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BRL Abstracts Database |
Your search for ultrasound produced 3296 results. Page 8 out of 330
Title |
A flexible blood flow phantom capable of independently producing constant and pulsatile flow with a predictable spatial flow profile for ultrasound flow measurement validations. |
Author |
Hein IA, O'Brien WD Jr. |
Journal |
IEEE Trans Biomed Eng |
Volume |
|
Year |
1992 |
Abstract |
The validation of the ultrasound time-domain correlation method of measuring blood flow has required the development of a flexible blood flow phantom capable of generating predictable flow profiles under a wide variety of conditions. The purpose of the phantom is to generate flow with well-known flow properties and not to mimic actual in vivo vessels. This paper describes a flow phantom which can independently generate both constant and pulsatile flow over a wide range of flow rates with a spatially fully developed laminar flow profile. It incorporates a computer-controlled pulsatile pump, which can produce different temporal pulsatile waveforms. The flow phantom also supports multiple vessels, different vessel sizes, as well as different attenuating media. The fluid most commonly used in the phantom is Sephadex mixed in water, and the probability density function of ultrasound reflected from Sephadex is experimentally determined and compared with that of blood. Examples of different constant and pulsatile flow experiments using the phantom are presented. |
Title |
A focused ultrasound method for simultaneous diagnostic and therapeutic applications - a simulation study. |
Author |
Konofagou EE, Thierman J, Hynynen. |
Journal |
Phys Med Biol |
Volume |
|
Year |
2001 |
Abstract |
Similar to other therapeutic methods, ultrasound surgery requires an imaging modality to monitor the extent of tissue damage during treatment. In this paper, we have considered the method of ultrasound-stimulated acoustic emission (USAE) that uses two ultrasonic beams at high frequency (1.7 MHz) (same as that used for ablation) to locally excite the tissue by generating a low-frequency (1–50 kHz) radiation force. Recording of the tissue response at several locations yields an image. The amplitude of the tissue response depends on the mechanical and acoustic tissue properties, namely its stiffness and absorption. These two properties were initially hypothesized to have counteractive effects on the response amplitude, i.e., the amplitude should increase with absorption and decrease with stiffness. To check this hypothesis as well as the degree to which these properties influence the response, finite-element simulations of a uniform lesion formed inside a homogeneous medium were used. The results show that, as expected, the displacement amplitude decreased with increasing lesion stiffness at lower frequencies (except at resonance) while, contrary to our initial hypothesis, it increased with stiffness at relatively higher frequencies (>22 kHz). At resonance, a frequency upshift occurred with increasing stiffness but was found to be highly spatially variant and system dependent, i.e., not yielding a uniform lesion response when imaged. On the other hand, the absorption increase led to a uniform linear increase of the mechanical response amplitude of the lesion. Therefore, at higher frequencies, increase of the two parameters had a synergistic effect on the tissue response to the applied radiation force. This study showed that relatively higher frequencies constitute the optimal range in the use of USAE for coagulation monitoring. A preliminary experimental verification in vitro is also provided. |
Title |
A focusing radiator for the generation of superhigh intensity ultrasound at 1 Mc. |
Author |
Rozenberg LD, Sirotyuk MG. |
Journal |
Sov Phys Acoust |
Volume |
|
Year |
1963 [1963] |
Abstract |
A simplified design for an ultrasonic focusing system using lead zirconate titanate ceramic (LZT) is described. At a frequency of one megacycle the device is capable of producing an average intensity of 43-10(3)W/cm(^2) in a focal spot 1.6 mm in diameter; this corresponds to a pressure of 725 atm at the focal point. |
Title |
A Fourier approach to diffraction of pulsed ultrasonic waves in lossless media. |
Author |
Guyomar D, Powers J. |
Journal |
J Acoust Soc Am |
Volume |
|
Year |
1987 |
Abstract |
A method based on a Fourier domain approach is presented for computing the diffraction of a pulsed ultrasound wave from a rigidly baffled source in lossless media. The propagation from a planar source is dependent on the total impulse response which is just the Green's function. Computing the spatial transform of the point spread function gives the propagation transfer function which multiplies the spatial spectrum of the spatial excitation to produce the spatial spectrum of the propagated wave. The propagation transfer function can then be considered to be a time-varying spatial filter. The results are valid for separable arbitrary time excitation and planar spatial distributions of the source. The solution is amenable to including the effects of a finite receiver. Results of different simulations using this method are included. |
Title |
A frequency domain model for generating b-mode images with array transducers. |
Author |
Li Y, Zagzebski JA. |
Journal |
IEEE Trans UFFC |
Volume |
|
Year |
1999 |
Abstract |
A frequency domain B-mode imaging model applicable to linear and phased array transducers was developed for simulating ultrasound images of random media. Computations are based on an approximation that is less restrictive than the Fresnel approximation. The model is compared with the exact time domain impulse response method, regarded as the 'gold standard'. In a typical application, errors in simulated rf waveforms are less than 1% regardless of the steering angle for distances greater than 2 cm, yet computation times are on the order of 1/150 of those using the exact method. This model takes into account the effects of frequency-dependent attenuation, backscattering, and dispersion. Modern beam-forming techniques such as apodization, dynamic aperture, elevational focusing, multiple transmit focusing, and dynamic receiving focusing also can be simulated. |
Title |
A fundamental criticism of hydrophone-in-water exposure measurement. |
Author |
Duck FA, Bacon DR. |
Journal |
Ultrasound Med Biol |
Volume |
|
Year |
1988 |
Abstract |
No abstract available, letter to the editor. |
Title |
A general model for the absorption of ultrasound by biological tissues and experimental verification. |
Author |
Jongen HA, Thijssen JM, van den Aarssen M, Verhoef WA. |
Journal |
J Acoust Soc Am |
Volume |
|
Year |
1986 |
Abstract |
In this paper, a closed-form expression is derived for the absorption of ultrasound by biological tissues. In this expression, the viscothermal and viscoelastic theories of relaxation processes are combined. Three relaxation time distribution functions are introduced, and it is assumed that each of these distributions can be described by an identical and simple hyperbolic function. Several simplifying assumptions had to be made to enable the experimental verification of the derived closed-form expression of the absorption coefficient. The simplified expression leaves two degrees of freedom and it was fitted to the experimental data obtained from homogenized beef liver. The model produced a considerably better fit to the data than other, more pragmatic models for the absorption coefficient as a function of frequency that could be found in the literature. Scattering in beef liver was estimated indirectly from the difference between attenuation in in vitro liver tissue as compared to absorption in a homogenate. The frequency dependence of the scattering coefficient could be described by a power law with a power of the order of 2. A comparable figure was found in direct backscattering measurements, performed at our laboratory with the same liver samples [Van den Aarssen et al., J. Acoust. Soc. Am. (to be published)]. A model for scattering recently proposed by Sehgal and Greenleaf [Ultrason. Imag. 6, 60-80 (1984)] was fitted to the scattering data as well. This latter model enabled the estimation of a maximum scatterer distance, which appeared to be of the order of 25 micron. |
Title |
A generalized westervelt equation for nonlinear medical ultrasound. |
Author |
Taraldsen G. |
Journal |
J Acoust Soc Am |
Volume |
|
Year |
2001 |
Abstract |
A model equation is derived for nonlinear medical ultrasound. Unlike the existing models, which use spatial coordinates, material coordinates are used and hence a model for a heterogeneous medium is able to be derived. The equation is a generalization of the Westervelt equation, and includes the nonlinearity, relaxation, and heterogeneity of soft tissue. The validity of the generalized Westervelt equation as a model for a Piola-Kirchoff for the acoustic pressure is discussed. In the second case it turns out that the model follows from two geometric approximations which are valid when the radius of curvature of the phase fronts is much larger than the particle displacements. The model is exact for plane waves and includes arbitrary nonlinearity in the stress-strain relation. |
Title |
A geometric approach to the filtered backpropagation algorithm. |
Author |
Oristaglio ML. |
Journal |
Ultrason Imaging |
Volume |
|
Year |
1983 |
Abstract |
We show how the filtered backpropagation algorithm [1] for parallel-beam diffraction tomography can be implemented geometrically by weighted straight-line projections. The resulting algorithm is formally equivalent to the original approach using Fourier transforms [2], but reorganizes the calculations to separately backpropagate each plane-wave component of the filtered data. In principle, at least, the projection method also suggests the design of an ultrasound scanner that records the wavefield with line detectors. |
Title |
A graphical description of scattering. |
Author |
Greenleaf JF. |
Journal |
Ultrasound Med Biol |
Volume |
|
Year |
1986 |
Abstract |
The relationship between Fourier transforms of the refractive index of objects and the Fourier transform of the scattered ultrasonic waves are reviewed. Both the Born and Rytov approximations in the first order are used to linearize the Helmholtz wave equation. Both forward and backward scatter geometries are illustrated. The relationship between these coherent wave considerations and the clinically used echo and transmission modalities is discussed. |
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