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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 270 out of 330
| Title |
The use of ultrasound methods to defect changes in breast tissue which precede the formation of a malignant tumor. |
| Author |
Kelly-Fry E. |
| Journal |
Acoust Holography |
| Volume |
|
| Year |
1977 |
| Abstract |
No abstract available. |
| Title |
The use of ultrasound-stimulated acoustic emission in the monitoring of modulus changes with temperature. |
| Author |
Konofagou EE, Thierman J, Hynynen. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
2003 |
| Abstract |
It has been previously shown that the amplitude of the ultrasound-stimulated acoustic emission (USAE) signal is sensitive to tissue temperature and, therefore, can help detect it. Its amplitude, however, is sensitive to both acoustical and mechanical parameters, that at most frequencies have opposite effects due to temperature. In this paper, we explore the feasibility of using a frequency shift of the resonant peaks of the USAE signal for monitoring the tissue stiffness variation with temperature. In a numerical simulation, the variation of the frequency shift at different temperatures is shown. Then, in a series of experiments involving a gel phantom and porcine muscle tissue, the frequency shift variation is shown to follow the known stiffness changes due to temperature. It is also shown that this shift indicates reversible changes as well as the onset of thermal coagulative necrosis. The necrosis is marked by a monotonically increasing positive frequency shift. It was thus shown that the USAE spectrum peaks undergo a negative shift (or, downshift) when the stiffness decreases and a positive shift (or, upshift) when the stiffness increases. The experimental frequency shifted around a peak at 22.1–22.5 kHz within a range of −250 to 80 Hz and −200 to 250 Hz for the gel and muscle tissue for the temperatures of 25–70 and 30–70 °C, respectively. Simulation and ex vivo experimental results indicate that the USAE frequency shift method can help decouple the mechanical from the acoustical parameter dependence as well as detect the onset of thermal coagulative necrosis. |
| Title |
The usefulness of a contrast agent and gradient-recalled acquisition in a steady-state imaging sequence for magnetic resonance imaging-guided noninvasive ultrasound surgery. |
| Author |
Hynynen K, Darkazanli A, Damianou CA, Unger E, Schenck JF. |
| Journal |
Invest Radiol |
| Volume |
|
| Year |
1994 |
| Abstract |
RATIONALE AND OBJECTIVES:
The ability of magnetic resonance imaging to detect small temperature elevations from focused ultrasound surgery beams was studied. In addition, the value of a contrast agent in delineating the necrosed tissue volume was investigated.
MATERIALS AND METHODS:
Gradient-recalled acquisition in a steady state (GRASS) T1-weighted images were used to follow the temperature elevation and tissue changes during 2-minute sonications in the thigh muscles of 10 rabbits. The effects of the treatment on the vascular network was investigated by injecting a contrast agent bolus before or after the sonication.
RESULTS:
The signal intensity decreased during the sonication, and the reduction was directly proportional to the applied power and increase in temperature. The signal intensity returned gradually back to baseline after the ultrasound was turned off. Injection of the contrast agent increased the signal intensity in muscle, but not in the necrosed tissue. The dimensions of the delineated tissue volume were the same as measured from the T2-weighted fast-spin-echo images and postmortem tissue examination.
CONCLUSIONS:
These results indicate that magnetic resonance imaging can be used to detect temperature elevations that do not cause tissue damage and that contrast agent can be used to delineate the necrosed tissue volume. |
| Title |
The variation of heating depth with therapeutic ultrasound frequency in physiotherapy. |
| Author |
Demmink JH, Helders PJM, Hobaek H, Enwemeka C. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2003 |
| Abstract |
In patient treatment, different ultrasound (US) frequencies are attributed to differences in penetration and, as an effect of that, to different heating depths in tissues. A set of 13 experiments was carried out with US frequencies of 0.86, 2 and 3 MHz. A dynamic treatment protocol and a stationary treatment protocol were used. The temperature increase patterns were thermally imaged with a 1-min interval during an insonation of 5 min. At every data point, the temperature in the reference image was subtracted from the thermal image after 1, 2, 3, 4 and 5 min. In every difference thermal image, the distance between the US applicator and the deepest point of heat increase was measured. Results show that US frequencies do not affect the depth limit for the different temperature ranges, in either the static treatment protocol or the dynamic treatment protocol.
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| Title |
The velocity of sound through tissues and the acoustic impedance of tissues. |
| Author |
Ludwig GD. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1950 |
| Abstract |
The velocity of sound through various animal organ tissues and through living human tissues is measured, using an ultrasonic pulse method, at 1.25 and 2.5 Mc. The effect of anisotropy (fiber direction) on velocity is determined with beef muscle. Values obtained with the beam traversing the tissue perpendicularly to the long axis of the muscle bundles do not differ significantly from those found with the energy directly parallel with the muscle fibers. Velocity through living human tissues, consisting mostly of muscle, is measured by transmitting the ultrasound through various thicknesses of the arm, leg, and thigh. Specific gravities of the tissues are measured. The characteristic acoustic impedances, calculated from the density and velocity data, vary between 1.5x10(^5) and 1.7x10(^5) g/cm(^2)/sec. The imaginary component of tissue impedance is calculated and found to be negligible at the frequencies at which these measurements are made. |
| Title |
The velocity of ultrasound in human blood under varying physiologic parameters. |
| Author |
Bradley EL III, Sacerio J. |
| Journal |
J Surg Res |
| Volume |
|
| Year |
1972 |
| Abstract |
No abstract available. |
| Title |
The-effective directivity characteristic of a pulsed ultrasound transducer and its measurement by semi-automatic means. |
| Author |
Bamber J, Phelps J. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1977 |
| Abstract |
A simple electronic apparatus for plotting the effective directivity characteristics of pulsed, or continuously excited, ultrasound (1-10 MHz) transducers has been built. Beam profiles are plotted by recording a dc signal, derived from the peak amplitude of either the echo from a steel ball or the signal from a small hydrophone, as a function of position in the acoustic field. An additional capability of direct plotting of isoamplitude contours provides a particularly easy and informative means of recording and displaying the acoustic field. The apparatus has been used to investigate the relationship, in grey scale imaging systems, between dynamic range compression characteristics and effective display resolution. It is shown that, so long as the gain characteristics of the system remain constant, the display resolution capability is inversely related to the strength of the scatterers to be visualized. |
| Title |
Thecnical principles of ultrasound. |
| Author |
Blake GM, Wahaer HW. |
| Journal |
Basic Principles of Ultrasound |
| Volume |
|
| Year |
1999 |
| Abstract |
No abstract available. |
| Title |
Theoretical analysis of an ultrasonic interferometer for procise measurements at high pressures. |
| Author |
Sarvazyan AP, Chalikian TV. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1991 |
| Abstract |
Expressions making it possible to calculate the transmission coefficient of an ultrasonic interferometer, its Q factor and errors of interferometric measurements are presented. These expressions are obtained on the basis of a one-dimensional model which takes into account the non-ideal reflection of acoustic waves from the transducers, diffraction losses and electromechanical properties of transducers. Fixed path ultrasonic interferometers which are widely used for high precision measurements of ultrasound absorption and velocity in liquids, include air-backed transducers. Air-backing limits the application of these interferometers for the measurements under the high pressures. The influence of non-gaseous backing of transducers on the characteristics of an interferometer is theoretically analysed by means of suggested expressions. Frequency dependences of main parameters of the interferometer are obtained. The possibility of high precision measurements of velocity and absorption of ultrasound in liquids under the high pressures by means of fixed path interferometers with liquid-backed transducers is shown. |
| Title |
Theoretical and experimental studies on the influence of ultrasound on immobilized enzymes. |
| Author |
Schmidt P, Rosenfeld E, Millner R, Czerner R, Schellenberger A. |
| Journal |
Biotechnol Bioeng |
| Volume |
|
| Year |
1987 |
| Abstract |
Experimental investigations on alpha-amylase and glucoamylase bound to porous polystyrene show that the activity of immobilized enzymes can be raised in the presence of an ultrasonic field. The maximum activity increase in a flow cuvette at 7.6 MHz and a sound intensity of 5 kW/m2 amounts to more than 200% under the given experimental conditions. A mathematical model based on the differential equation for the interior and exterior substrate transport is set up and solved numerically. From the theoretical considerations and the experiments it is evident that the mechanism of the ultrasonic effect can be explained in terms of a reduction of the unstirred diffusion layer around the matrix particles. |
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