Peder C. Pedersen

Impedance‐matching properties of an inhomogeneous matching layer with continuously changing acoustic impedance

This paper analyzes the impedance‐matching properties of an inhomogeneous layer whose specific acoustic impedance varies smoothly across the layer, from the effective value of the transducer specific acoustic impedance to the value of the output medium specific acoustic impedance. In this paper we first derive the energy reflection and transmission coefficients for an inhomogeneous layer with no attenuation and constant propagation speed, and the results are presented for selected profiles. Next, the performance of an inhomogeneous matching layer with an exponential impedance profile is compared with a quarter‐wavelength matching plate. Finally, the general case in which both sound speed and density of the inhomogeneous matching layer may vary is analyzed; the specific acoustic impedance profile as a function of travel time is found to be a principal factor, determining the transmission and reflection properties. The spatial functions of the acoustic parameters are determined for the case where both density and sound speed vary exponentially with travel time.

Evaluation of angle-dependent spectral distortion for infinite, planar elastic media via angular spectrum decomposition

The angle‐dependent spectral distortion for an infinite planar interface (IP‐ASD) is examined for the case of fluid–fluid and fluid–solid (elastic) interfaces. The results are based on computation of the angular spectrum (plane wave) decomposition of the acoustic pressure field as radiated by an equivalent image source transducer, for each frequency of interest. The exact incident‐angle‐dependent reflection coefficient of the interface may thus be used in the computation, versus approximate methods previously required [D. P. Orofino and P. C. Pedersen, J. Acoust. Soc. Am. 92, 2883–2899 (1992)]. Comparisons of the previous approximate IP‐ASD results to the current exact results are made. IP‐ASD results for elastic parallel plates are also presented for a variety of plate orientations and geometries. For the special case of normal incidence, a comparison of the IP‐ASD result to a previously published result is made. The IP‐ASD is additionally examined for a reduced subset of plane wave components that appro...

Multirate Digital Signal-Processing Algorithm to Calculate Complex Acoustic Pressure Fields

An efficient algorithm to compute complex (magnitude and phase) acoustic pressure field data that uses a multirate digital processing architecture is presented. The algorithm is based on the discretization of the velocity potential function, sampled at a rate that varies as a function of field point location and transducer geometry. The algorithm can be used to determine accurate magnitude and phase information at any field point location, and for any transducer geometry with a closed‐form velocity potential function, including planar pistons, spherically focused pistons, and planar annular array transducers (e.g., the nondiffracting or J0‐Bessel transducer). Numerical simulations based on this algorithm are presented together with exact field calculations wherever possible in order to make absolute error comparisons. Additionally, results based on a standard Gaussian quadrature integration scheme are presented in order to compare computational speed and accuracy in the near field. Results indicate improv...

Inverse acoustic scattering for one‐dimensional lossy media

An algorithm for the reconstruction of the physical parameters of a layer of an attenuating medium is presented. The parameters are to vary only along the thickness of the layer. The input data for the algorithm consist of the reflection coefficients of two incident time‐harmonic plane waves intersecting the layer at two different angles. Using scattered data, the density variation of the medium is determined, as is the spatial variation of its attenuation coefficient. The frequency variation of the attenuation is assumed to be known. The reconstruction algorithm is based on the impediography formula modified to account for non‐normal incident waves and for the attenuation of the medium. Several numerical examples of the algorithm are presented using computer‐generated input data for media with specified density and attenuation variations.

Wireless Image Streaming in Mobile Ultrasound

This work evaluates the feasibility of using 802.11 g ad hoc and 3G cellular broadband networks to wirelessly stream ultrasound video in real-time. Telemedicine ultrasound applications in events such as disaster relief and first-response triage can incorporate these technologies, enabling onsite medical personnel to receive assistance with diagnostic decisions by remote medical experts. The H.264 scalable video codec was used to encode echocardiographic video streams at various image resolutions (video graphics array [VGA] and quarter video graphics array [QVGA]) and frame rates (10, 15, 20, and 30 frames/s). The video stream was transmitted using 802.11 g and 3G cellular technologies, and pertinent transmission parameters such as data rate, packet loss, delay jitter, and latency were measured. 802.11 g permits high frame rate and VGA resolution and has low latency and jitter, but it is suitable only for short communication ranges, whereas the 3G cellular network allows medium to low frame rate streaming at QVGA image resolution with medium latency. However, video streaming can take place from any location with 3G service to any other site with Internet connectivity. The transmitted ultrasound video streams were subsequently recorded and evaluated by physicians with expertise in medical ultrasonography who evaluated the diagnostic value of the received video streams relative to the original videos. They expressed the opinion that image quality in the case of both 802.11 g and 3G was fully to adequately preserved, but missed frames could momentarily decrease the diagnostic value. This research demonstrates that 3G and 802.11 g wireless networks combined with efficient video compression make diagnostically valuable wireless streaming of ultrasound video feasible.

Numerical Techniques for the Inverse Acoustical Scattering Problem in Layered Media

Techniques for reconstruction of medium properties, mainly acoustic impedance or permittivity, based on the backscattered energy from an interrogating wave, have applicability in a wide range of scientific fields, such as in seismology, underwater acoustics, medical ultrasound, remote sensing and optics. Consequently, the classical inversion problem has in recent years been the subject of many investigations, especially in electromagnetics Cl-3H and geophysics C4,5D. Theoretical treatment of the inverse acoustical scattering problem has been presented in papers and books [6-8].

Direct acoustic scattering for one‐dimensional lossy media

The scattering of acoustic waves by inhomogeneities within a one‐dimensional lossy medium is investigated. The inhomogeneities may be spatial variations in the attenuation coefficient of the lossy medium and/or variations in its density and wave velocity. The frequency dependence of the attenuation coefficient is modeled after loss mechanisms governed by viscosity, heat conduction, and single and multiple relaxation processes. Two methods previously developed for lossless media—the transmission matrix method and the forward scattering or impediography method—are extended to treat the determination of the reflection and transmission coefficients of an inhomogeneous lossy medium. These scattering coefficients are numerically determined for several one‐dimensional media with varying density and attenuation profiles.

The Inverse Acoustical Scattering Problem for Layered Media in the Presence of Broad-Band Acoustic Noise and with Limited Transducer Bandwidth

The inverse problem in acoustics (or acoustical imaging) is based on the theory of acoustic wave propagation, absorption, and scattering. The techniques for processing and display of the medium parameters, based on scattered acoustic energy may be divided into 2 general categories: (1) techniques based upon transmitted, or forward scattered ultrasound energy; (2) techniques based upon reflected, or backscattered, ultrasound energy. The reconstruction techniques, discussed in this paper, are based on backscattered acoustic energy.

Analytical and Experimental Comparisons Between the Frequency-Modulated-Frequency-Shift Measurement and the Pulsed-Wave-Time-Shift Measurement Doppler Systems

In previous publications, a new echo‐ranging Doppler system based on transmission of repetitive coherent frequency‐modulated (FM) sinusoids in two different implementations was presented. One of these implementations, the frequency‐modulated–frequency‐shift measurement (FM–fsm) Doppler system is, in this paper, compared with its pulsed‐wave counterpart, the pulsed‐wave–time‐shift measurement (PW–tsm) Doppler system. When using transmitted PW and FM signals with a Gaussian envelope, the parallelism between the two systems can be stated explicitly and comparison can be made between the main performance indices for the two Doppler systems. The performance of the FM and PW Doppler systems is evaluated by means of numerical simulation and measurements of actual flow profiles. The results indicate that the two Doppler systems have very similar levels of performance.

One-Dimensional Inverse Acoustic Scattering for Lossy Media

In our previous work, we developed numerical techniques to reconstruct the acoustic impedance, density, and sound velocity profiles of a one-dimensional lossless inhomogeneous medium and investigated the effect of noise, limited transducer bandwidth, and deconvolution on the algorithms [1–2]. In addition, we studied the direct problem in lossy media and extended the previously developed transmission matrix method and impediography method [3] to lossy media [4].