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Fifth ultrasonics in biophysics and bioengineering conference
Fifth ultrasonics in biophysics and bioengineering conference Illinois,
USA, l-4
June 1982
This year’s conference was devoted to a thorough discussion of ultrasonically induced cavitation and was organized by the Bioacoustics Laboratory, University of Illinois, US. After a brief welcome by L.A. Frizzell, University of Illinois, the meeting was opened by W.L. Nyborg, University of Vermont, Burlington, with a comprehensive review of non-linear acoustic phenomena associated with activities of small gaseous bodies. Second-order effects including acoustic streaming, radiation force and radiation torque resulting from ultrasonically generated low-level cavitation were discussed with particular reference to biological effects of ultrasound. Video tape recording of the experimental investigation into the dynamic response of gasfilled micropores to ultrasound at different acoustic intensity levels was also demonstrated. Transient cavitation by short, 1 ps, pulses at 2.2 MHZ and 5 x 10’ Pa, typical for clinical devices, was then discussed by H.G. Flynn, University of Rochester, whose computations suggest that generation of transient cavities by short pulses can occur, and furthermore, at a given frequency only nuclei below resonance size will transform into transient cavities. Chemical effects associated with ultrasonically induced cavitation were the subject of a separate session in which K.K. Suslick, University of Illinois, outlined sonochemistry of non-aqueous solvents and underlying, both primary and secondary mechanisms of sonochemical reactions. He indicated that, for example, in the chemistry of iron carbonyls Fe(CO)s, ultrasound can be used as a convenient sonocatalyst to produce multiple ligand dissociations. Sonochemistry in an aqueous medium was then discussed by C.M. Sehgal, Mayo Clinic, Rochester, who described the methods of investigating the chemical architecture of the species by sonoluminescence and other sonochemical reactions. The session on experimental studies was opened by K. Morton, Institute of Cancer Research, Sutton, UK, who gave details of experimental evidence for ultrasonically induced cavitation in guinea pigs in uivo, which occurred with cw ultrasound intensity above 80 mW cm-‘. For 2 ms pulsed ultrasound, threshold was observed at 240 mW cm-’ (spatial peak, temporal peak). Referring to the work of G. ter Haar, S. Daniels, K.L. Eastaugh and C.R. Hill, K. Morton pointed out that free gas bubbles were induced within living mammalian tissue by a 0.75 MHz ultrasound
46
irradiation at 680 mW cm-‘. In the discussion that followed the point was made that bubbles may constitute abiological hazard either directly or indirectly by tissue disruption, inducing ischaemia, and activation of blood clotting mechanics. A.R. Williams, University of Manchester, UK, pointed out that non-acoustic parameters, like heart beating and stirring, have a substantial influence on nucleation processes in biological media. Ch. Christman, Bureau of Radiological Health, Rockville, USA, described the principle and performance of a bubble detector facilitating detection of bubbles in viuo. Experimental results on cavitational effects in vivo were summarized by L. Frizzell, University of Illinois, who concentrated on the biological effects induced by the ultrasonic intensities of the order of several hundred W cmm2 (spatial average, time average). Absolute calibration methods and applications of the miniature PVDF polymer hydrophone probe in ultrasonic dosimetry and cavitation detection were then briefly outlined by P.A. Lewin, Technical University of Denmark. He raised the question whether instantaneous peak-pressure was a more indicative exposure parameter than intensity for the characterization of the interaction between ultrasound and biological media. F. Dunn, University of Illinois, presented experimental evidence for transient cavitation in mammalian tissue at the SaTa intensity levels of 1000 W cmm2. Bubble growth in tissue by rectified diffusion was discussed in detail by L. Crum, University of Mississippi, who described nucleation mechanisms and concluded that evidence accumulating indicates that in the natural body system bubbles are already present and that these bubbles will respond even at relatively low acoustic intensities. The last session of the meeting was concerned with speculation on potential clinical implications of ultrasonically induced cavitation and was opened by F. Dunn, University of Illinois, who faced the difficult task of summarizing the most important statements made by the previous speakers and raising points to initiate the discussion by participants. From the lively discussion which followed it emerged that there is growing evidence that conditions exist in the body for cavitation, that much additional work is required to develop instrumentation suitable for monitoring transient and stable cavitation in viva, and also that further research is necessary to assess the influence of microsecond acoustic pulses on biological systems. P.A. Lewin
ULTRASONICS.
JANUARY
1983
USA, l-4
June 1982
This year’s conference was devoted to a thorough discussion of ultrasonically induced cavitation and was organized by the Bioacoustics Laboratory, University of Illinois, US. After a brief welcome by L.A. Frizzell, University of Illinois, the meeting was opened by W.L. Nyborg, University of Vermont, Burlington, with a comprehensive review of non-linear acoustic phenomena associated with activities of small gaseous bodies. Second-order effects including acoustic streaming, radiation force and radiation torque resulting from ultrasonically generated low-level cavitation were discussed with particular reference to biological effects of ultrasound. Video tape recording of the experimental investigation into the dynamic response of gasfilled micropores to ultrasound at different acoustic intensity levels was also demonstrated. Transient cavitation by short, 1 ps, pulses at 2.2 MHZ and 5 x 10’ Pa, typical for clinical devices, was then discussed by H.G. Flynn, University of Rochester, whose computations suggest that generation of transient cavities by short pulses can occur, and furthermore, at a given frequency only nuclei below resonance size will transform into transient cavities. Chemical effects associated with ultrasonically induced cavitation were the subject of a separate session in which K.K. Suslick, University of Illinois, outlined sonochemistry of non-aqueous solvents and underlying, both primary and secondary mechanisms of sonochemical reactions. He indicated that, for example, in the chemistry of iron carbonyls Fe(CO)s, ultrasound can be used as a convenient sonocatalyst to produce multiple ligand dissociations. Sonochemistry in an aqueous medium was then discussed by C.M. Sehgal, Mayo Clinic, Rochester, who described the methods of investigating the chemical architecture of the species by sonoluminescence and other sonochemical reactions. The session on experimental studies was opened by K. Morton, Institute of Cancer Research, Sutton, UK, who gave details of experimental evidence for ultrasonically induced cavitation in guinea pigs in uivo, which occurred with cw ultrasound intensity above 80 mW cm-‘. For 2 ms pulsed ultrasound, threshold was observed at 240 mW cm-’ (spatial peak, temporal peak). Referring to the work of G. ter Haar, S. Daniels, K.L. Eastaugh and C.R. Hill, K. Morton pointed out that free gas bubbles were induced within living mammalian tissue by a 0.75 MHz ultrasound
46
irradiation at 680 mW cm-‘. In the discussion that followed the point was made that bubbles may constitute abiological hazard either directly or indirectly by tissue disruption, inducing ischaemia, and activation of blood clotting mechanics. A.R. Williams, University of Manchester, UK, pointed out that non-acoustic parameters, like heart beating and stirring, have a substantial influence on nucleation processes in biological media. Ch. Christman, Bureau of Radiological Health, Rockville, USA, described the principle and performance of a bubble detector facilitating detection of bubbles in viuo. Experimental results on cavitational effects in vivo were summarized by L. Frizzell, University of Illinois, who concentrated on the biological effects induced by the ultrasonic intensities of the order of several hundred W cmm2 (spatial average, time average). Absolute calibration methods and applications of the miniature PVDF polymer hydrophone probe in ultrasonic dosimetry and cavitation detection were then briefly outlined by P.A. Lewin, Technical University of Denmark. He raised the question whether instantaneous peak-pressure was a more indicative exposure parameter than intensity for the characterization of the interaction between ultrasound and biological media. F. Dunn, University of Illinois, presented experimental evidence for transient cavitation in mammalian tissue at the SaTa intensity levels of 1000 W cmm2. Bubble growth in tissue by rectified diffusion was discussed in detail by L. Crum, University of Mississippi, who described nucleation mechanisms and concluded that evidence accumulating indicates that in the natural body system bubbles are already present and that these bubbles will respond even at relatively low acoustic intensities. The last session of the meeting was concerned with speculation on potential clinical implications of ultrasonically induced cavitation and was opened by F. Dunn, University of Illinois, who faced the difficult task of summarizing the most important statements made by the previous speakers and raising points to initiate the discussion by participants. From the lively discussion which followed it emerged that there is growing evidence that conditions exist in the body for cavitation, that much additional work is required to develop instrumentation suitable for monitoring transient and stable cavitation in viva, and also that further research is necessary to assess the influence of microsecond acoustic pulses on biological systems. P.A. Lewin
ULTRASONICS.
JANUARY
1983