Page 39 - Spring 2018

National Institutes of Health (note the plural Institutes) and slowly an extramural research program of grants evolved. Funding was mostly from the military (Navy Bureau of Ships, Office of Naval Research, and Air Force Aero Medical Laboratory) and the money focused largely on transducer developments.
Another problem for which Bill Fry successfully obtained funding significantly benefitted Floyd because it provided him with the experience and importance of carefully char- acterizing and calibrating acoustic fields in water. The skills acquired with this project were front and center in Floyd’s future academic career as he successfully pursued ground- breaking tissue-based ultrasonic exposure and dose re- search. The research involved the development of a sonar transducer for the Navy that would have a continuously varying resonant frequency (Fry et al., 1951, 1955; Hall and Fry, 1951; Welkowitz and Fry, 1954; Dunn et al., 1956). This was a completely new and challenging problem and, if suc- cessful, was viewed by the Navy as a capability for poten- tially avoiding sonar detection. A column of mercury was intimately coupled to the transducer such that the trans- ducer’s resonance frequency changed continuously with the column’s length; the mercury column effectively increased the thickness of the transducer crystal. Floyd assembled the 100 individual piezoelectric elements and was responsible for then measuring the shape of the sound field as a function of frequency (charactering it) and calibrating the relation- ship between the input voltage and the transmitted sound pressure level. A cubic foot of enormously expensive, triply distilled mercury was required to provide an effective bond with the piezoelectric elements. To avoid this expense, Floyd purchased less expensive, newly mined mercury ore and de- signed and constructed a still to process the metal.
Floyd meticulously acquired calibrated data from the vari- able-frequency transducer using a World War II signal gen- erator and power amplifier as well as Navy-provided sonar detectors. The analyses showed that the frequency varied by a factor of 2 (22-44 kHz), as was required by the original design. However, the Navy wanted to verify the findings and required that the transducer be tested using sophisticated Navy instrumentation at a Navy facility in Connecticut called Dodge Pond. A university truck was outfitted to safely transport the transducer, and it took Floyd and Frank three to four days to drive the truck to Dodge Pond for the test- ing that required about a week. The observations from the Dodge Pond measurements were essentially the same as the measurements at Illinois.
Curiosity, Imagination, and a Movie
Floyd had to have inherited curiosity and imagination from his BRL experiences that fed him for a lifetime. From col- leagues and students, Floyd demanded curiosity, critical thinking, imagination, thoughtfulness, and creative prob- lem solving—not much different from how Floyd described those early BRL days when he was closely involved with a significant medical ultrasound research project. In 1955, a movie was produced in the BRL titled Neuro Sonic Surgery. This movie captured the details of a large project that a couple of years later led successfully to human neurosonic surgery (O’Brien and Dunn, 2015). Floyd was the movie’s technical director. The movie largely documented Floyd’s laboratory interactions as a graduate student with the Fry brothers’ ma- jor project and showed how significantly Floyd was involved in and contributed to the work, including (1) developing a precision motion-controlled positioning system to guide the focused ultrasound beam to generate complex geometric tis- sue lesions; (2) calibrating the ultrasound field using newly developed measurement techniques (no national or inter- national measurement standards existed then); (3) estimat- ing the ultrasonic intensity delivered in vivo at the focus to generate spatially precise tissue lesions; and (4) conducting a wide range of carefully controlled ultrasound tissue bio- physical experiments to either temporarily or permanently alter tissue.
Determining Thresholds:
Functional Outcome
Floyd’s PhD thesis (Dunn, 1956) represented a seminal study that yielded ultrasonic intensity relationships from a functional in vivo end point rather than from a histologi- cal (tissue structure) end point. At the time when Floyd was working on his thesis, the neurosonic surgery studies were also being conducted and were able to demonstrate precise and reproducible lesions in brain tissue for which histol- ogy was used for the lesion outcome measure. This, in itself, did not help to identify the mechanism responsible for the lesion. The ultrasonic mechanisms being mentioned fre- quently at that time were thermal and cavitation, and when theory and/or experiment appeared to rule out these two mechanisms, the literature often mentioned a nonthermal, mechanical mechanism (because sound is a mechanically propagated wave). It was viewed as essential then that the physical mechanism of the action of high-intensity ultra- sound on tissue be understood so that its full medical po- tential could be realized. To this end, Floyd chose for his thesis and follow-up research an ideal subject: day-old mice.
Spring 2018 | Acoustics Today | 37