Electronic neutron personal dosimetry with superheated drop detectors.

The prototype of an electronic personal neutron dosemeter based on superheated drop detectors is presented. This battery operated device comprises a neutron sensor, bubble-counting electronics and a temperature controller ensuring an optimal dose equivalent response. The neutron sensor is a 12 ml detector vial containing an emulsion of about 50,000 halocarbon-12 droplets of 100 microns diameter. The temperature controller is a low-power, solid-state device stabilising the emulsion at 31.5 degrees C by means of an etched foil heater. The microprocessor controlled counting electronics relies on a double piezo-electric transducer configuration to record bubble formation acoustically via a comparative pulse-shape analysis of ambient noise and detector signals. The performance of the dosemeter was analysed in terms of the requirements presently developed for neutron personal dosemeters. The detection threshold is about 1 microSv, while the personal dose equivalent response to neutrons in the thermal to 62 MeV range falls within a factor 1.6 of 13 bubbles per microSv.

The effects of surfactant additives on the acoustic and light emissions from a single stable sonoluminescing bubble.

The localized concentration of energy during a single bubble collapse is manifested in two forms, which are the emission of an acoustic pulse, and the emission of a light pulse. Through precise control of experimental parameters, one can levitate a single bubble in a standing wave field and measure the magnitude of the acoustic and light pulses resulting from the violent collapse of the cavity. The information acquired from such measurements provides better understanding of the mechanisms that are responsible for the emissions, which may lead to the practical application of controlled bubble implosions. An experimental apparatus was developed to measure the acoustic and light emissions from a single, stable sonoluminescing bubble. Two surfactant additives were studied to determine the effects on the bubble emissions. Triton X-100, which has previously been shown to provide free interfacial motion, reduced the magnitude of both the acoustic and light pulses from the bubble. The protein bovine serum albumin (BSA) which has been shown to hinder interfacial motion, allowed the bubble to be driven to higher acoustic pressures, and resulted in an increase in the magnitude of the light pulses from the bubble. Images of the sonoluminescing bubble indicate that the collapse remains spherical in the cases presented, and that bubble translation can be correlated with weak acoustic and light emissions.

Radiation force on a spherical object in the field of a focused cylindrical transducer.

An exact solution of the radiation force on a spherical object, when positioned on the acoustic axis of a cylindrical transducer, is provided. The solution is valid for any type of sphere of any size. The radiation force function allows the calibration of high-frequency focused ultrasound fields from radiation force measurements and expands the utility of the elastic sphere radiometer developed by Dunn et al. [Acustica 38, 58-61 (1977)]. Numeral results reveal an oscillatory behavior of the radiation force function for small spheres near the transducer surface and this behavior may present an opportunity for particle sorting based on the mechanical properties of the particle and other types of manipulation.

Inertial cavitation produced by pulsed ultrasound in controlled host media.

The purpose of this work is to provide measurements in well-characterized media in order to better understand inertial (transient) cavitation phenomena. Focused transducers with megahertz center frequencies (2.5 MHz, 4.3 MHz) and a clinical diagnostic ultrasound system (4.0 MHz) was used to generate pulsed ultrasound to induce cavitation. An improved active cavitation detector which utilizes a highly focused transducer with much higher center frequency (30 MHz) was used to measure the threshold of inertial cavitation. In order to study the effect of the concentration of nucleation agents on cavitation thresholds, experiments were conducted in distilled water seeded with polystyrene particles. Inertial cavitation thresholds were measured for various concentrations of polystyrene particles. It was observed that the threshold decreased from 2.5 MPa at concentration of about 10(6) particles/ml to 1.6 MPa at a concentration of about 10(9) particles/ml. The effect of the concentration is not significant for smaller changes of concentration. Measurements of the cavitation thresholds were then made in specially developed phantom materials to study the effect of viscosity on the cavitation threshold when surface tension and other mechanical properties of the materials are kept relatively constant. Experimental results show that the threshold increases with increasing viscosity, consistent with theoretical predictions. Cavitation was also detected in water seeded with polystyrene particles using a clinical ultrasound system at an acoustic pressure of 3.84 MPa. Results are comparable with those obtained in the laboratory using a 4.3-MHz focused transducer.

Radiation force on a spherical object in an axisymmetric wave field and its application to the calibration of high-frequency transducers.

The general formulation for the radiation force on a spherical object in an axisymmetric acoustic field is provided. The sphere is described in general by three parameters: its density, compressional wave speed, and shear wave speed. Other types of spheres, including the rigid and immovable sphere and the infinitely soft sphere (void), are treated as limiting cases. Specialized formulations of the radiation force function are provided for several types of incident waves of common interest. A low-frequency expansion for each case is provided for comparison with results from the literature. Among the solutions provided are the scattering function of an elastic sphere in a focused acoustic field and the radiation force on the sphere. The radiation force function is used to calibrate high-frequency transducers. Experimental data are provided for a focused transducer for frequencies up to 10 MHz, where the size of the elastic sphere is comparable to or longer than the (-3)-dB beamwidth of the sound field.

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.

In vitro measurements of inertial cavitation thresholds in human blood.

Inertial cavitation thresholds were measured in human blood exposed to pulsed ultrasound. Freshly drawn blood, bank blood and aqueous dilutions of both were used in this experimental study. Micrometer-sized polystyrene particles were used as extra potential nuclei in some samples. Focused transducers with megahertz center frequencies (2.5 MHz, 4.3 MHz) were employed to generate pulsed ultrasound to induce cavitation. Specially designed cells for hosting the blood samples were made to adapt to the experimental environment. Cavitation threshold measurements were achieved by using an active cavitation detection scheme which utilizes a highly focused transducer with a much higher center frequency (30 MHz). In 50% diluted blood samples, when no polystyrene particles were added to the samples, the threshold for cavitation was about 4.1 MPa at 2.5 MHz, while no cavitation was detected at 4.3 MHz. Generally, the measured thresholds decrease in samples with lower volume concentration of red blood cells or when polystyrene particles were added to the samples. Results show that the measured thresholds in some circumstances are in the range of output pressure of diagnostic ultrasound instrumentation; but for whole, freshly drawn blood, our apparatus was unable to detect cavitation, even at 6.3 MPa.

An interferometric technique for B/A measurement.

An isentropic phase method is described for measuring in vitro the acoustic nonlinearity parameter B/A of several aqueous buffers, protein solutions, lipid oils, and emulsions. The technique relies upon the use of an acoustic interferometer to measure the small changes in sound speed that accompany a rapid hydrostatic pressure change of between one and two atmospheres. Average accuracies of 0.85% are attainable with this method.

Method for determining the reliable prediction(s) of compositions of tissue phantoms.

In a previous paper, we showed that mixture laws can be used to predict the compositions of tissue phantoms. Due to the variation in the performance of the mixture laws when they are applied to phantoms, however, it is difficult to determine which predictions of the composition are most reliable. In this paper, we study the causes of the variation in the performances of different sets of mixture laws, and propose a criterion for choosing a reliable predictor. The predictions selected with the proposed criterion agree very well with the known compositions of phantoms. The potential uses of the mixture methodology and the criterion to tissue characterization are discussed.

Transient thermal and mechanical response of water subject to ionizing radiation.

The ultrafast transient (10(-14) to 10(-12)S) thermal and mechanical response of water subject to ionizing radiations of different linear energy transfers has been investigated in order to understand the initial events which lead to cell mutation and lethality. Based on computational fluid dynamics, the production of a "thermal spike" around the trajectory of a charged particle and subsequent diffusion of deposited heart are calculated for particles with linear energy transfer (LET) of 4, 40, and 400 keV/microns. A radiation damage region (that is, the so-called "thermal core") is identified, and the transient behavior of the thermal core is studied. The local and transient environment has a dimension of nanometers, a scale which is of critical interest in understanding mechanisms of radiation damage in cells. The radius of the thermal core, Dd, at temperatures (or internal energy density) of up to 1,000 K, is observed to increase with LET, L, as Dd (in nanometers) = C4.L (in keV/microns)0.6, where, for example, C4 = 0.50 for T = 800 degrees C.

Acoustic cavitation produced by microsecond pulses of ultrasound: a discussion of some selected results.

Because of its extensive utilization in clinical practice, and because the subjects examined are often fragile and sensitive to trauma, the safety of diagnostic ultrasound has always been of concern. Of the various mechanisms through which ultrasound could act in a manner deleterious to a patient, acoustic cavitation, should it occur, appears to possess significant potential for biological damage. This paper reviews several recent reports of progress by our two groups and demonstrates the conditions under which cavitation has been observed by microsecond pulses of ultrasound. Although these results give no indications that diagnostic ultrasound may pose a true risk to a patient, they do indicate that in vivo cavitation may occur under certain conditions.

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.

Acoustic microcavitation: enhancement and applications.

Experimental investigation into acoustic microcavitation is extended to include "cavitation activity" in addition to the threshold measurements using acoustic detectors. The primary setup incorporates two detectors: an unfocused, untuned 1-MHz transducer, which serves as a passive detector, and a focused 30-MHz transducer used in pulse-echo mode as the active detector. Cavitation itself is brought about by a focused piezoelectric transducer driven in pulse mode. The active detector is arranged confocally with respect to the cavitation transducer. Both the interrogating pulse and the cavitation pulse arrive simultaneously at the common focus which is the region of cavitation. Cavitation is conducted primarily at 0.75 MHz and 1% duty cycle in clean water using microparticles to seed the events. Cavitation activity appears to be directly proportional to the number density of the particles present in the cavitation medium. The fact that the active detector affects the cavitation process can be further exploited to seek interesting applications leading possibly to submicronic particle counting and testing of surface characteristics of silica particles used in liquid chromatography.

Acoustic microcavitation: its active and passive acoustic detection.

In this work acoustic microcavitation in water is studied primarily at 0.75 MHz and 1% duty cycle. To detect cavitation, two kinds of acoustic detectors are used. The first one is an unfocused, untuned 1-MHz receiver transducer that serves as a passive detector. The other one is a focused 30-MHz transducer that is used in pulse-echo mode and is called the active detector. Cavitation itself is brought about by a focused PZT-8 crystal driven in pulse mode. The active detector is arranged confocally with respect to the cavitation transducer. Both the interrogating pulse and the cavitation pulse arrive simultaneously at the common focus, which is the region of cavitation. With the test chamber filled with clean water, no cavitation is observed, even when the cavitation transducer is driven to give its peak output of 22 bar peak negative. Cavitation is, however, observed when polystyrene microparticles are added to the host water. Our view of how these smooth, spherical, monodispersed microparticles give rise to cavitation is described with some estimates. An attempt has been made to understand whether the presence of "streaming" affects the thresholds, and it has been found that the active detector field affects the cavitation process.

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.

Applications of mixture laws for predicting the compositions of tissue phantoms.

This paper generalizes the mixture law methodology for ultrasonic tissue characterization by combining mixture laws used by Apfel and Seghal et al. in producing nine different combinations. After applying these combinations of mixture laws to 10 tissue phantoms, it is shown that three of the combinations consistently give better predictions of the composition of the phantoms than others. The results verify directly that the phase shift parameter law is probably not a good choice in predicting tissue composition. The potential uses of the mixture methodology are discussed.

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.

Mechanical characterization of microparticles by scattered ultrasound.

A technique for determining the compressibility and density of individual microparticles in suspension is described. The particles have diameters on the order of 10 microns Ultrasonic tone bursts of 2-microseconds duration and 30-MHz center frequency scatter from individual particles as they traverse the confocal zone of two transducers. The resulting scattered tone bursts are detected at 90 degrees and 180 degrees (backscattering). The received rf signals are demodulated, peak detected, digitized, and stored in computer memory. Using Rayleigh scattering theory, the compressibility and density of a particle can be computed given knowledge of the particle size and host fluid properties. Results of experiments with latex microspheres are presented and compared with calculations based on long-wavelength (Rayleigh) and elastic scattering theory.

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.