I arrived at the Massachusetts Institute of Technology (MIT) in 1950, the war was over, and new technology was in the offing, for the benefit of mankind. One of the foremost beneficiaries was to be medicine. But there was not yet a common language between M.D.s and Ph.D.s; the term "bioengineering" was not yet coined, and interdisciplinary research was a new modus operandi. Radar and sonar signal processing had made great strides during the war, but much of this was still shrouded in secrecy. Newly available isotopes gave strong impetus to the new field of nuclear medicine. Such was the stage on which the joint MGH/MIT program on brain tumor detection was launched by the Massachusetts General Hospital (MGH), combining re-sources with MIT.

Prior to my arrival in the USA, I had been involved, at the Siemens electromedical laboratories in Erlangen, Germany, in ultrasonic propagation experiments in ani-mal tissues (1946 -1949). Measurements were made at various frequencies near 1 MHz, which held promise for reflections at interfaces and for differential absorption through the tissue layers traversed (Hueter 1948). It was hoped that this would lead to diagnostic or therapeutic applications of an energy that could be easily beamed. Ultrasonic metal inspection using A-scan presentations was finding general acceptance in industry in the late 1940s, notably through the Sperry "reflectoscope." Crude reflectograms could be obtained from various parts of the human body, but were difficult to interpret and lacked stability (early work by J. Wild and Howry).

However, medical diagnosticians at that time envisioned "ultrasonograms" that would resemble the familiar roent-genograms (X-ray films), hopefully with better contrast and delineation of the soft tissues. Clearly, they were not looking for complex pulse reflection trains on an A-scope, but for a two-dimensional (2-D) rendering of internal body topography, for an analogue to the X ray.

This amounted to what I would like to call the "visualization paradigm" that, for many years, biased the preferences for tissue visualization in ultrasonic diagnostics.

At the MIT/MGH project, I collaborated with T. Ballantine and R. H. Bolt (Hueter and Bolt 1951) in exploiting some earlier (1945) attempts at ultrasonic cerebral ventricu-lography by the Dussik brothers in Austria. The original objectives of the MGH/ MIT study were strongly influenced by the abovementioned visualization paradigm. It turned out that ultrasonic mapping of the brain tissues within the human skull was prone to great error due to the very large bone contrast encountered at useful diagnostic frequencies. Efforts to compensate for bone effects by use of the different absorptions at different frequencies-a very modern kind of a scheme were marginally successful at that stage of the computational art.

However, in the course of these pursuits, a good many basic data essential for tissue characterization were assem-bled and proved useful for later diagnostic work on other body regions accessible through natural windows. This research benefited from lively interaction between the var-ious groups at Urbana, Minnesota, Denver and Boston. A particularly important product of this early work at MIT/MGH was the establishment of guidelines for dosimetry (Ballantine et al. 1954). The need for this had been recognized by my mentor, R. Pohlman, during the early postwar years at Siemens, in Erlangen, Germany. Pohlman was a superb experimenter and a man of many inventions. He showed me how to take advantage of E. Hiedemann's ultrasonic Schlieren techniques for the probing of ultrasonic fields. Among several clever patents in the application of US, he came up with an ingenious design for a device to measure ultrasonic in-tensity (in W/cm 22 ) by radiation pressure. It was pro-duced and sold by Siemens as an ultrasonic dosimeter, one of which I brought with me to the USA at that time. Our work at MGH on controlled lesions in animal brains with focused US (in parallel with W. Fry's group in Urbana) was much concerned with dosimetry and the underlying questions of the "mechanism of action" of US of any intensity on cell tissues. At a symposium held by the American Society of Chemical Engineers in Colum-bus, Ohio, I summarized the state of our knowledge in this area based on the literature available up to 1951 (Hueter 1951; Hueter et al. 1953).

Later, in 1956, D. E. Goldman and I pulled together all the then available data on ultrasonic propagation in mammalian tissues for publication in the Journal of the Acoustical Society of Amer-ica (Goldman and Hueter 1956).

From then (Rosenblith and Hueter 1954) until the present time, including the recent controversy on damage thresholds in sonic lithotripsy, there has been a lively debate in the literature on the role of heating vs. mechan-ical (or cavitational) effects. Only recently (in the 1990s), some more light is being shed on the physics of bubble collapse through the study of sonoluminescence from a single bubble (e.g., Putterman 1995). During my 6 y at MIT/MGH, I benefited from many discussions at the Acoustical Society of America and other meetings, with my fellow players in the biomedical ultrasonics field who, likewise, enjoyed NIH sponsorship of their research. Schwann and Carstensen, Fry, Howry and Nyborg are names that stand out. My own research interests focused on the behaviour of tissues as viscoelas-tic materials with frequency-dependent relaxational properties. This work is documented in a report to the Aeromedical Laboratory of ASAF (Hueter 1958).

Some of the early pioneers and their contributions are listed in Table 1 (Ballantine et al. 1956; Hueter 1956, 1972; Hueter and Bolt 1955; Hueter and Fry 1960). In doing this work at the Acoustics Laboratory of MIT, I came to realize that the art of industrial flaw detection, as well as tissue visualization, with US would benefit con-siderably from the unleashing of the technologies (still partly classified) developed in radar and sonar during World War II. Thus, when an opportunity came to join the Submarine Signal Division of Raytheon and to expand my knowledge in the area of signal processing and transducer design, I switched my allegiance from spon-sorship by the National Institutes of Health to that of the U.S. Navy. This turned out to be the beginning of a new career in industrial research and development, with a strong management flavor.

Finally, in 1960, I moved with my family from Boston to Seattle to work for Honeywell until my retirement in 1982. It was the combination of these industrial advances (Howry's radar background) with the results of the spreading research activities in ultrasonic tissue charac-terization (Schwan, Wild, Dunn et al.) that finally brought to life the present era of widespread diagnostic ultrasonics. In the 20 y it took to make this new modality acceptable, if not indispensable, the visualization para-digm had finally been overcome; medical doctors be-came used to "windows" and "sector scans," to comput-er- corrected imagery, to color-coding and to use of the Doppler effect. The grandfathers of the pioneering effort in medical ultrasonics, such as Loomis, Schmitt, Harvey, and Pohlman, would be pleased by the progress made, even though it took almost half a century to get there.

Excerpted from: "T. F. HUETER: FOUNDATIONS AND TRENDS IN THE 1950s" by T. F. Heuter in the article: "BIOLOGICAL EFFECTS OF ULTRASOUND: DEVELOPMENT OF SAFETY GUIDELINES. PART I: PERSONAL HISTORIES" by Wesley Nyborg. Ultrasound in Med. & Biol., Vol. 26, No. 6, pp. 911964, 2000

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