Robin O. Cleveland

Time‐domain modeling of finite‐amplitude sound in relaxing fluids

A time‐domain computer algorithm that solves an augmented Burgers equation is described. The algorithm is a modification of the time‐domain code developed by Lee and Hamilton [J. Acoust. Soc. Am. 97, 906–917 (1995)] for pulsed finite‐amplitude sound beams in homogeneous, thermoviscous fluids. In the present paper, effects of nonlinearity, absorption and dispersion (both thermoviscous and relaxational), geometrical spreading, and inhomogeneity of the medium are taken into account. The novel feature of the code is that effects of absorption and dispersion due to multiple relaxation phenomena are included with calculations performed exclusively in the time domain. Numerical results are compared with an analytic solution for a plane step shock in a monorelaxing fluid, and with frequency‐domain calculations for a plane harmonic wave in a thermoviscous, monorelaxing fluid. The algorithm is also used to solve an augmented KZK equation that accounts for nonlinearity, thermoviscous absorption, relaxation, and diffraction in directive sound beams. Calculations are presented which demonstrate the effect of relaxation on the propagation of a pulsed, diffracting, finite‐amplitude sound beam.

Numerical simulations of heating patterns and tissue temperature response due to high-intensity focused ultrasound

The results of this paper show-for an existing high intensity, focused ultrasound (HIFU) transducer-the importance of nonlinear effects on the space/time properties of wave propagation and heat generation in perfused liver models when a blood vessel also might be present. These simulations are based on the nonlinear parabolic equation for sound propagation and the bio-heat equation for temperature generation. The use of high initial pressure in HIFU transducers in combination with the physical characteristics of biological tissue induces shock formation during the propagation of a therapeutic ultrasound wave. The induced shock directly affects the rate at which heat is absorbed by tissue at the focus without significant influence on the magnitude and spatial distribution of the energy being delivered. When shocks form close to the focus, nonlinear enhancement of heating is confined in a small region around the focus and generates a higher localized thermal impact on the tissue than that predicted by linear theory. The presence of a blood vessel changes the spatial distribution of both the heating rate and temperature.

In Vivo Pressure Measurements of Lithotripsy Shock Waves in Pigs

Stone comminution and tissue damage in lithotripsy are sensitive to the acoustic field within the kidney, yet knowledge of shock waves in vivo is limited. We have made measurements of lithotripsy shock waves inside pigs with small hydrophones constructed of a 25-microm PVDF membrane stretched over a 21-mm diameter ring. A thin layer of silicone rubber was used to isolate the membrane electrically from pig fluid. A hydrophone was positioned around the pig kidney following a flank incision. Hydrophones were placed on either the anterior (shock wave entrance) or the posterior (shock wave exit) surface of the left kidney. Fluoroscopic imaging was used to orient the hydrophone perpendicular to the shock wave. For each pig, the voltage settings (12-24 kV) and the position of the shock wave focus within the kidney were varied. Waveforms measured within the pig had a shape very similar to those measured in water, but the peak pressure was about 70% of that in water. The focal region in vivo was 82 mm x 20 mm, larger than that measured in vitro (57 mm x 12 mm). It appeared that a combination of nonlinear effects and inhomogeneities in the tissue broadened the focus of the lithotripter. The shock rise time was on the order of 100 ns, substantially more than the rise time measured in water, and was attributed to higher absorption in tissue.

The effect of polypropylene vials on lithotripter shock waves

In studies to understand the mechanisms responsible for shock wave lithotripsy (SWL) cell injury, we observed that shock waves (SWs) are influenced by the shape of the specimen vial. Lytic injury to kidney cells treated in a Dornier HM3 lithotripter was higher (p < 0.0001) when SWs entered the vial through the flat end (cap end) compared to the round end. Measurements of the acoustic field within polypropylene vials were carried out using both lithotripter SWs and pulsed ultrasound (US) in the megahertz frequency range. We compared pressure amplitudes inside the round and flat vials and found significant differences. When SWs entered through the round end, the average peak positive pressure was 40% of free-field pressure, due mostly to a dramatic reduction in pressure off axis. The average peak pressure inside the flat vial was twice that of the round vial. Experiments with US demonstrated that sound field focusing was induced by the curved interface of the round vial. Ray analysis for the round vial indicates the presence of hot spots on axis and cold spots off axis, in qualitative agreement with pressure profiles. We conclude that the shape of the specimen vial is an important factor that should be considered in model systems of SWL cell injury.

Ultrasound-guided localized detection of cavitation during, lithotripsy in pig kidney in vivo

It is supposed that inertial cavitation plays a significant role in tissue damage during extracorporeal shock wave lithotripsy (ESWL). In this work we attempted to detect cavitation in tissue. In vivo experiments with pigs were conducted in a Dornier HM3 electrohydraulic lithotripter. Kidney alignment was made using fluoroscopy and B-mode ultrasound. Cavitation was detected by a dual passive cavitation detection (DPCD) system consisting of two confocal spherical bowl PZT transducers (1.15 MHz, focal length 10 cm, radius 10 cm). An ultrasound scanhead was placed between the transducers, an hyperechoic spots in the image indicated pockets of bubbles during ESWL. A coincidence-detection algorithm and the confocal transducers made it possible to localize cavitation to within a 4 mm diameter region. The signals from both the collecting system and kidney tissue were recorded. The targeting of the DPCD focus was confirmed by using the DPCD transducers as high intensity focused ultrasound (HIFU) sources at HIFU durations below the lesion formation threshold. In this HIFU regime, a bright spot appears in the B-mode image indicating the position of the DPCD focus. In this way we could confirm that refraction and scattering in tissue did not cause a misalignment. The tissue region interrogated was also marked with a lesion produced by HIFU. Clear cavitation signals were detected from the collecting system and from pools of blood that formed near the kidney capsule and weak signals were recorded from tissue during the ESWL treatment.

Fdtd Simulation of Transcranial Focusing Using Ultrasonic Phase-Conjugate Arrays

Focusing ultrasonic fields onto selected targets within the body with good accuracy and sufficient intensity to allow controlled necrosis of a region of tissue is a topic of great interest in medical acoustics. Several techniques have been demonstrated through simulation and laboratory experiment to achieve promising results [1, 2, 3]. The purpose of the present paper is to demonstrate the use of acoustic phase conjugation, or more narrowly in this context, time reversal as a valid focusing technique in biological tissue, which does not require a priori knowledge of the inhomogeneous propagation medium’s properties. Another goal of this paper is to demonstrate the effectiveness of the finite-difference time-domain (FDTD) method for the simulation of linear acoustic propagation in an inhomogeneous propagation medium.

The role of cavitation in therapeutic ultrasound

Ed Carstensen has made many contributions to biomedical ultrasound but among those that are becoming more and more relevant to current clinical practice are those that determine the conditions under which cavitation is induced in vivo. For many years, it was assumed that the medical ultrasound devices were unable to induce cavitation in living tissue because either the acoustic conditions were not sufficient or the nucleation sites that are required were too small. With the advent of lithotripters and high‐intensity focused ultrasound (HIFU) devices, cavitation generation in vivo is commonplace. Our current research at the University of Washington has focused on the role that cavitation plays in stone comminution and tissue damage during lithotripsy, as well as the enhancement or reduction of desirable coagulative necrosis during HIFU application. During HIFU application, we find enhanced heating that results from nonlinear acoustic wave propagation (a key Carstensen contribution) leads to vapor bubble fo...

Bubbles generate unexpected stresses

Modern medicine is taking greater and greater advantage of the diverse capabilities of ultrasound. High-intensity ultra-sound in particular is a promising technique for treating various medical conditions, but we still do not fully understand how the ultrasound induces physical changes in body tissue. Some ingenious experiments by Guy Delacretaz and colleagues at Ecole Polytechnique Federale Lausanne in Switzerland, and Northwestern University in Illinois, US, have provided important new insights into the problem (Appl. Phys. Lett. 1997 70 3510). They have studied the effect of ultrasound in a tissue-like gel, and have found some surprising results.