Index of content:
Volume 125, Issue 3, March 2009
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
Chirp excitation technique to enhance microbubble displacement induced by ultrasound radiation force125(2009); http://dx.doi.org/10.1121/1.3075548View Description Hide Description
Ultrasound radiation force has been proposed to increase the targeting efficiency in ultrasonic molecular imaging and drug delivery. A chirp excitation technique is proposed to increase the radiation force induced microbubble displacement and might potentially be used for enhancing the targeting efficiency of microbubble clouds. In this study, a modified Rayleigh–Plesset equation is used to estimate the radius-time behavior of insonified microbubbles, and the translation of insonified microbubbles is calculated by using the particle trajectory equation. Simulations demonstrate that the chirp excitation is superior to the sinusoidal one in displacing microbubbles with a wide-size distribution, and that the performance is dependent on the parameters of the chirp signal such as the center frequency and frequency range. For Gaussian size distributed microbubble clouds with mean diameter of and variance of 1, a 2.25 MHz chirp with frequency range of 1.5 MHz induces about 59.59% more microbubbles over a distance of during insonification, compared to a 2.25 MHz sinusoidal excitation with equal acoustic pressure.
125(2009); http://dx.doi.org/10.1121/1.3075764View Description Hide Description
Gas-filled quasi-spherical resonators are excellent tools for the measurement of thermophysical properties of gas and have also been retained for the determination of the Boltzmann constant with a low uncertainty, which can be derived from measurements of both the speed of sound in a noble gas and the volume of the resonator. To achieve this, a detailed modeling of the acoustic field in quasi-spherical resonators is of importance. Several phenomena and perturbations must be taken into account, including, among inertia and compressibility, heat conduction, viscosity, the shape of the resonator, small irregularities on the wall, and so on. The aim of this paper is to provide improvements to the current models of the acoustic field in such resonator. Namely, the model given here takes into account all the different perturbing elements together in a unique formalism, including the coupling between the different perturbing elements and the resulting modal coupling in a consistent manner. The first results obtained from this analytical model on a simple configuration show that the effect of modal coupling is small but should not be neglected regarding the accuracy required here, even if several improvements could still be provided to this new unified model.
125(2009); http://dx.doi.org/10.1121/1.3068445View Description Hide Description
In conventional biomedical photoacoustic tomography (PAT), ultrasonic pulses generated through the absorption of nanosecond pulses of near-infrared light are recorded over an array of detectors and used to recover an image of the initial acoustic pressure distribution within soft tissue. This image is related to the tissue optical coefficients and therefore carries information about the tissue physiology. For high resolution imaging, a large-area detector array with a high density of small, sensitive elements is required. Such arrays can be expensive, so reverberant-field PAT has been suggested as a means of obtaining PATimages using arrays with a smaller number of detectors. By recording the reflections from an acoustically reverberant cavity surrounding the sample, in addition to the primary acoustic pulse, sufficient information may be captured to allow an image to be reconstructed without the need for a large-area array. An initial study using two-dimensional simulations was performed to assess the feasibility of using a singledetector for PAT. It is shown that reverberant-field data recorded at a single detector are sufficient to reconstruct the initial pressure distribution accurately, so long as the shape of the reverberant cavity makes it ray-chaotic. The practicalities of such an approach to photoacoustic imaging are discussed.
125(2009); http://dx.doi.org/10.1121/1.3068447View Description Hide Description
A laser-based ultrasonic technique suitable for characterization of the microstructural state of metal foils is presented. The technique relies on the measurement of the intrinsic attenuation of laser-generated longitudinal waves at frequencies reaching 1 GHz resulting from ultrasonic interaction with the sample microstructure. In order to facilitate accurate measurement of the attenuation, a theoretical model-based signal analysis approach is used. The signal analysis approach isolates aspects of the measured attenuation that depend strictly on the microstructure from geometrical effects. Experimental results obtained in commercially cold worked tungsten foils show excellent agreement with theoretical predictions. Furthermore, the experimental results show that the longitudinal wave attenuation at gigahertz frequencies is strongly influenced by the dislocation content of the foils and may find potential application in the characterization of the microstructure of micron thick metal foils.