Robert E. Apfel

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.

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

Transient cavitation is a discrete phenomenon that relies on the existence of stabilized nuclei, or pockets of gas within a host fluid, for its genesis. A convenient descriptor for assessing the likelihood of transient cavitation is the threshold pressure, or the minimum acoustic pressure necessary to initiate bubble growth and subsequent collapse. An automated experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz. A fluid jet was used to convect potential cavitation nuclei through the focal region of the insonifying transducer. Potential nuclei tested include 1-microns polystyrene spheres, microbubbles in the 1- to 10-microns range that are stabilized with human serum albumin, and whole blood constituents. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Measurements of the transient cavitation threshold in water, in a fluid of higher viscosity, and in diluted whole blood are presented. These experimental measurements of cavitation thresholds elucidate the importance of ultrasound, host fluid, and nuclei parameters in determining these thresholds. These results are interpreted in the context of an approximate analytical theory for the prediction of the onset of cavitation.

THE SUPERHEATED DROP DETECTOR

Abstract The Superheated Drop Detector (SDD) is a new tool for radiation spectrometry, area monitoring, and dosimetry that may find use in nuclear science, safety, and medicine. It is based on the principle of the bubble chamber, but unlike the bubble chamber, offers continuous radiation sensitivity, portability and adaptability, direct reading capability, and low cost. Moderately superheated drop detectors are insensitive to gamma rays and X-rays for energies less than 6 MeV, but sensitive to fast neutrons and have an energy threshold that can be adjusted by varying temperature, pressure, or drop composition.

Acoustic radiation pressure produced by a beam of sound

The second‐order force produced by a sound beam directed normally at a plane target is calculated. Previous theories on acoustic radiation pressures associated with plane acoustic waves are examined critically and erroneous results, where they exist, are noted and rectified. A number of general relations are established using a new approach which avoids the necessity of dealing with detailed solutions of the governing nonlinear equations. Some of the concepts inferred from known solutions obtained by previous authors require drastic revision in the light of the present study. Specifically, the notion that Rayleigh radiation pressure depends on the nonlinearity of the medium (while Langevin radiation pressure does not) is not true in the case where the medium is bound by a partially reflecting wall. Again, that the concept that Rayleigh radiation pressure depends on the acoustic field only through the energy density of the field is shown to be false. In one instance it is shown to depend also on how the fi...

An improved theory for the prediction of microcavitation thresholds

An approximate analytical formulation is presented that allows for the calculation of acoustic pressure thresholds for transient cavitation over a variety of frequencies and host fluid parameters. Specifically, R.E. Apfels (1986) theory is extended to include an estimate of the time delay associated with the Laplace pressure, 2 sigma /R/sub 0/, where sigma is the surface tension and R/sub 0/ is the initial radius. Also presented is a correction factor for the time-averaged pressure difference, across the bubble wall during growth. An optimum size distribution of nuclei for the predisposition of a sample to microcavitation is exhibited. The role of transient cavitation in medical ultrasound is discussed.<<ETX>>

Direct evidence of cavitation in vivo from diagnostic ultrasound

Recent increases in the pressure output of diagnostic ultrasound scanners have led to an interest in establishing thresholds for bioeffects in many organs including the lungs of mammals. Damage may be mediated by inertial cavitation, yet there have been no such direct observations in vivo. To explore the hypothesis of cavitation-based bioeffects from diagnostic ultrasound, research has been performed on the thresholds of damage in rat lungs exposed to 4.0-MHz pulsed Doppler and color Doppler ultrasound. A 30-MHz active cavitation detection scheme complementing these studies provides the first direct evidence of cavitation in vivo from diagnostic ultrasound pulses.

An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound

An acoustic backscattering technique for detecting transient cavitation produced by 10-microseconds-long pulses of 757-kHz ultrasound is described. The system employs 10-microseconds-long, 30-MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented.

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.

The Role of Impurities in Cavitation‐Threshold Determination

We have extended the theoretical work of Harvey et al. [J. Cellular Comp. Physiol. 24, 1–22 (1944)] and Strasberg [J. Acoust. Soc. Amer. 31, 163–176 (1959)] in order to consider the conditions that must exist in a liquid for a vapor cavity to be nucleated from an imperfectly wetted solid impurity (mote) in the liquid. It is found that, for sufficiently small and readily wetted motes, the tensile stress required for nucleation increases with increasing surface tension and decreasing mote size but is almost independent of the gas content and history of the liquid. On the other hand, for sufficiently large and imperfectly wetted motes, the gas content of the liquid and its history are crucial, whereas the mote size and the liquid‐vapor surface tension play no role in determining the conditions for nucleation. The qualitative predictions of this theory of moted‐induced nucleation bring some semblance of order to a wide variety of observations of statically induced cavitation and low‐frequency acoustic cavitat...

Acoustic cavitation prediction

We have derived an approximate expression for the threshold pressure for transient acoustic cavitation and have used this result with published expressions for the thresholds for gas bubble nucleation and for rectified diffusion in synthesizing cavitation prediction charts for water covering a frequency range from 10 kHz to 2.5 MHz, and for gas saturation percentages 10%, 50%, and 100%. This synthesis, which is applicable to other liquids, may provide guidance about the phenomena associated with acoustic cavitation to those who are either trying to minimize or to optimize its effects.