R. Glynn Holt

Measurements of bubble-enhanced heating from focused, mhz-frequency ultrasound in a tissue-mimicking material

Time-resolved measurements of the temperature field in an agar-based tissue-mimicking phantom insonated with a large aperture 1-MHz focused acoustic transducer are reported. The acoustic pressure amplitude and insonation duration were varied. Above a critical threshold acoustic pressure, a large increase in the temperature rise during insonation was observed. Evidence for the hypothesis that cavitation bubble activity in the focal zone is the cause of enhanced heating is presented and discussed. Mechanisms for bubble-assisted heating are presented and modeled, and quantitative estimates for the thermal power generated by viscous dissipation and bubble acoustic radiation are given.

Temporal and Spatial Detection of HIFU-Induced Inertial and Hot-Vapor Cavitation with a Diagnostic Ultrasound System

The onset and presence of inertial cavitation and near-boiling temperatures in high-intensity focused ultrasound (HIFU) therapy have been identified as important indicators of energy deposition for therapy guidance. Passive cavitation detection is commonly used to detect bubble emissions, where a fixed-focus single-element acoustic transducer is typically used as a passive cavitation detector (PCD). This technique is suboptimal for clinical applications, because most PCD transducers are tightly focused and afford limited spatial coverage of the HIFU focal region. A Terason 2000 Ultrasound System was used as a PCD array to expand the spatial detection region for cavitation by operating in passive mode, obtaining the radiofrequency signals corresponding to each scan line and filtering the contribution from scattering of the HIFU signal harmonics. This approach allows for spatially resolved detection of both inertial and stable cavitation throughout the focal region. Measurements with the PCD array during sonication with a 1.1-MHz HIFU source in tissue phantoms were compared with single-element PCD and thermocouple sensing. Stable cavitation signals at the harmonics and superharmonics increased in a threshold fashion for temperatures >90 degrees C, an effect attributed to high vapor pressure in the cavities. Incorporation of these detection techniques in a diagnostic ultrasound platform could result in a powerful tool for improving HIFU guidance and treatment.

Nucleating cavitation from laser-illuminated nano-particles

Vapor bubble generation from laser-illuminated gold nano-particles has been investigated as a means of providing nucleation sites for cavitation induced by high-intensity focused ultrasound (HIFU). Pulses from a 532-nm Nd:Yag laser were synchronized with a pulsed 1.1-MHz HIFU source in an acrylamide phantom seeded with 82-nm-diameter gold particles. Emissions from bubble collapses were detected by a 15-MHz focused transducer at a laser pulse energy and HIFU focal pressure of 0.10 mJ and 0.92 MPa, respectively. In comparison, a HIFU peak focal pressure of 4.50 MPa was required to nucleate detectable cavitation without laser illumination.

Deformation and location of an acoustically levitated liquid drop

A theoretical method to determine the location and static deformation of an acoustically levitated liquid drop in air is presented. The interaction between drop and sound field, involving nonspherical acoustic scattering and drop volume variation, is the crux of this analysis, which is valid for drops with aspect ratio as large as 2. Numerical calculations are presented of drop shape and location as functions of sound pressure, surface tension, and drop volume in both gravity (1g) and gravity‐free (0g) environments. The numerical results agree well with our experimental measurements and those of other researchers.A theoretical method to determine the location and static deformation of an acoustically levitated liquid drop in air is presented. The interaction between drop and sound field, involving nonspherical acoustic scattering and drop volume variation, is the crux of this analysis, which is valid for drops with aspect ratio as large as 2. Numerical calculations are presented of drop shape and location as functions of sound pressure, surface tension, and drop volume in both gravity (1g) and gravity‐free (0g) environments. The numerical results agree well with our experimental measurements and those of other researchers.

Acoustically forced oscillations of air bubbles in water: Experimental results

An experimental technique for measuring the time‐varying response of an oscillating, acoustically levitated air bubble in water is developed. The bubble is levitated in a resonant cell driven in the (r,θ,z) mode of (1,0,1) at a frequency fd≊24 kHz. Linearly polarized laser light (Ar–I 488.0 nm) is scattered from the bubble, and the scattered intensity is measured with a suitable photodetector positioned at some known angle from the forward, subtending some solid acceptance angle. The output photodetector current, which is linearly proportional to the light intensity, is converted into a voltage, digitized, and then stored on a computer for analysis. For spherical bubbles, the scattered intensity Iexp(t) as a function of radius R and angle θ is calculated theoretically by solving the boundary value problem (Mie theory) for the water/bubble interface. The inverse transfer function R(I) is obtained by integrating over the solid angle centered at some constant θ. Using R(I) as a look‐up table, the radius vers...

Dynamics and control of cavitation during high-intensity focused ultrasound application

In this paper the results of two studies related to high-intensity focused ultrasound (HIFU) and cavitation are reported. The first study described used polyacrylamide phantoms to gain insight into the behavior of cavitation activity in the focal region of the HIFU transducer. Results indicate that cavitation is the source of a previously observed enhanced heating effect in HIFU. The second study discussed used agar-graphite phantoms to see if changing the duty cycle of the driving could effect some measure of control over the cavitation activity; the results indicate that it can.

Mitigation of Damage to Solid Surfaces From the Collapse of Cavitation Bubble Clouds

The collapse of transient bubble clouds near a solid surface was investigated to test a scheme for mitigation of cavitation-induced damage. The target was a porous ceramic disk through which air could be forced. Transient cavitation bubbles were created using a shock-wave lithotripter focused on the surface of the disk. The dynamics of bubble clouds near the ceramic disks were studied for two boundary conditions: no back pressure resulting in surface free of bubbles and 10 psi (0.7 atm) of back pressure, resulting in a surface with a sparse (30% of area) bubble layer. Images of the cavitation near the surface were obtained from a high-speed camera. Additionally, a passive cavitation detector (3.5 MHz focused acoustic transducer) was aligned with the surface. Both the images and the acoustic measurements indicated that bubble clouds near a ceramic face without a bubble layer collapsed onto the boundary, subsequently leading to surface erosion. When a sparse bubble layer was introduced, bubble clouds collapsed away from the surface, thus mitigating cavitation damage. The erosion damage to the ceramic disks after 300 shock waves was quantified using micro-CT imaging. Pitting up to 1 mm deep was measured for the bubble-free surface, and the damage to the bubble surface was too small to be detected.

Monitoring the Development of HIFU‐Induced Cavitation Activity

One of the limitations of HIFU treatment for tissue necrosis is the difficulty in achieving a predictable lesion shape and size in a short amount of time. Simply increasing the HIFU intensity cannot solve this problem, as it leads to the formation of a so‐called “tadpole”—shaped lesion. Bubble shielding of the incident HIFU is one of the mechanisms implicated in the development of these deformed lesions; at super‐nucleation pressures inertial cavitation will commence and the scattering from bubbles in the path of the ultrasound propagation will reflect the HIFU energy back towards the transducer. In the past we have employed a single focused, passive broadband transducer (PCD) to detect inertial cavitation activity at the HIFU focus in agar‐graphite tissue phantoms. At sufficiently‐high pressures, the inertial cavitation signal decreases over time, giving rise to the notion that bubble shielding is at fault for the signal decrease. Here we present evidence that bubble shielding is not the only mechanism b...

Numerical simulation of superoscillations of a Triton-bearing drop in microgravity

Large-amplitude nonlinear oscillations of an axially symmetric water drop of volume 7.33 cm 3 , initial aspect ratio 3.4, with surfactant Triton X-100 of 1.4 x 10 -4 g ml -1 (1 CMC), in microgravity are compared with predictions of the boundary-integral method. The small shear viscosity of the bulk phase, as well as the surface dilatational viscosity and surface shear viscosity are considered. When a very specific set of material properties is assumed, numerical simulations of the drop oscillations are in good agreement with the experimental results of drop oscillations measured in space during the second United States Microgravity Laboratory, USML-2

Surface‐controlled drop oscillations in space

A series of experiments probing the effects of surfactants was performed by Bob Apfel and his research group in the 1990s. Several laboratory experiments were carried out in uni‐axial acoustic levitators. Two experiments were carried out in a triple‐axis levitator called the Drop Physics Module, which was carried on Space Shuttle Columbia as part of the First and Second United States Microgravity Laboratory missions. Liquid drops containing aqueous solutions of soluble surfactants were acoustically positioned and deformed (and in some cases rotated) in order to excite shape mode oscillations. The results of these experiments allowed the inference of surface rheological properties (Gibb’s elasticity, surface viscosity coefficients) as functions of surfactant type and concentration. The highlights of this effort will be presented in a semi‐technical fashion. [Work supported by NASA.]