Volume 128, Issue 1, July 2010
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
128(2010); http://dx.doi.org/10.1121/1.3442575View Description Hide Description
The crawling wave experiment, in which two harmonic sources oscillate at different but nearby frequencies, is a development in sonoelastography that allows real-time imaging of propagating shear wave interference patterns. Previously the crawling wave speed was recovered and used as an indicator of shear stiffness; however, it is shown in this paper that the crawling wave speed image can have artifacts that do not represent a change in stiffness. In this paper, the locations and shapes of some of the artifacts are exhibited. In addition, a differential equation is established that enables imaging of the shear wave speed, which is a quantity strongly correlated with shear stiffness change. The full algorithm is as follows: (1) extract the crawling wave phase from the spectral variance data; (2) calculate the crawling wave phase wave speed; (3) solve a first-order PDE for the phase of the wave emanating from one of the sources; and (4) compute and image the shear wave speed on a grid in the image plane.
128(2010); http://dx.doi.org/10.1121/1.3409370View Description Hide Description
The predicted efficiency of a simple thermoacoustic waste heat power conversion device has been investigated as part of a collaborative effort combining a thermoacoustic engine with a piezoelectric transducer.Symko et al. [Microelectron. J.35, 185–191 (2004)] at the University of Utah built high frequency demonstration engines for this application, and Lynn [ASMDC report, accession number ADA491030 (2008)] at the University of Washington designed and built a high efficiency piezoelectric unimorph transducer for electroacoustic conversion. The design presented in this paper is put forward to investigate the potential of a simple high frequency, air filled, standing wave thermoacousticdevice to be competitive with other small generator technologies such as thermoelectric devices. The thermoacoustic generator is simulated using a low-amplitude approximation for thermoacoustics and the acoustic impedance of the transducer is modeled using an equivalent circuit model calculated from the transducer’s mechanical and electrical properties. The calculations demonstrate that a device performance of around 10% of Carnot efficiency could be expected from the design which is competitive with currently available thermoelectric generators.
Displacement analysis of diagnostic ultrasound backscatter: A methodology for characterizing, modeling, and monitoring high intensity focused ultrasound therapy128(2010); http://dx.doi.org/10.1121/1.3436554View Description Hide Description
Accurate monitoring of high intensity focused ultrasound (HIFU) therapy is critical for widespread clinical use. Pulse-echo diagnosticultrasound (DU) is known to exhibit temperature sensitivity through relative changes in time-of-flight between two sets of radio frequency (RF) backscattermeasurements, one acquired before and one after therapy. These relative displacements, combined with knowledge of the exposure protocol, material properties, heat transfer, and measurement noise statistics, provide a natural framework for estimating the administered heating, and thereby therapy. The proposed method, termed displacement analysis, identifies the relative displacements using linearly independent displacement patterns, or modes, each induced by a particular time-varying heating applied during the exposure interval. These heating modes are themselves linearly independent. This relationship implies that a linear combination of displacement modes aligning the DU measurements is the response to an identical linear combination of heating modes, providing the heating estimate. Furthermore, the accuracy of coefficient estimates in this approximation is determined a priori, characterizing heating, thermal dose, and temperature estimates for any given protocol. Predicted performance is validated using simulations and experiments in alginate gel phantoms. Evidence for a spatially distributed interaction between temperature and time-of-flight changes is presented.