatively recent development. Probably fewer than 500 physicians in the United States have had direct experience in this field. Yet ultrasonic diagnosis cannot properly be classified as an investigational technique, since clinical decisions based on such findings are being made routinely in scores of leading hospitals. The method does not involve the use of ionizing radiation or the injection of foreign substances into human tissue. It makes use of high-frequency (therefore inaudible) sound transmitted in brief pulses, separated by relatively long “silent” intervals. Pulsed ultrasound, a t the energy levels used for diagnostic purposes, has been demonstrated to be harmless to human tissues.. Examinations are usually performed at power levels that could be increased one thousand times before approaching the number of watts per square centimeter of tissue at which deleterious effects occur. Diagnostic ultrasonography is totally painless ; examinations require neither general nor local anesthesia. In contrast to roentgenography, in which the film is placed behind the tissue to be examined, ultrasonic information is picked up a t the original point of transmission, in the form of echoes from internal structures. Echoes are generated at each interface along the path of the sound beam. An interface is a site a t which one tissue differs from adjacent tissues in acoustic characteristics. One of the unique characteristics of diagnostic ultrasonography is that echoes are received not only from soft-tissue-to-bone interfaces, but also from many softtissue-to-soft-tissue, and soft-tissue-to-fluid interfaces. Since ultrasound behaves in a manner similar to that of light, only structures that lie a t or very near a right angle to the sound beam will reflect sound back to the transducer.