Reflection and transmission coefficient approximations for P, S1 and S2 waves in triclinic media

SUMMARY The conventional assumptions, in most published approximations of the reflection and transmission (R/T) coefficients of plane waves at a plane interface between two anisotropic half-spaces, confine their applications to weakly anisotropic and/or weak contrast models. We consider the horizontal interface enclosed by two triclinic half-spaces to approximate the R/T coefficients normalized by the vertical energy flux. The homogeneous background medium can be anisotropic with arbitrary symmetry to better simulate the strongly anisotropic media. The second-order approximations are proposed to accommodate the strong contrast interface. We also consider an isotropic background medium under the weak anisotropy assumption. The obtained approximations can be applied to P, S1 and S2 waves, except for the transmission coefficients between the S1 and S2 waves. The S-wave transmission coefficients are insensitive to the model parameter contrasts and predominately rely on the S-wave polarization directions in the half-spaces above and below the interface. The proposed approximations are tested numerically.

Skull’s Photoacoustic Attenuation and Dispersion Modeling with Deterministic Ray-Tracing: Towards Real-Time Aberration Correction

Although transcranial photoacoustic imaging has been previously investigated by several groups, there are many unknowns about the distorting effects of the skull due to the impedance mismatch between the skull and underlying layers. The current computational methods based on finite-element modeling are slow, especially in the cases where fine grids are defined for a large 3-D volume. We develop a very fast modeling/simulation framework based on deterministic ray-tracing. The framework considers a multilayer model of the medium, taking into account the frequency-dependent attenuation and dispersion effects that occur in wave reflection, refraction, and mode conversion at the skull surface. The speed of the proposed framework is evaluated. We validate the accuracy of the framework using numerical phantoms and compare its results to k-Wave simulation results. Analytical validation is also performed based on the longitudinal and shear wave transmission coefficients. We then simulated, using our method, the major skull-distorting effects including amplitude attenuation, time-domain signal broadening, and time shift, and confirmed the findings by comparing them to several ex vivo experimental results. It is expected that the proposed method speeds up modeling and quantification of skull tissue and allows the development of transcranial photoacoustic brain imaging.

A Revisit of the Coupling Loss Factors in Determining Power Flows in Structures

Coupling loss factor is one of the most important parameters used in the statistical energy analysis (SEA). It regulates the energy flows between two subsystems. Theoretically, the coupling loss factors are usually calculated from the wave transmission coefficients derived using two semi-infinite systems. While the final system equation in the SEA is based on the powerful energy conservation principle, in the process the coupling loss factors are assumed to be unaffected by the configuration changes. In other words, the coupling loss factors which are experimentally or analytically determined under some ideal conditions are considered to be unchanged by the more complicated coupling conditions under a system configuration. The validity of this treatment needs to be carefully studied because there is a belief that the SEA method can be readily extended to lower frequencies so long as the coupling loss factors can be somehow satisfactorily determined. In this study, wave transmission coefficients are obtained by calculating the energy flows for a three-beam system coupled together in T-shape. Separately, the wave transmission coefficients are estimated by using energy flows calculated for each possible pairing of two beams. By comparing the results from these two different approaches, some insightful information has been obtained regarding the characteristics of the coupling loss factors in determining power flows in structures.