Finite-amplitude effects on ultrasound beam patterns in attenuating media

Some problems relevant to medical ultrasonics are addressed through experimental measurements of focused, pure-tone beam patterns under quasilinear conditions where significant nonlinearities are manifested. First, measurements in water provide a comparison of the beam patterns of the fundamental and nonlinearly generated harmonics against recent theoretical predictions of others. The radial beamwidths, presence and spacing of sidelobes, axial distances to peak pressures, focal shock parameter, time-domain waveform asymmetry, and post-focal falloff of the fundamental through fifth harmonics are discussed relative to various models under preshock conditions (sigma less than 1). Second, the focused sources are placed in a more attenuating fluid to mimic the behavior of these fields in tissue. The changes in beam characteristics are examined relative to measurements at the same intensities in water, and relative to theoretical predictions. The results suggest that, given a known linear (low-intensity) focused beam pattern in water, guidelines can be followed to predict the beam pattern of the fundamental and higher harmonics at higher intensities in water, and then in attenuating media such as tissue.     

Frequency-domain wave equation and its time-domain solutions in attenuating media

Our purpose in this paper is to describe the wave propagation in media whose attenuation obeys a frequency power law. To achieve this, a frequency-domain wave equation was developed using previously derived causal dispersion relations. An inverse space and time Fourier transform of the solution to this algebraic equation results in a time-domain solution. It is shown that this solution satisfies the convolutional time-domain wave equation proposed by Szabo [J. Acoust. Soc. Am. 96, 491-500 (1994)]. The form of the convolutional loss operator contained in this wave equation is obtained. Solutions representing the propagation of both plane sinusoidal and transient waves propagating in media with specific power law attenuation coefficients are investigated as special cases of our solution. Using our solution, comparisons are made for transient one-dimensional propagation in a medium whose attenuation is proportional to frequency with recently obtained numerical solutions of Szabo's equation. These show good agreement.     

Ultrasound detection through turbid media

Optical coherence-domain reflectometry and laser-based ultrasound detection have been combined with the use of adaptive optics to detect ultrasound through turbid media. The dynamic hologram in a photorefractive quantum-well device performs as a coherence gate that eliminates multiply scattered background. Quadrature homodyne detection conditions are selected by the choice of center wavelength of the pulse spectrum, requiring no active stabilization or feedback. A depth resolution of 30 microm was achieved, with a pulse duration of nominally 120 fs for ultrasound detection through turbid media up to optical thicknesses of 11 mean free scattering lengths.     

Neutrinos in electromagnetic fields and moving media

The history of the development of the theory of neutrino-flavor and neutrino-spin oscillations in electromagnetic fields and in a medium is briefly surveyed. A new Lorentz-invariant approach to describing neutrino oscillations in a medium is formulated in such a way that it makes it possible to consider the motion of a medium at an arbitrary velocity, including relativistic ones. This approach permits studying neutrinospin oscillations under the effect of an arbitrary external electromagnetic field. In particular, it is predicted that, in the field of an electromagnetic wave, new resonances may exist in neutrino oscillations. In the case of spin oscillations in various electromagnetic fields, the concept of a critical magnetic-field-component strength is introduced above which the oscillations become sizable. In considering neutrino oscillations in moving matter, it is shown within the Lorentz-invariant formalism that the relativistic motion of matter significantly affects the character of neutrino oscillations and can radically change the conditions under which the oscillations are resonantly enhanced. Possible new effects in neutrino oscillations are discussed for the case of neutrino propagation in relativistic fluxes of matter.     

A model for spatial coherence from directive ambient noise in attenuating, dispersive media

As a complement to experimental efforts in seismics and acoustics to infer geo-acoustic properties of the propagation environment from the second order statistics of ambient noise measurements, a set of exact, explicit, closed form expressions for the cross-spectral density and spatial coherence of diffuse random wave fields is presented. Taken together, the expressions are well suited for modeling broadband, diffuse wave coherence in realistic scenarios involving directive, ambient noise from local (i.e., volume) and distant (i.e., plane wave) source features in an open, dispersive, attenuating medium.     

Approximating moments of acoustic fields in random media

Starting from a path integral representation of the fourth moment of an acoustic field, two types of approximations are considered in detail: a 'zero-kink' approximation and a 'single-kink' approximation. The expressions 'zero-kink' and 'single-kink' describe families of paths which are used to approximate path integrals over all continuous paths. The single-kink approximation reduces the propagation problem to a phase screen problem with fluctuations on the phase screen containing information about fluctuations along entire paths from source to receiver.     

Approximating moments of acoustic fields in random media

Abstract

Starting from a path integral representation of the fourth moment of an acoustic field, two types of approximations are considered in detail: a ‘zero-kink’ approximation and a ‘single-kink’ approximation. The expressions ‘zero-kink’ and ‘single-kink’ describe families of paths which are used to approximate path integrals over all continuous paths. The single-kink approximation reduces the propagation problem to a phase screen problem with fluctuations on the phase screen containing information about fluctuations along entire paths from source to receiver.     

Extremely narrow switching fields in oriented Ba-ferrite particulate media

Oriented particulate Ba-ferrite (BF) media have been known to exhibit very narrow switching field distributions, due to a confluence of reasons based on the particle morphology and the direction of their magnetic anistropy. In this study we report on extremely narrow switching fields observed in longitudinally oriented BF rigid disk coatings, which are primarily the result of strong positive magnetostatic interactions between adjacent platelets in the oriented assemblies. In the transverse direction, the interactions are negative, and the switching fields are large.     

Domain wall depinning in random media by ac fields

The viscous motion of an interface driven by an ac external field of frequency omega(0) in a random medium is considered here in the nonadiabatic regime. The velocity exhibits a smeared depinning transition showing a double hysteresis which is absent in the adiabatic case omega(0)-->0. Using scaling arguments and an approximate renormalization group calculation we explain the main characteristics of the hysteresis loop. In the low frequency limit these can be expressed in terms of the depinning threshold and the critical exponents of the adiabatic case.     

Electro-and magnetostatic fields in media with a nonuniform velocity

A motion-induced magnetic or electric field is calculated by the integral method in the first order of smallness in the ratio of the motion velocity to the speed of light for homogenous media with an arbitrary stationary velocity distribution that are placed in static electric or magnetic fields. For the case of rotation of a sphere, the validity of the results is corroborated by comparing with a solution obtained by joining the fields in the moving and quiescent parts of the medium. Estimates suggest the feasibility of experimentally observing this effect of continuum electrodynamics.     

Spinor fields with zero mass in unbounded isotropic media

The Dirac equation for massless fields in unbounded media has solutions similar to the focus wave mode solutions of Maxwell's equations leading to infinite dynamical invariants. We define the splash wave mode solutions as a weighted superposition of the focus wave modes, and discuss the conditions to be fulfilled by the weight functions to make the dynamical invariants bounded. We leave open the physical interpretation of these solutions.     

Reference-wave solutions for high-frequency fields propagating in random media

Ray theory plays an important role in determining the propagation properties of high-frequency fields and their statistical measures in complicated random environments. According to the ray approach, the field at the observer can be synthesized from a variety of field species arriving along multiple ray trajectories resulting from refraction and scattering from boundaries and from scattering centers embedded in the random medium. For computations of the statistical measures, it is desirable therefore to possess a solution for the high-frequency field propagating along an isolated ray trajectory. For this reason, a new reference-wave method was developed to provide an analytic solution of the parabolic-wave equation.     

Ultrasound tagged light imaging in turbid media in a reflectance geometry

A combination of light and focused ultrasound waves provides a unique way to obtain directly three-dimensional absorption data in a turbid medium. We present the combination of an ultrasound wave and light in which both the input and the output optodes are on the same side of the sample (reflectance geometry). This technique permits local detection in depth of the presence of a purely absorbing object, without further mathematical processing. It is a promising technique for medical imaging and monitoring of tissues.     

Effects of local fields on spontaneous emission in dielectric media

The local-field renormalization of the spontaneous emission rate in a dielectric is explicitly obtained from a fully microscopic quantum-electrodynamical, many-body derivation of Langevin-Bloch operator equations for two-level atoms embedded in an absorptive and dispersive, linear dielectric host. We find that the dielectric local-field enhancement of the spontaneous emission rate is smaller than indicated by previous studies.     

Reference-wave solutions for the high-frequency fields in inhomogeneous-background random media

Ray theory plays an important role in determining the propagation properties of high-frequency fields and their statistical measures in complicated random environments. For computations of the statistical measures it is therefore desirable to have a solution for the high-frequency field propagating along an isolated ray trajectory. A new reference wave is applied to obtain an analytic solution of the parabolic wave equation that describes propagation along the ray trajectory of the deterministic-background medium. The methodology is based on defining a paired-field measure as a product of an unknown field propagating in a disturbed medium and the complex-conjugate component propagating in a medium without random fluctuations. When a solution of the equation for the paired-field measure is obtained, the solution of the deterministic component can be extracted from the paired solution to determine the solution of the unknown field in an explicit form.