Thresholds for transient cavitation produced by pulsed ultrasound in a controlled nuclei environment

Current status of research on biophysical effects of ultrasound

This overview of bioeffects of ultrasound presents some key aspects of selected papers dealing with biophysical end-points. Its purpose is to establish a basis for exposure and dosimetric standards for medical ultrasonic equipment. It is intended to provide essential background resource material for the medical/scientific community, and more specifically for scientific working groups. This document was prepared by members of the Safety Committee of the World Federation for Ultrasound in Medicine and Biology. It was produced as a resource document in response to a request for information by Working Group 12 (Ultrasound exposure parameters) of the International Electrotechnical Commission Technical Committee 87, Ultrasonics. IEC TC 87, WG12 is the working group responsible for generating international standards for the classification of equipment by its acoustic fields based on safety thresholds. Our paper is intended to update and supplement information on the thermal mechanism provided in the publication, "WFUMB Symposium on Safety and Standardisation in Medical Ultrasound: Issues and Recommendations Regarding Thermal Mechanisms for Biological Effects of Ultrasound" (WFUMB 1992). It also provides an overview of trends in research into nonthermal mechanisms as a preliminary to the next WFUMB Symposium on Safety of Medical Ultrasound when this subject will be examined in detail by a select group of international experts. The WFUMB-sponsored workshop will take place in Utsunomiya, Japan during 11-15th July, 1994. The purpose of the meeting is to evaluate the scientific literature and to formulate internationally accepted recommendations on the safe use of diagnostic ultrasound that may be endorsed as official policy of the WFUMB. It should be noted that the current publication is not intended for review or endorsement as an official WFUMB document. It is produced as a scientific paper by individuals who are members of the WFUMB Safety Committee, and it therefore represents the opinions of the authors. Nevertheless, during the preparation of this document, contributions were received from members of the International Electrotechnical Commission Technical Committee 87 as well as many other individual experts, and the authors sincerely acknowledge their support.

In vitro detection of cavitation induced by a diagnostic ultrasound system

Experiments were performed to determine whether a clinical diagnostic scanner, a Hewlett-Packard (HP) 77020A, could produce cavitation in water containing suspensions of either 0.245-mum polystyrene spheres or Albunex, 1-10 mum albumin-coated microbubbles. Two calibrated, phased-array HP imaging transducers with 2.5- and 5.0-MHz operating frequencies were driven in M-mode (single cycle) and Doppler mode 4 cycles by the HP imaging system. Cavitation was detected in the water with polystyrene spheres at 2.5 MHz in both M-mode and Doppler mode at a peak negative acoustic pressure of 1.1 MPa or greater. Insonification at 5.0 MHz in either mode did not produce a detectable amount of cavitation, even with peak negative pressures as high as 1.2 MPa. Cavitation was not detected in water with the Albunex spheres at either frequency.

Gauging the likelihood of cavitation from short-pulse, low-duty cycle diagnostic ultrasound

Although no deleterious effects form diagnostic ultrasound have been reported in epidemiologic studies and surveys of widespread clinical usage (Ziskin and Petitti 1988), the conditions for the onset of transient cavitation must be investigated in the total evaluation of potential risks associated with diagnostic ultrasound applications. An extension of the results from the approximate theory developed by Holland and Apfel (1989) is applied in this paper to a population of nuclei to predict the onset of cavitation in host fluids with physical properties similar to those of biological fluids. From this analysis and from results of recent in vitro cavitation experiments, an index is developed which can gauge the likelihood of substantial microbubble growth in the presence of short-pulse, low-duty cycle diagnostic ultrasound.

Lung damage from exposure to pulsed ultrasound

Motivated by a recent finding that threshold pressures for hemorrhage in mouse lung exposed to the fields of an electrohydraulic lithotripter were less than 2 MPa, we extended the exposures to pulsed ultrasound. Sharply defined thresholds of the order of 1 MPa were found with 10 microseconds length pulses and roughly twice that value for 1 microsecond pulses. The thresholds at 4 MHz are greater than at 1 MHz. The thresholds are comparable for focused and unfocused fields. As would be expected for a cavitation-like phenomenon, temporal average intensity is a very poor predictor of this effect. In the extreme case, lesions were found at temporal average intensities on the order of 1 mW/cm2.