Index of content:
Volume 129, Issue 5, May 2011
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
129(2011); http://dx.doi.org/10.1121/1.3557036View Description Hide Description
The ultimate goal of quantitative ultrasound (QUS) imaging methods based on backscatter coefficient (BSC) estimates is to obtain system-independent structural information about samples. In the current study, three BSC estimation methods were compared and evaluated using the same backscattered pressure datasets in order to assess their consistency. BSC estimates were obtained from two phantoms with embedded glass spheres and compared to theoretical BSCs calculated using size distributions estimated using optical microscopy. Effective scatterer diameter and concentration estimates of the glass spheres were also obtained from the estimated BSCs. One estimation method needed to be compensated by more than an order of magnitude in amplitude in order to produce BSCs comparable to the other two methods. All calibration methods introduced different frequency-dependent effects, which could have noticeable effects on the bias of QUS estimates derived from experimental BSCs. Although in most cases the experimental QUS estimates obtained with all three methods were observed to differ by less than 10%, larger differences are expected depending on both the pressure focusing gain of the transducer (proportional to the ratio of the square of the aperture radius to the product of the wavelength and focal length) and ka range used in the estimation.
129(2011); http://dx.doi.org/10.1121/1.3557040View Description Hide Description
At the fluid/porous-medium interface the pseudo-Rayleigh (pR) and pseudo-Stoneley (pSt) waves exist. The relation with the corresponding poles in the slowness plane is not unambiguous, depending on the choice of branch cuts. For a point-force excitation, the far-field Green’s functions are computed using vertical branch cuts (method I) implying that the pR- and pSt-poles obey the radiation condition. Then, a separate pseudo interface wave is entirely captured by the corresponding pole residue because the loop integral along a branch cut contributes to a body wave only. When hyperbolic branch cuts are used (method II) the poles lie on the “principal” Riemann sheet. Then, also the loop integrals necessarily contribute to the pR-wave because the pR-pole is different from that in method I. They do not contribute to the pSt-wave when the pSt-pole lies on the principal Riemann sheet because the pole is identical to that in method I. When the pSt-pole has migrated to another Riemann sheet, however, the pSt-wave is fully captured by the loop integrals. In conclusion, the phase velocity and attenuation of a separate pseudo interface wave can be computed from the pole location in method I, but should be extracted from the full response in method II.
129(2011); http://dx.doi.org/10.1121/1.3559699View Description Hide Description
Elastodynamic response of anisotropiclaminate composite structures subjected to a force loading is evaluated based on the integral representations in terms of Green’s matrices. Explicit and asymptotic expressions for guided waves generated by a given source are then obtained from those integrals by means of series expansions and the residue technique. Unlike to conventional modal expansions, such representations keep information about the source, giving an opportunity for a quantitative near- and far-field analysis of generated waves. An effective computer implementation is achieved by the use of fast and stable algorithms for the Green matrix, pole, and residue calculations. The potential of the model is demonstrated by examples of anisotropy manifestation in the directivity of radiated waves. The effect of main energy outflow in the direction of either upper- or inner-ply orientation depending on the source size and frequency is discussed.
129(2011); http://dx.doi.org/10.1121/1.3570946View Description Hide Description
A two dimensional simulation study was performed to investigate the photoacoustic signal properties of non-aggregated and aggregated erythrocytes. Spatial distributions of non-aggregated blood samples were generated by employing a Monte Carlo method and aggregated blood samples were simulated using a hexagonal packing scheme. For the non-aggregating case photoacoustic signals demonstrated a monotonic rise with hematocrit. For the aggregating case it was found that spectral (<20 MHz) intensity increased (11 dB at 15.6 MHz) when the aggregate size increased. This study strongly suggests that the assessment of erythrocyte aggregation level in human blood might be possible by using a photoacoustic spectroscopic method.
129(2011); http://dx.doi.org/10.1121/1.3569730View Description Hide Description
This work was undertaken to help understand and interpret the ultrasonicwave reflection response of Portland cement paste as it transforms from a fluid-like suspension to a solid in the first hours after mixing. A high impact polystyrene buffer (delay line) was used to measure small changes in the P- and S-wave reflection coefficients. Two materials were studied: a non-hydrating colloidal alumina suspension whose microstructure was manipulated between dispersed and flocculated states by adjusting the pH and a coarse silicasuspension that readily sedimented. The S-wave reflection coefficient clearly distinguished between dispersed and flocculated states. Sedimentation of particles in dispersed suspensions was distinguished using the P-wavereflection coefficient. Based on these findings, the observed P- and S-wave responses from hydrating Portland cement paste are interpreted in terms of flocculation and sedimentation processes.