摘要:

A pattern recognition algorithm and instrumentation forinvivoultrasound human liver differentiation are presented. An available 16‐MHz microprocessor‐based data acquisition and analysis system with 6‐bit resolution is used to capture, digitize, and store the backscattered ultrasound signal. The algorithm is based on a multilayer perceptron neural network using the backpropagation training procedure. The network is implemented to differentiate between normal and abnormal liver. Data earlier obtained from 18 volunteers with normal liver history and from 12 volunteers with liver abnormalities are used to test the algorithm. The power spectra of the backscattered signal from depths of 5, 6.5, and 8 cm in the liver are calculated. The acoustic attenuation coefficient is calculated by the log spectral difference technique over the frequency range from 1.5 to 4.5 MHz. The change of speed of sound with frequency (dispersion) is estimated over the 3‐MHz bandwidth. The attenuation and velocity dispersion are used as differentiation features. The results show that of the 22 tested cases, the system differentiated correctly 19 and 20 cases when using the attenuation and the velocity dispersion, respectively. The average magnitude of dispersion of liver is estimated to be 1.67±0.1 m/s/MHz and about 2.3±0.18 m/s/MHz in the normal and abnormal cases, respectively. The overall performance of the system for liver differentiation is 91% for normal cases, and 86% for abnormal cases. The data files are also differentiated using the nearest neighbor statistical classifier. The results show that of the 30 tested cases, 23 files are differentiated correctly using the attenuation coefficient.

ISSN:0001-4966    

DOI:10.1121/1.410487

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The split‐step Padé solution [J. Acoust. Soc. Am.93, 1736–1742 (1993)] is generalized to simultaneously permit arbitrarily large range steps (in range‐independent regions) and dense sampling in range. Although the range step size is limited by the rate of range dependence, range steps of many wavelengths are possible for many range‐dependent ocean acoustics problems. At each range point in a dense grid, the acoustic pressure is defined over depth in terms of a linear combination of solutions of tridiagonal systems. On a multiprocessor computer, each of the tridiagonal systems is solved on a different processor, and the acoustic field can be generated throughout a large range‐independent region in the time required by standard finite‐difference solutions to take one range step. The generalized split‐step Padé solution is also applicable to elastic media.

ISSN:0001-4966    

DOI:10.1121/1.410488

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A simple spectral representation is derived for the Doppler‐shifted field in general stratified waveguides, including those with elastic bottoms and ice covers. By transforming the field representation for a moving, finite bandwidth source into the moving receiver’s frame of reference, a spectral integral representation is obtained for the dynamic transfer functions, which may then be transformed into time domain responses by standard Fourier synthesis. The spectral representation of the Doppler‐shifted field is more general than earlier modal formulations since it is not based on any assumptions concerning the invariance of mode shapes, and it properly accounts for modal cutoff effects. However, it will be demonstrated that its approximate modal expansion is consistent with earlier modal formulations. The spectral formulation has been implemented in an existing wave‐number integration code, and applied to illustrate the fundamental physics of the source/receiver dynamics in stratified waveguides. Examples include moving source/receivers in shallow water, and the acoustic emission produced by propagating ice fractures in the Arctic.

ISSN:0001-4966    

DOI:10.1121/1.410489

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Recently Porter and Jensen [J. Acoust. Soc. Am.94, 1510–1516 (1993)] reported on a seemingly benign propagation problem in which two wide‐angle split‐step parabolic equations (PEs) performed poorly compared to other PEs. That is, for a source and receiver located within a leaky surface duct, anomalously high transmission losses were predicted for ranges at which the leakage energy is refracted back into the duct. Moreover, both equations displayed greater sensitivity to the choice of reference wave number than the standard PE. In this paper, a new propagation operator is formulated that retains the computational efficiency of the split‐step algorithm but is considerably more accurate. Two rapid numerical solution algorithms valid in both two and three dimensions are then introduced and applied to the leaky surface duct. To implement the simplest second‐order method a tridiagonal finite‐difference system of equations is solved at each range step in addition to the usual split‐step Fourier computations. Higher‐order procedures instead involve applying the Lanczos algorithm solely to the higher‐order terms in the wide‐angle expansion, thereby circumventing the convergence difficulties associated with the direct Lanczos evaluation of the Helmholtz propagator [Hermanssonetal., IEEE J. Light. Technol.10, 772–776 (1992)].

ISSN:0001-4966    

DOI:10.1121/1.410490

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

On reflection from a plane interface there is a wave shift which in underwater acoustics is often called the effective depth; there is also a beam shift or beam displacement. These shifts are briefly introduced and then the relation between them is spelled out. In application to modal propagation in a range‐independent medium, it is shown that a wave shift sum is invariant with respect to depth and provides an eigenvalue equation. Beam shift sums for the cycle distance and cycle time are useful but only approximately invariant, however their ratio defines the truly invariant group velocity. Propagation in a range‐dependent medium is analyzed assuming the adiabatic relation; the wave shift sum is now invariant with respect to range as well. This process corresponds to a ray invariant with wave reflections at a fictitious surface defined by the wave shift. A description in terms of beam shifts is more complicated, but shown to be compatible. A simple upslope example is included.

ISSN:0001-4966    

DOI:10.1121/1.410491

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A formalism is presented that demonstrates that the mean Green’s function for the acoustic field between two rough interfaces can be expressed as a Green’s function associated with two flat interfaces with effective reflection coefficients. This result incorporates all orders of the fluctuations in the half‐space scattering amplitudes associated with each interface considered separately. From the mean Green’s function modal attenuations can be found. To lowest order in the surface height fluctuations it is shown that it is not sufficient to use mean half‐space scattering amplitudes as effective reflection coefficients. The formalism is designed to provide approximations for the Green’s function in layered media which are based on previously developed approximations for half‐space scattering amplitudes.

ISSN:0001-4966    

DOI:10.1121/1.410428

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Dispersion analysis of interface waves (Scholte/Stoneley/Rayleigh) has been used extensively to determine geoacoustic models of near‐bottom ocean sediments. In the traditional approach, a single seismic trace is analyzed to obtain group velocity dispersion using the multiple filter technique. A dispersion curve obtained in this manner reflects the averaged influence of the sediment acoustic properties over the entire path from source to receiver. Thus a geoacoustic model derived from this dispersion curve is not a true vertical section in the vicinity of the receiver unless the sediment properties do not change over the entire propagation path. However, when a linear array of receivers is used to obtain data simultaneously at a number of different ranges, it is possible to derive both group and phase velocity dispersion curves that correspond to propagation over a short portion (typically 5 m) of the total path at many different ranges. With this kind of data, the range‐dependent variation of geoacoustic properties in the sediment may be assessed. Results are presented based on data obtained with a 24‐receiver array and an explosive source, both deployed on the seafloor. Analysis of data from a number of shallow water sites suggests that there is a considerable amount of lateral inhomogeneity even over very short ranges.

ISSN:0001-4966    

DOI:10.1121/1.410429

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A method for estimating range‐averaged sound‐speed and sound‐slowness profiles from single‐slice tomographic travel‐time measurements is demonstrated. The method directly yields the range average of the equivalent symmetric profile and the asymmetry of the sound channel at the source and receiver. In the absence of independent information, the measurements themselves indicate whether they are consistent with a range‐independent sound channel. The inversion method is applied to a simulated pulse arrival sequence (generated by ray tracing), and the recovered sound speed agrees with that used for the simulation. Using climatology (or other independent information) for the sound speed below the sound‐channel axis would allow an estimate of the range average of the profile above the sound‐channel axis. The method yields the range average of sound slowness without linearization and gives the range average of the sound speed to first order.

ISSN:0001-4966    

DOI:10.1121/1.410430

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A recent 1000‐km acoustic pulse transmission experiment in the Pacific revealed unexpected fluctuations on received wavefronts, including a dominant rapid variation, called the broadband fluctuation, with time scales less than 10 minutes and spatial scales of less than 60 m; a distinct breakdown of the geometrical optics wavefront pattern and broadening of the wavefront near the transmission finalé; and a coherent wavefront motion with a timescale near the semi‐diurnal tidal period. Parabolic‐equation numerical simulations have been carried out which utilize environmental data and which take into account internal‐wave‐induced sound‐speed perturbations obeying the Garrett–Munk (GM) spectral model. It is shown that the effects of internal waves can account for the broadband fluctuations, the breakdown of the geometrical optics pattern, and the wavefront broadening. The sensitivity of these fluctuations to internal‐wave energy and modal content is examined. The spectral energy in the GM model at tidal periods proves insufficient to explain the tidal period coherent fluctuations, strongly suggesting the influence of an internal tide during the experiment. The simulations allow the estimation of the average travel‐time bias caused by internal waves. The simulation results for travel‐time wander and bias are compared with analytic calculations based on the path‐integral technique.

ISSN:0001-4966    

DOI:10.1121/1.411331

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

This paper comments on ‘‘Effects of propagation on the operation of a synthetic aperture sonar’’ by J. H. Tarng and C. C. Yang [J. Acoust. Soc. Am.82, 1403–1408 (1987)]. Shortcomings in those analyses are identified and corrections and a discussion are provided. This is accomplished through the application of standard synthetic aperture analysis methods common to both radar and sonar, combined with a ray tracing algorithm that models the ocean having an arbitrary sound‐speed profile.

ISSN:0001-4966    

DOI:10.1121/1.410431

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP