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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 187 out of 330
| Title |
Nonlinear effects in ultrasound propogation. |
| Author |
Baker AC ( Duck FA , Baker AC , Starritt HC eds.). |
| Journal |
Book Chapter |
| Volume |
|
| Year |
1998 |
| Abstract |
In a fluid,ultrasound propagates as longitudinal waves of alternate compressions and rarefactions. To a first approximation the wave travels at a constant speed (c) and so its shape remains unchanged as it propagates. This level of approximation corresponds to the simplest possible form of wave equation and is widely applicable to many acoustic systems (e.g. normal sound levels in air and most sonar systems in water). The method of linear systems theory are appropriate to the solutions of problems in these fields and great use is made of methods such as superposition and linear scaling of solutions. The introduction of a frequency-dependent absorption causes no great difficulties either since the system is linear. The linear wave equation depends on two main assumptions: firstly that the particle velocity (u) of the wave is infinitesimal (or at least small compared to c) and secondly that the pressure-density relationship of the fluid is linear.
If the acoustic amplitude is sufficiently high then assumptions of linearity are no longer valid and will introduce significant errors. The resulting wave has compressional phases that travel at a speed (c+βµ0), which is faster than the speed of rarefactions (c-βµ0); β is a parameter characterizing the non linearity of the medium (the non linearity parameter is often expressed as B/A=2(β-1):measurement methods and typical values are given in Chapter 4). Note that the finite particle velocity and the non linearity of medium both produce the same effect. Thus we get a distortion that will cause a waveform that is initially sinusoidal to become more like a sawtooth(figure 2.1).The amount of distortion will increase with distance propagated and shock-like waveforms are commonly encountered,with an abrupt increase from peak negative pressure to peak positive pressure as the wave passes any point. In terms of frequency content, the waveform distortion is equivalent to harmonic generation at integer multiples of the original frequency.Thus,energy is pumped to higher frequencies where the absorption losses will be higher causing , among other things , increased intensity loss which can lead to enhanced heating and streaming effects. In a real beam of ultrasound there will also be diffraction effects which interact with non linearity and absorption to further complicate matters. A general history of non linearity in fluids can be found in the book by Beyer (1984). |
| Title |
Nonlinear pressure fields due to focused circular apertures. |
| Author |
Baker, AC. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1992 |
| Abstract |
The use of high amplitude focused ultrasound fields is widespread in medical diagnosis and therapy but there has been relatively little work published that compares experimental measurements with appropriate theory for such symptoms. In this paper, comparisons are make between the measured continuous-wave pressure field of a focused circular aperture operating at 2.25 MHz and a numerical solution of the nonlinear parabolic wave equation. The measurements were made in water using a 38-mm diam plane circular transducer as the acoustic source with perspex lenses providing focusing, focal lengths of 440, 216, and 142mm were examined. Results are presented for the amplitudes of the fundamental, second, and third harmonics along and across the acoustic axis of the source. In general, the agreement between experiment and theory is good. |
| Title |
Nonlinear propagation applied to the improvement of resolution in diagnostic medical ultrasound. |
| Author |
Ward B, Baker AC, Humphrey VF. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1997 |
| Abstract |
Medical B-mode scanners operating under conditions typically encountered during clinical work produce ultrasonic wave fields that undergo nonlinear distortion. In general, the resulting harmonic beams are narrower and have lower sidelobe levels than the fundamental beam, making them ideal for imaging purposes. This work demonstrates the feasibility of nonlinear harmonic imaging in medical scanners using a simple broadband imaging arrangement in water. The ultrasonic system comprises a 2.25-MHz circular transducer with a diameter of 38 mm, and a polymer lens with a focal length of 262 mm. These components are arranged coaxially giving an imaging geometry similar to that used in many commercial B-scanners, but with a receiver bandwidth sufficient to record the first four harmonics. A series of continuous wave and pulse echo measurements are performed on a wire phantom to give 1-D transverse pressure profiles and 2-D B-mode images, respectively. The reflected beamwidths w(n) decreases as w(n)/w(1) = 1/(n^0.78), where n is the harmonic number, and the reflected sidelobe levels fall off quickly with increasing n. In imaging terms, these effects correspond to a large improvement in lateral resolution and signal-to-clutter ratio for the higher harmonics. |
| Title |
Nonlinear propagation in doppler ultrasound. |
| Author |
Li S, McDicken WN, Hoskins PR. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1993 |
| Abstract |
The nonlinear propagation of ultrasound was demonstrated using Doppler transducers on two commercial duplex machines. The influence of nonlinear propagation on Doppler measurements was studied on both a flow phantom and a string phantom. It was found that although the pulse waveforms showed clearly different degrees of nonlinear distortion, no effect due to the nonlinearity could be detected on the received Doppler spectrum both in terms of the maximum frequency or underlying Doppler spectral profile. |
| Title |
Nonlinear propagation of ultrasound in liquid media. |
| Author |
Dunn F, Law WK, Frizzell LA. |
| Journal |
Ultrasound Interact Biol Med |
| Volume |
|
| Year |
1983 |
| Abstract |
No abstract available. |
| Title |
Nonlinear propagation of ultrasound in liquid media. |
| Author |
Dunn F, Law WK, Frizzell LA. |
| Journal |
Ultrasound Interact Biol Med |
| Volume |
|
| Year |
1983 |
| Abstract |
No abstract available. |
| Title |
Nonlinear properties of microbubbles and applications to medical ultrasound imaging. |
| Author |
uhlendorf V, Fritzsch T, Reinhardt M, Scholle FD. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1998 |
| Abstract |
No abstract available. |
| Title |
Nonlinear response of microbubbles to pulsed diagnostic ultrasound. |
| Author |
Hilgenfeldt S, Lohse D, Zomack M. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1998 |
| Abstract |
No abstract available. |
| Title |
Nonlinearities in the bioeffects of ultrasound. |
| Author |
Carstensen EL. |
| Journal |
J. Acoust. Soc. Am |
| Volume |
|
| Year |
1998 |
| Abstract |
Biological effects of ultrasound involve nonlinear phenomena (1) in the propagation of sound itself, (2) in the generation of acoustic cavitation, and (3) in the biochemistry, physiology, and pathology of the biological system. Most nonthermal bioeffects of interest to users of diagnostic ultrasound require acoustic pressures great enough that the wave can become distorted by nonlinear propagation. Under limiting conditions this process can increase the absorption parameter of weakly absorbing media by orders of magnitude and make the absorption parameter of a material such as water as great as the linear absorption coefficient of liver tissue. At low amplitudes the response of the bubbles is dominated by the acoustic pressure. At a critical acoustic pressure, however, the inertia of the surrounding medium becomes controlling. At this threshold of acoustic pressure a 10% or 20% increase in acoustic pressure leads to an increase in the collapse pressure in the bubble by orders of magnitude. The rates of biochemical processes, including denaturation of biological macromolecules, are exponential functions of the temperature. Whether the physical process of heating by ultrasound is linear or nonlinear, this leads to a very strong nonlinear dependence of thermal tissue damage and teratological effects upon the levels of ultrasound. |
| Title |
Nonlinearity parameter B/A imaging by using ultrasound echo signals. |
| Author |
Akiyama I. |
| Journal |
Proc Twelfth ISNA |
| Volume |
|
| Year |
1990 |
| Abstract |
Nonlinearity parameter B/A is expected to indicate the pathological states of biological tissues and offer the significant information for clinical diagnosis. In this study imaging technique of the nonlinearity parameter B/A is presented and the experiments of imaging the phantom based on agar-gel and the bovine and pig liver was conducted by using the finite amplitude ultrasonic pulses of center frequency 1 MHz and the infinitesimal amplitude ultrasonic pulses of center frequency 2 MHz. The discussion of the resultant images is described. |
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