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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 89 out of 330
| Title |
Diffraction correction methods for pulse-echo acoustic attenuation estimation. |
| Author |
Céspedes I, Ophir J. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1990 |
| Abstract |
We describe computer simulations and water tank experiments for the estimation of the attenuation coefficient in scattering media. The efficacies of the Axial Beam Translation (ABT) and Inverse Diffraction Filtering (IDF) methods in reducing diffraction errors in such estimates are compared throughout the radiation field of a plane transducer. The effect of phase aberration due to the body wall and the effect of the scattering properties of the media are considered. Consistent improvement in the estimation due to ABT is demonstrated as well as unreliable improvement due to IDF which is sensitive to the presence of phase aberration and to changes in scattering.
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| Title |
Diffraction effects and their role in the precise measurements of velocity and attenuation of ultrasound. |
| Author |
Krasnushkin PE. |
| Journal |
Acustica |
| Volume |
|
| Year |
1968 |
| Abstract |
A detailed investigation is given of the diffraction effects in ultrasonic devices with plane radiators used for measurements of propagation constants in gases and liquids. |
| Title |
Diffraction effects in reflected ultrasound spectral estimates. |
| Author |
Kuc R, Regula DP Jr. |
| Journal |
IEEE Trans Biomed Eng |
| Volume |
|
| Year |
1984 |
| Abstract |
This paper explores the effect of diffraction on estimates of the spectra of signals reflected from interfaces located at a given range from the transducer. When an ultrasound pulse is transmitted into a medium, the power spectra of reflected signals vary as a function of range from the transducer. These spectral changes are caused by diffraction effects, due to the finite-sized aperture, and by the properties of the medium. When the properties of the medium are to be determined from the backscattered signals, the diffraction effects tend to mask these properties, especially in the near field. Estimates of the diffraction effects are determined by accurately estimating the reflected signal spectra at a set of ranges from the transducer. The observed signals are reflections from media that are composed of randomly distributed scatterers and have a linear-with-frequency attenuation. The two random media considered are the in vivo liver and a tissue-equivalent phantom. The results indicate that the diffraction effects produce a high-frequency enhancement in the reflected spectra which diminishes with range. Similar results are observed for the reflections from plane and randomly roughened surfaces located at the same ranges. Simulated results using a transducer model verify the plane surface results. The effect of diffraction is examined in the clinical problem of estimating the slope of the acoustic attenuation coefficient of the in vivo liver. Diffraction introduces a bias into the observed slope estimate. The bias is a function of the range interval over which the reflected data are acquired. |
| Title |
Diffraction of light by ultrasonic waves of various standing wave ratios. |
| Author |
Cook BD, Hiedemann EA. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1961 |
| Abstract |
The theory for the diffraction of light by plane ultrasonic waves of various standing wave ratios is derived. The liquid medium disturbed by the ultrasound is considered to act as an optical phase grating. By evaluating the diffraction integral for the light amplitude, expressions for the Doppler shift, the time dependent, and the time average light intensities are found for the diffraction spectrum. Experimental measurements using two adjacent ultrasonic waves progressing in opposite directions to simulate the desired optical phase grating indicate that the theory is valid.. |
| Title |
Diffraction of light by ultrasound in anharmonic crystals. |
| Author |
Melngailis J, Maradudin AA, Seeger A. |
| Journal |
Phys Rev |
| Volume |
|
| Year |
1963 |
| Abstract |
Due to anharmonic effects an initially sinusoidal ultrasonic wave of finite amplitude gradually distorts as it passes through a crystal. In the present paper we calculate the asymmetry in the diffraction pattern formed by passing monochromatic light through a cubic crystal perpendicular to the direction of propagation of such a distorted longitudinal ultrasonic wave. The solution of the nonlinear wave equation for the crystal, obtained by iteration to first order in the nonlinear terms, is used in the diffraction integral to obtain an expression for the light amplitude in the diffraction pattern. The first-order intensities are computed for NaCl. In a typical case, a wave of strain amplitude 2X10(-5) produces a 10% difference in intensity between the first positive and the first negative orders. These results suggest a method for obtaining the third-order elastic constants of transparent crystals. |
| Title |
Diffraction of ultrasound by soft tissues: The inhomogenous continuous model. |
| Author |
Sehgal CM, Greenleaf JF. |
| Journal |
Proc Thirteenth Int Symp Acoust Imaging - Minneapolis |
| Volume |
|
| Year |
1983 |
| Abstract |
The theory of wave propagation in continuous-inhomogeneous media is utilized to explain scattering of ultrasound by soft tissues. When a number of scattering volume elements are present, as opposed to one, the scattered energy from a given volume element subsequently encounters other scattering volume elements. As a result of this, the sound energy is diffused instead of being propagated as well-defined waves. This means that the phase relationships are lost and the radiation becomes incoherent. If one were to freeze the ensemble to one instant and observe all the volume elements, each of them would appear as a secondary source emitting radiation in a different phase. Using this concept and assuming that soft tissues do not possess any sharp discontinuities, an equation for attenuation due to scattering is obtained which predicts a near linear frequency dependence for scattering-attenuation The velocity fluctuations and correlation length for a variety of tissues are estimated from the values of scattering -attenuation using the postulated model. On the basis of these values, ray, Born, and Rytov approximations used to describe sound propagation through soft tissues, are evaluated. |
| Title |
Diffraction of ultrasound in elastically anisotropic NaCl and in some other materials. |
| Author |
Papadakis EP. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1963 |
| Abstract |
Ultrasonic diffraction in NaCl and KCl single crystals, as well as in steel, has been studied by the pulse-echo method from 8-45 Mc/sec with half inch disk transducers. The diffraction is of the Fresnel type, so that the curves of loss due to diffraction can be plotted with a normalized abscissa S=z/(a*a/lambda), where z is the path length in the specimen, a the transducer radius, and lambda the ultrasonic wavelength. In general, the loss curve is not monotonic in S. The shape of the curve is dependent upon the specimen material, the crystallographic direction of ultrasonic propagation, and the transducer plating configuration. The position of the last local maximum in the loss curve is a function of the anisotropy of the specimen. In particular, its position is steel (isotropic case) case agrees with the theoretical position for isotropic materials. |
| Title |
Diffraction of ultrasound radiating into an elastically anisotropic medium. |
| Author |
Papadakis EP. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1964 |
| Abstract |
The diffraction loss-distance characteristics in crystals have been computed for the single-slit transducer geometry in ultrasonic pulse-echo measurements. The characteristics are functions of elastic anisotropy and show several local maxima and minima before becoming monotonic at large distances from the transducer. By applying scale factors to the ordinates and abscissas of the characteristics, they were made applicable to the case of a circular transducer. The scale factors were chosen to bring the computed single-slit characteristic for isotropic materials into coincidence with the well-known isotropic-diffraction characteristic for circular-piston sources. The scaled crystal characteristics served to predict the positions of the local loss maxima to 15% and their amplitudes to 25% or better in four experiments. Certain crystals produce less diffraction loss per unit length than others because of the dependence of diffraction on anisotropy. The scaled diffraction loss-distance characteristics can be used for computing diffraction corrections for ultrasonic attenuation. |
| Title |
Diffuse targets for improved contrast in beamforming adapted to target. |
| Author |
Ellis MA, Viola F, Walker WF. |
| Journal |
Proc Ultrason Symp IEEE |
| Volume |
|
| Year |
2007 |
| Abstract |
Adaptive beamforming (ABF) algorithms reduce the contribution of undesired off-axis signals, while maintaining a desired response along a specific look direction. Previously, we have described a novel ABF that can be applied to medical ultrasound, entitled time-domain optimized near-field estimator (TONE) and demonstrated its ability to achieve significant gains in spatial resolution when compared to conventional, delay-and-sum beamforming (CBF). However, several characteristics of TONE hinder its adoption into medical ultrasound, including the discretized appearance of its images as well as clouds surrounding bright targets, which reduce the overall image contrast. Here, we have altered the system model of TONE from a grid of discrete hypothetical targets to a grid of diffuse hypothetical targets in order to suppress these clouds and the discretized appearance of the image. This modification imposes no additional computational cost at the time of beamforming. We performed a series of simulations and experiments, using both point targets and anechoic regions within speckle backgrounds, to test the performance of diffuse-TONE (dTONE) and compare it to both TONE and CBF. In all instances, dTONE showed a reduced appearance of these clouds and improved image contrast. We also analyzed the performance of dTONE in the presence of varying amounts of electronic noise, finding dTONE to be more robust than TONE to decreases in signal-to-noise ratio (SNR). |
| Title |
Digital acquisition and interactive processing of ultrasonic echoes. |
| Author |
Robinson DE, Williams BG, Horn PR. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1976 |
| Abstract |
No abstract available. |
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