摘要:

Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off‐peak response of the imaging system’s point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phasd array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the Muller–Buffington correction technique, [R. A. Muller and A. Buffington, J. Opt. Soc. Am.64(9), 1200–1209 (1974)] used to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two‐dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.

ISSN:0001-4966    

DOI:10.1121/1.397889

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The problem of the scattering of a plane wave by a general slender body of arbitrary cross section with a constant surface impedance is treated by a formalism based on the matched asymptotic method. It is shown that the scattered pressure may be determined by the resolution of a set of 2‐D problems when using the slender‐body approximation that is valid for wavelengths of the order of magnitude or greater than the characteristic cross‐section length of the body. In the case of axisymmetric bodies, an expression for the scattered field can be obtained explicitly. The contributions of the insonified end section are analyzed from a paraboloidal geometry model, and simplified solutions are proposed for rigid and soft conditions. Some theoretical and numerical results for the farfield directivity are presented to illustrate the theory.

ISSN:0001-4966    

DOI:10.1121/1.397890

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The eigenfrequencies of a hard‐walled acoustic cavity resonator are found by solving the Helmholtz equation with Neumann boundary conditions. A cavityCwith an internal hard obstacleBof vanishing size was considered. Because the perturbation is never small in the neighborhood of the obstacle even in the limit of a small obstacle, a modified perturbation theory is required. A formalism was developed and tested by calculating the eigenfrequencies of a cylinder with an internal sphere on the axis. The results are compared with theoretical and experimental values determined by other investigators. The difference between the perturbation calculations and the numerical results varies from less than 10−5to about 0.02 of the unperturbed eigenfrequency, depending on the size of the perturbing sphere and the mode.

ISSN:0001-4966    

DOI:10.1121/1.397891

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

Two approaches involving the spatial and temporal Fourier transforms have been used to derive time‐ and space‐dependent Green’s functions pertinent to the propagation of sound waves in a fluid that is moving with a constant velocityv. The two approaches give rise to differing interpretations of the observations made by a stationary observervis‐à‐visthose made by an observer moving with the fluid. The properties of the causal and the noncausal Green’s functions are analyzed, and are shown to be equivalent.

ISSN:0001-4966    

DOI:10.1121/1.397892

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

When an axisymmetric, bifrequency transducer mounted in a rigid baffle is excited at acoustic Mach numbers that are a relatively large fraction, the result is a dual frequency sound beam that exhibits harmonic and intermodulation distortion. The present analysis of this problem develops a perturbation solution based on a wave equation that consistently accounts for nonlinearity and diffraction. The linearized problem is described by a King integral for the sound beam at each primary frequency. Asymptotic analysis using Laplace’s method of integration is used to find the second‐order potential. The method of renormalization then leads to a uniformly accurate expression for the acoustic pressure. A technique for improvement in computational efficiency is developed by interfacing the King integral predictions to a farfield model for quasispherical waves. Propagation curves for parametric arrays obtained from the model compare favorably with experimental observations.

ISSN:0001-4966    

DOI:10.1121/1.397893

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The development of a systematic method for finding the reflected and transmitted fields due to a point source in the presence of a plane, penetrable interface is presented. This approach is based on the classical method of steepest descent, but the plane‐wave reflection and transmission coefficients are allowed to influence the location of the saddle points and their steepest descent paths. As a consequence, saddle points are, in general, complex, and complicated processes such as the reflected lateral wave field and the evanescent field in the bottom are incorporated in the saddle point formulation. The geometric interpretation of the saddle point criterion is derived in terms of eigenrays and their characteristics, from which expressions are obtained for the ray displacement upon reflection and transmission. Summaries of the eigenray structure of the reflected and transmitted fields are given for low‐frequency situations.

ISSN:0001-4966    

DOI:10.1121/1.397894

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A ray method for finding the acoustic field due to a point source in the presence of a plane, penetrable interface is extended to two simple models for shallow‐water ocean environments: the flat, isovelocity waveguide (the Pekeris model) and the sloping, isovelocity waveguide (the penetrable wedge). In both cases, the sound speed in the bottom is assumed larger than that in the water. The total field is expressed as a sum of ray fields, each of which takes the form of a plane wave integral. The integrals are solved using the method of steepest descent, where the plane wave reflection and transmission coefficients are allowed to influence the location of the saddle points. For a Pekeris waveguide in which three modes are trapped, agreement between the ray model and the SAFARI model is nearly perfect at all ranges, while the discrete normal mode solution is in error when a mode is near cutoff. The ray model is accurate even when the water depth is half of the acoustic wavelength. For the penetrable wedge problem, the plane wave integral for the ray field is developed, and the origin of the multiple lateral wave fields is examined. Excellent agreement between the ray model and a two‐way coupled mode model is demonstrated. Examples of the eigenray structure in both the flat and sloping waveguides are given.

ISSN:0001-4966    

DOI:10.1121/1.397895

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

The small‐angle‐of‐propagation limit and the method of matched asymptotics are applied to derive an efficient model for solving realistic underwater acoustics problems involving both propagation and scattering from a submerged object. The propagation and scattering aspects of the waveguide scattering problem are decoupled by approximating the waveguide Green’s function on the surface of the scatterer. For low frequencies, the small‐angle limit also allows one to approximate the incident field with a horizontally propagating plane wave and the scattered field with an azimuthally specular point‐source field. With these approximations, scattering calculations can be performed efficiently in the time domain. Calculations involving the three‐dimensional parabolic equation and the time‐domain parabolic equation are presented.

ISSN:0001-4966    

DOI:10.1121/1.397896

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A model is described that has been developed to evaluate the scatter produced by a high‐frequency acoustic pulse that originates from a stationary and arbitrarily located source; is incident on and scattered from an under‐ice surface characteristic of pack ice regions of the interior Arctic; and is detected by a stationary and arbitrarily located receiver. Measured, two‐dimensional, under‐ice, acoustic profile data and several empirical results that relate various geometric parameters of large‐scale under‐ice relief features (e.g., ice keels) are used to construct a three‐dimensional, under‐ice surface model consisting of first‐year ice keels and sloping, flat ice regions. A first‐year ice keel is modeled as an ensemble of randomly oriented ice blocks on a planar surface inclined at some slope angle with respect to a horizontal plane at sea level. Ice blocks are modeled as layered, viscoelastic solids with smooth rectangular faces or facets. A region of flat ice is modeled as a smooth planar surface whose slope angle is less than some arbitrary minimum slope angle. The Helmholtz–Kirchhoff integral equation and the Kirchhoff approximation are used to evaluate the scattered pressure field. Time is partitioned into bins and, beginning at the bin corresponding to the time the scattered pressure field of each scattering facet arrives at the receiver, the scattered pressure field is added coherently in all bins spanning the temporal duration of the incident pulse.

ISSN:0001-4966    

DOI:10.1121/1.397897

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP

摘要:

A model that has been developed to calculate the scatter produced by a high‐frequency acoustic pulse that originates from a stationary and arbitrarily located source; is incident on and scattered from an under‐ice surface characteristic of pack ice regions of the interior Arctic; and is detected by a stationary and arbitrarily located receiver is used to calculate a variety of acoustic data. Scattering from small‐scale surface roughness produced by ice blocks as well as from large‐scale surface roughness produced by ice keels is calculated and discussed. The effects of physical parameters on the scatter of a high‐frequency plane wave from an ice block whose physical and geometric parameters are characteristic of those found in the Arctic are calculated and discussed briefly. The ice block scattering model is modified to calculate the near field target strength of a circular ice piston as a function of incidence angle. When measured and modeled facet target strength data are compared, it is shown that, near normal incidence, measured and modeled target strength levels agree reasonably well, but that the details of their dependence on incidence angle are different and that differences between the measured and modeled target strength data increase as frequency increases. Monostatic reverberation time series, facet target strength distributions, and correlation data are calculated and compared with measured data for three Arctic sites, and it is shown that measured and modeled data agree reasonably well. For the Chukchi Sea site, a variety of monostatic and bistatic time series, distribution, and correlation data are calculated and discussed for ice keels and flat ice features as well as for the entire under‐ice surface. It is shown that the predominant large‐scale, under‐ice scatterers are ice keels; that ice keels increase reverberation levels; that the facets of ice blocks from which ice keels are composed produce high‐level echos or glints; that generally the spatial location of these high‐level echoes depends on source–receiver location, although some may persist from one location to the next; and that high‐level echos from ice facets are one of the principal means by which ice keels increase high‐frequency, under‐ice reverberation.

ISSN:0001-4966    

DOI:10.1121/1.397845

出版商:Acoustical Society of America    

年代:1989

数据来源: AIP