Some 35 years after the brothers Paul-Jacques and Pierre Curie discovered piezoelectricity, ultrasonic imaging was developed by Paul Langevin. He used the "echo method" in which underwater ultrasonic echoes were bounced off submerged objects. During this work, ultrasonic energy was observed to have a deterimental biological effect "... fish placed in the beam in the neighborhood of the source ... were killed immediately, and certain observers experienced a pain ... on plunging the hand in this region." These observations were confirmed a decade later by R. W. Wood and A. L. Loomis. Although ultrasound biological effect studies continued, the importance of characterizing the exposure conditions was not recognized. Intensity levels undoubtedly were much higher than those currently used in clinical medicine. It was not until the early 1950's that ultrasonic exposure conditions were controlled and specified so that studies could focus on the mechanisms by which ultrasound influenced biological materials. Two classes of mechanisms were identified, namely, those associated with conversion of ultrasonic energy into heat, and cavitation and other nonthermal mechanisms.
In the late 1940's and the early 1950's, pioneering work was initiated to image the human body by ultrasonic techniques. These engineers and physicians were well aware of the deleterious effects of ultrasound at sufficiently high levels, so it must be presumed that they endeavered to keep the exposure levels reasonably low. Yet, it was not until the early 1970's that major efforts were made to assess the risk of ultrasonic energy.
Over the past three decades, diagnostic ultrasound has become a sophisticated technology. However, our understanding of the potential risks has not changed appreciably from that of the 1950's. However, it is very encouraging that human injury has never been attributed to clinical practice of diagnostic ultrasound.
After a historical introduction, the lecture focuses on the risk of using ultrasound in medical practice. Assessing the risk includes determining which biological systems are most sensitive to ultrasound, and the exposure levels that impose a significant risk on these systems. This approach requires a greater understanding of energy absorption locally and regionally (dosimetry), and understanding of ultrasound-biological material interaction phenomena (biophysics). These topics are covered in the lecture.
1997 IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 44:727
© 1997 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved.