Ultrasonic echo sounding has been established for several years in industrial practice for the detection of flaws in metals. We have applied the principle of this technique to reveal the boundaries between the different tissue structures within the body. Ultrasonic energy is mechanical, vibrational energy whose frequency is beyond the range of hearing and, at the high frequencies which we are employing, namely 2·5 and 1·5 Mc/s, it can be focussed into a narrow, almost parallel beam which, apart from its greater ability to penetrate tissue, is transmitted and reflected in much the same way as a beam of light. The physical nature, however, of this ultrasonic energy is totally different from that of electromagnetic radiation but this difference in itself may provide scope for enlarging the field of diagnostic radiology. The sound is transmitted into the patient in exceedingly short pulses or “wave packets”. Such a pulse is in some ways similar to a single quantum of X-ray energy except, of course, that it is simply a pressure wave, which gradually loses its energy by partial reflection from the tissue interfaces it crosses until finally it dies away completely. During this period echoes from the various tissue interfaces are being recorded by electronic means. Only after all the echoes have been detected, and the pulse has completely died away, is another pulse transmitted, and the whole process repeated. This happens 50 times a second.
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Donald et al. (1961) studied this question.