Low Acoustic Pressure Research Articles

Observations of insonified contrast agents in vitro and in vivo

Ultrasound contrast agents, which may include a gas core and lipid or albumin shell, play an increasingly important role in medical imaging. However, a complex set of phenomena have limited quantitative evalua- tion of tissue perfusion based on echoes from these microbubbles. With the use of optical microscopy, mechanisms of microbubble destruction during insonation are evaluated, as well as the effects of ultrasonic radiation force. Depending on conditions such as acoustic intensity and dissolved gas concentration in the media, several mechanisms can change the microbubble and promote its destruction during insonation. Slow gas diffusion from the bubble is observed with low-intensity transmission, but with several pulses at a higher intensity, the albumin shell may weaken sufficiently to release the gas core. It is demonstrated that the primary acoustic radiation force has a significant effect on microbubbles when the ultrasound is transmitted with a low acoustic pressure and high pulse repetition frequency. Deflection of the contrast agent streamline to the wall of a small vessel is shown both in vitro and in vivo. Such manipulation of the contrast agents may be desired in drug delivery applications, as it provides a method of concentrating bubbles in a specific area.

Reviews Of Acoustical Patents

The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.

Intensity-modulated fiber-optic microphone

A fiber-optic microphone has been developed that can detect low acoustic sound pressure levels. The microphone is an extrinsic intensity-modulated fiber-optic level sensor. The optical mechanisms involved and the effect on performance of fiber placement with respect to the optical element will be discussed. Plots of frequency response, sensitivity, and a tape recording made using the microphone will be presented.

Response of propellant combustion to a turbulent acoustic boundary layer

An analysis of the transitional and turbulent reactive acoustic boundary layer on a homogeneous, solid-propellant surface is conducted to investigate potential mechanisms of combustion instability. The theoretical approach utilizes a low Reynolds number turbulence closure model and a finite-difference solution procedure for an equation system of parabolic form. A new technique is developed for the condensed-phase thermal layer, in which the propellant space is mapped onto the gas space and efficiently solved using the same adaptive numerical grid. Results are obtained at an acoustic pressure node in the absence of a mean axial flow. The results indicate that acoustically induced transition can occur at relatively low acoustic pressure amplitudes, propellant response to harmonic axial velocity fluctuations is both rectified and displays a mean augmentation (D-C shift), and that nominal propellant combustion parameters lead to increasing susceptibility to acoustic transition at elevated mean pressures. As OQ Bg cp €« / H h h^ k ks L® n q R Reac Res Ret Ru r

Theoretical analysis of a push-pull fiber-optic hydrophone

A push-pull fiber-optic hydrophone is analyzed theoretically by closed-form approximations. The sensor is a fiber-optic Mach-Zehnder interferometer with a single-mode optical fiber from each arm wound on a separate tube. The two tubes are coaxial, with one tube inside the other. The acoustic medium is applied to the inside (outside) of the smaller (larger) diameter tube with the fiber wound on the tube's opposite side. This configuration is desired so that the acoustic signal modulates the optical phase of the arms with equal magnitude 180° out of phase. This gives the sensor twice the sensitivity of an analogous single tube sensor. The heat sinked interferometer arms with equal and opposite sensitivities are in relative close proximity to each other. This promotes good noise rejection to achieve a low minimum detectable acoustic pressure due to the interferometer's common-mode rejection.

Measurement of cavitation acoustic pressure and bubble dynamics in liquid helium II

Acoustic cavitation nucleated in bulk He II at a temperature of 2.10°K was photographed at 6000 frames‐per‐second. The vapor bubbles were formed at pressure antinodes near the axis of a PZT4 cylindrical transducer operating at 52 KHz with a wavelength of 4.4 mm. The diffraction pattern of light traveling along the cylinder axis was used to adjust the frequency for maximum acoustic intensity and to measure the acoustic pressure. For a pressure of 280:± 70 mbar rms, spherical bubbles originating in the liquid grew to 1 mm diameter in less than 0.3 msec. They fragmented during the next 0.6 msec to form smaller nonspherical cavities which persisted for tens of milliseconds. The smaller nonspherical cavities could also be observed at lower acoustic pressures where they originated on the surface of a probe and not in the bulk fluid. Previous photographs of He II cavitation [A. Mosse, M.L. Chu, and R.D. Finch, J. Acoust. Soc. Am. 47, 1258 (1970)] showed similar nonspherical cavities without revealing their origin.

Acoustic Measurements in the Finite-Amplitude Beam of a Plane Piston

A plane piston source of radius 1 cm is driven in water at a fundamental frequency of 2.58 Mc. Acoustic excess pressures near the source are 0.2–6 atm. The first, second, and, to some extent, the third, harmonics have been mapped to distances of 40 cm along and across the beam axis. Both a wire probe as described by A. B. Wood [J. Acoust. Soc. Am. 31, 1213 (1959)] and a barium-titanate cylinder 116 in. long and wide have been used, with consistent results. The probes are calibrated by integrating their response across the axis and by comparing with that of a large plane receiver previously calibrated by the reciprocity method. Another check is to compare with established theory the axial response of the receiver at the fundamental frequency and low acoustic pressures (correcting for finite size of the probes). Patterns at higher intensities are complicated and quite different from first to higher harmonics, as expected. Only partial comparisons with theory are feasible. [Work supported in part by the Office of Naval Research and performed at Brown University, where R. K. G. was a visitor for the summer of 1962.]