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BRL Abstracts Database |
Your search for ultrasound produced 3296 results. Page 24 out of 330
Title |
Absorption and attenuation in soft tissues: I - Calibration and error analyses. |
Author |
Parker KJ, Lyons ME. |
Journal |
IEEE Trans UFFC |
Volume |
|
Year |
1988 |
Abstract |
Error estimations are developed for pulse decay absorption and radiation force insertion loss attenuation measurements. In absorption measurements, significant difficulty lies with accurate determination of peak intensity, especially where sharply focused beams are utilized. An intensity calibration is developed based on radiation force measurement of total power and main lobe beam patterns, using embedded thermocouples and short bursts of ultrasound. The main lobe measurements are highly reproducible, but sidelobes (which heavily influence estimates of total power) are easily corrupted by noise. Thus a theoretical extension of the main lobe beam pattern to include sidelobes is utilized to estimate peak focal intensity. The theory is based on circular baffled piston sources with apodizing lenses, an ideal condition that was closely approximated in a specially constructed experimental apparatus. The approach enables estimates of peak intensity in situ with typical uncertainty of less than five percent and resulting absorption coefficient uncertainty of 10 percent. Similar analyses of attenuation measurement uncertainties show that errors of three percent or less are possible where repeated measurements of radiation force insertion loss are made on relatively homogeneous materials. Comparisons of accurate measurements of both attenuation and absorption coefficients of tissues enable a clearer understanding of the dominant mechanisms of ultrasound-tissue interaction. |
Title |
Absorption and dispersion of ultrasound in biological meida. |
Author |
Dunn F, Edmonds PD. Fry WJ. |
Journal |
Book Chapter |
Volume |
|
Year |
1968 |
Abstract |
No Abstract Available. |
Title |
Absorption and scatter of encapsulated gas filled microspheres: Theoretical considerations and some measurements. |
Author |
de Jong N, Hoff L, Skotland T, Bom N. |
Journal |
Ultrasonics |
Volume |
|
Year |
1992 |
Abstract |
Albunex is an ultrasound contrast agent for use in echocardiology and other areas. It is capable of passing the lung circulation after intravenous injection. A theoretical model is developed for some acoustic properties, particularly the scatter and absorption, of this contrast agent, considering the individual microspheres as air bubbles surrounded by a thin shell. The attenuation, the sum of absorption and scatter, of this contrast medium is measured with five transducers to cover the frequency range from 700 kHz to 8.5 MHz. It is concluded that the model correlates well with these acoustic measurements. When Albunex is used intravenously the backscatter enhancement in the left ventricle is caused mainly by the microspheres with diameters between 5 and 8 microns. |
Title |
Absorption coefficient of ultrasound in soft tissues and their biological conditions. |
Author |
Yosioka K, Oka M, Omura A, Hasegawa T. |
Journal |
Memoirs Inst Sci Ind Res Osaka Univ |
Volume |
|
Year |
1969 |
Abstract |
The measuring method and the results lately obtained in our laboratory are described concerning the ultrasonic absorption of fresh tissues, living brains, and blood as affected by physical and biological conditions. |
Title |
Absorption in liver at the focus of an ultrasonic shock wave field. |
Author |
Fry FJ, REilly CR, Dines KA, Etchison MR, Trauner EJ. |
Journal |
Ultrasound Med Biol |
Volume |
|
Year |
1991 |
Abstract |
This experimental study is an extension of a previous investigation of finite amplitude ultrasound absorption at 1 MHz fundamental frequency in freshly excised rat liver at 37°C (Fry et al. 1989). The work reported here includes measurements of the absorption as a function of intensity under a variety of conditions (temperature, pressure, and tissue state). The maximum intensity employed in the investigation (700 W/cm−2) corresponds to a shock parameter of approximately 1.7 based on previous characterization of the acoustic field which established that shock (σ = 1) occurred in the range 225 W/cm−2 to 275 W/cm−2 (Fry et al. 1989). Of the three temperatures chosen for the thermal study (30°C, 37°C, and 41°C), the only statistically significant differences (p < 0.05) in absorption were between the 30°C and 41°C data in the intensity range just above σ = 1 (300–500 W/cm−2). The intensity-dependent absorption coefficient was also determined for excised liver under hyperbaric conditions and for liver in situ at 37°C. At a pressure of 350 psi, the absorption was generally less than at atmospheric pressure. In situ liver at 37°C had a lower absorption above 200 W/cm−2 than freshly excised liver, but the difference was only significant between 200 and 300 W/cm−2 and between 600 and 700 W/cm−2. |
Title |
Absorption of finite amplitude focused.ultrasound. |
Author |
Dalecki D, Carstensen EL, Parker KJ, Bacon DR. |
Journal |
J Acoust Soc Am |
Volume |
|
Year |
1991 |
Abstract |
Predictions of the absorption of focused finite amplitude waves based on weak.shock theory have been tested experimentally. The characteristics of this.absorption are qualitatively different from those associated with small signal.losses. Under appropriate conditions, the absorption of finite amplitude.ultrasound is determined largely by source amplitude, field geometry, and the.nonlinear properties of the medium and is only weakly dependent upon the small.signal absorption coefficient of the material. These effects are seen most.dramatically in sharply focused sound fields. To emphasize nonlinear absorption.in an experimental test of these predictions, measurements of heating were made.in agar which has a very small linear absorption coefficient. Under appropriate.conditions, nonlinear losses can make the effective absorption coefficient of this.poorly absorbing material somewhat greater than the soft tissues of the body..Published erratum appears in J Acoust Soc Am 1991 Nov;90(5):2855. |
Title |
Absorption of finite amplitude ultrasound in tissues. |
Author |
Carstensen EL, McKay ND, Delecki D, Muir TG. |
Journal |
Acustica |
Volume |
|
Year |
1982 |
Abstract |
Absorption for the case of finite amplitude ultrasound coupled through a water path to a tissue sample is investigated. As a finite amplitude wave travels through the water path, it becomes rich in harmonics and is attenuated even though the low intensity (linear) absorption coefficient of the medium is very small. The absorption of this wave in the tissue is modeled as the sum of two mechanisms: First, the effective absorption in the tissue is increased because of the presence of harmonics in the incident wave, and second, the same nonlinear phenomena, which give rise to distortion and attenuation in the water path, continue in the tissue as long as the amplitude of the wave is great. The predictions of this model agree well with observations of the absorption of finite amplitude ultrasound in liver tissue. At 4.4 MHz and a source-sample separation of 11.5 cm,. the absorption parameter at 20 W/cm(^2) is approximately twice the linear (low intensity) absorption coefficient. |
Title |
Absorption of longitudinal and shear waves and generation of heat in soft tissues. |
Author |
Filipczynski L. |
Journal |
Ultrasound Med Biol |
Volume |
|
Year |
1986 |
Abstract |
The author analysed the oblique incidence of a longitudinal plane wave and the generation of a shear wave at a boundary between a soft tissue and a gas, considering a viscoelastic model (Voigt solid) of the medium. From the data measured by Frizzell et al. (J. Acoust. Soc. Am. 60, 1409-1411) for shear waves, the average velocity 30 m/s and the average absorption coefficient 7.7 X 10(3) cm-1 at the frequency of 5 MHz were assumed for calculations. The amplitude and the intensity of the generated shear wave were obtained and hence the rate of heat production per unit volume was determined. At the boundary, this quantity was found to be of the same order of magnitude for the generated shear wave as for the incident longitudinal wave. In the case of the shear wave, which propagates almost perpendicularly to the boundary, it decreases rapidly with the distance. Therefore, the temperature increase caused by shear waves was negligible in respect to longitudinal waves in spite of the extremely high absorption coefficient. This conclusion could be confirmed by solving the inhomogeneous equation of heat conductivity for the case under consideration. |
Title |
Absorption of longitudinal ultrasonic waves in aqueous polymer solutions. |
Author |
Borodin VN, Grigor'ev SB, Kuleshov AA, Mikhailov IG. |
Journal |
Sov Phys Acoust |
Volume |
|
Year |
1974 [1974] |
Abstract |
A brief survey of the experimental data and theoretical notions concerning the absorption of ultrasound in polymer solutions is presented, on the basis of which it is concluded that primarily two mechanisms are responsible for the absorption of ultrasound in polymer solutions; viscous friction between the polymer chains and solvent, and small-scale motions of the individual segments and side groups. The coefficient of absorption of longitudinal ultrasonic waves is measured at frequencies from 0.2 to 1000 MHz in aqueous solutions of polyvinyl alcohol (PVA) and gelatin. It is shown that the frequency dependence of the excess absorption represents relaxation curves with a broad spectrum of relaxation times, consistent with existing theoretical notions of sound absorption in polymer solutions. A number of characteristics are estimated for the solutions investigated. |
Title |
Absorption of sound in tissues. |
Author |
Carstensen EL. |
Journal |
Proc Second Int Symp Ultrason Tissue Character |
Volume |
|
Year |
1977 |
Abstract |
In spite of extensive applications of ultrasound in diagnosis, therapy and even surgery, there are still many problems to be solved in the basic physics of sound propagation in tissues. Absorption of longitudinal ultrasonic waves occurs primarily at the macromolecular level. There is evidence to indicate that this absorption can be profoundly modified by macromolecular interaction. The specific structural or chemical relaxation mechanisms responsible for the absorption are unknown. Microscopic inhomogeneities may lead to certain forms of relative motion viscous losses or thermal absorption. Macroscopic inhomogeneities in tissue affect sound propagation and can lead to artifacts in certain methods of measurement of tissue absorption. Shear waves are not important in the soft tissues of the body. |
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