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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 197 out of 330
| Title |
Phase and amplitude fluctuations in the propagation of acoustic waves.in lossless inhomogeneous continua with velocity, density and bulk.modulus variations. |
| Author |
Chivers RC. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1978 |
| Abstract |
No abstract available |
| Title |
Phase-abberration correction using signals from point reflectors and diffuse scatterers: Basic principles. |
| Author |
Flax SW, O'Donnell M. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1988 |
| Abstract |
Methods for correction of phase aberrations induced by near field variations in the index of refraction are explored. Using signals obtained from a sampled aperture (i.e., transducer array), phase aberrations can be accurately measured with a correlation approach similar to methods used in adaptive optics and radar. However, the specific method presented here has no need for a beacon or an ideal point reflector to act as a source for estimating phase errors. Indeed, the method detailed in this report uses signals from random collections of scatterers to accurately determine phase aberrations. Because there is no longer a need for a beacon signal, this method is directly applicable not only to medical ultrasound imaging, but also to any coherent imaging system with a sampled aperture, such as radar and sonar. |
| Title |
Phase-aberration correction using signals from point reflectors and diffuse scatterers: Measurement. |
| Author |
O'Donnell M, Flax SW. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1988 |
| Abstract |
A method for phase-aberration correction of phased-array images is tested using a model of near field velocity inhomogeneities. A set of grooved room-temperature vulcanizing (RTV) plates was constructed to simulate near field aberrations encountered in clinical ultrasound imaging. As expected, large image distortion was experienced when grooved plates producing significant aberrations were placed near to the surface of the array. However, an iterative aberration correction procedure based on cross-correlation measures between neighboring elements in a phased array using signals reflected from diffuse scatterers was able to significantly reduce the effects of these aberrations, producing images nearly identical to those generated in the absence of aberrations. These results suggest that a practical phase-aberration correction system can be constructed for medical ultrasound imaging, and possibly all coherent imaging systems using a sampled aperture. |
| Title |
Phase-aberration correction with a 3-D ultrasound scanner: Feasibility study. |
| Author |
Ivancevich NM, Dahl JJ, Trahey GE, Smith SW. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
2006 |
| Abstract |
We tested the feasibility of using adaptive imaging, namely phase-aberration correction, with two-dimensional (2-D) arrays and real-time, 3-D ultrasound. Because of the high spatial frequency content of aberrators, 2-D arrays, which generally have smaller pitch and thus higher spatial sampling frequency, and 3-D imaging show potential to improve the performance of adaptive imaging. Phase-correction algorithms improve image quality by compensating for tissue-induced errors in beamforming. Using the illustrative example of transcranial ultrasound, we have evaluated our ability to perform adaptive imaging with a real-time, 3-D scanner. We have used a polymer casting of a human temporal bone, root-mean-square (RMS) phase variation of 45.0 ns, full-width-half-maximum (FWHM) correlation length of 3.35 mm, and an electronic aberrator, 100 ns RMS, 3.76 mm correlation, with tissue phantoms as illustrative examples of near-field, phase-screen aberrators. Using the multilag, least-squares, cross-correlation method, we have shown the ability of 3-D adaptive imaging to increase anechoic cyst identification, image brightness, contrast-to-speckle ratio (CSR), and, in 3-D color Doppler experiments, the ability to visualize flow. For a physical aberrator skull casting we saw CSR increase by 13% from 1.01 to 1.14, while the number of detectable cysts increased from 4.3 to 7.7. |
| Title |
Phase-insensitive detection for measurement of backscattered ultrasound. |
| Author |
Johnston PH, Miller JG. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
1986 |
| Abstract |
The results of backscatter measurements in an inhomogeneous phantom obtained using phase-sensitive detection are compared with those obtained using phase-insensitive detection by means of a two-dimensional pseudo-array of point-like receivers. Phase-sensitive measurements were performed using a conventional 13-mm-diameter piezoelectric receiver at a scattering angle of approximately 170 degrees. Phase-insensitive measurements were obtained at the same sites by computing the zeroth-order two-dimensional spaitial moment of the ultrasonic energy measured by a pseudo-array. At all the sites the backscatter transfer function obtained using phase-insensitive detection was consistently larger in magnitude and exhibited smaller fluctuations with frequency than the corresponding phase-sensitive backscatter transfer function. Spatial averaging of the phase-sensitive results provided smoother backscatter transfer functions but could not recover the loss of magnitude resulting from phase-cancellation. These investigations suggest that phase-insensitive detection represents a means to improve the accuracy of backscatter measurements in inhomogeneous media. |
| Title |
Phased array ultrasound imaging through planar tissue layers. |
| Author |
Smith SW, Trahey GE, von Ramm OT. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1986 |
| Abstract |
Conventional ultrasound imaging devices are designed based on the assumption of a homogeneous tissue medium of constant acoustic VELOCITY = 1540 m/sec. However, the body consists of tissue layers of varying thicknesses and velocities which range from 1470 m/sec in fat to 3200 m/sec in skull bone. Refraction effects from these layers degrade ultrasound image quality. In this paper, pulse-echo ultrasound imaging is modeled as imaging an organ of interest through an intervening planar tissue layer, such as liver through fat in the abdomen or brain through skull bone in the adult head. Refraction effects from planar tissue layer interfaces are analyzed using Snell's law and measured using phantoms. We also introduce an on-line phased array correction technique based on planar tissue layers to restore ultrasound image quality. We conclude that fat/organ planar interfaces do not degrade image quality significantly. However, refraction effects at a skull/brain planar interface degrades resolution and target acquisition and introduces geometric distortion. Our plane layer phased array correction technique significantly improves image quality in phantoms through lucite aberrators and improves adult cephalic ultrasound image quality when used through the top of the adult skull. The correction technique is robust even in the presence of inaccurate estimates of skull thickness. |
| Title |
Phonophoresis: an in vivo evaluation using three topical anaesthetic preparations. |
| Author |
Williams AR. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1990 |
| Abstract |
An electrical sensory perception threshold technique has been developed for use with human volunteers. This technique has been used to reproducibly quantify the effects of three different commercially available topical anaesthetic preparations on superficial sensory cells (nociceptors) in the skin. Low intensities (0.25 W cm-2 SATA) of 1.1 MHz ultrasound had no detectable effects upon the rate of penetration of either one of the three anaesthetic preparations through human skin under conditions where temperature increases had been minimized. |
| Title |
Photoacoustic cell for ultrasound contrast agent characterization. |
| Author |
Alippi A, Bettucci A, Biagioni A, D'Orazio A, Germano M, Passeri D. |
| Journal |
Rev Sci Instrum |
| Volume |
|
| Year |
2010 |
| Abstract |
Photoacoustics has emerged as a tool for the study of liquid gel suspension behavior and has been recently employed in a number of new biomedical applications. In this paper, a photoacoustic sensor is presented which was designed and realized for analyzing photothermal signals from solutions filled with microbubbles, commonly used as ultrasound contrast agents in echographic imaging techniques. It is a closed cell device, where photothermal volume variation of an aqueous solution produces the periodic deflection of a thin membrane closing the cell at the end of a short pipe. The cell then acts as a Helmholtz resonator, where the displacement of the membrane is measured through a laser probe interferometer, whereas photoacoustic signal is generated by a laser chopped light beam impinging onto the solution through a glass window. Particularly, the microbubble shell has been modeled through an effective surface tension parameter, which has been then evaluated from experimental data through the shift of the resonance frequencies of the photoacoustic sensor. This shift of the resonance frequencies of the photoacoustic sensor caused by microbubble solutions is high enough for making such a cell a reliable tool for testing ultrasound contrast agent, particularly for bubble shell characterization. |
| Title |
Photoacoustic ultrasound. |
| Author |
Kruger RA. |
| Journal |
Med Phys |
| Volume |
|
| Year |
1994 |
| Abstract |
Differential absorption has been detected and localized in three-dimensions by recording the photoacoustic pulses that were produced when short-duration (approximately 1 microsecond) pulses of electromagnetic energy were absorbed regionally within a turbid medium. These absorption sites were localized with a spatial resolution of approximately 6 mm within a 20 x 20 x 7.5-cm3 volume of 0.3% Liposyn? solution, a highly scattering medium. A Xenon flashlamp, delivering a nominal 1 microsecond pulse of broadband light, was used to irradiate the Liposyn? solution. Photoacoustic echoes were recorded with a focused, ultrasonic transducer, tuned to a nominal frequency of 0.5 MHz. The spatial resolution that was demonstrated is consistent with the expected ultrasonic properties of the transducer. Improved spatial resolution can be expected with shorter-duration radiation exposure and higher-frequency transducer designs. The technique is generalizable to any electromagnetic energy range (including long-wavelength microwaves) that penetrates the medium and produces differential, regional absorption of energy. |
| Title |
Photoacoustic ultrasound: Pulse production and detection in 0.5% Liposyn. |
| Author |
Kruger RA, Liu P. |
| Journal |
Med Phys |
| Volume |
|
| Year |
1994 |
| Abstract |
Theoretical predictions and experimental measurements of photoacoustic pulse production within a 0.5% solution of Liposyn, a highly scattering, optical propagation medium, are reported. A simple model for photoacoustic energetics is developed that predicts photoacoustic signal pressure as a function of depth within a turbid medium following surface irradiation from an infrared source. The model is valid for very short irradiation duration. The model predicts that the acoustic pressure produced at a distance r from the center of a small, highly absorbing sphere of radius R consists of two, opposite polarity pulses, one originating from the near and one from the far side of the sphere. The magnitude of these biphasic pulses is expected to be proportional to the energy fluence (E) incident on the surface of the sphere and to the ratio, R/r. Furthermore, the energy fluence (E) that reaches the sphere is roughly proportional to e-mu effZ, where mu eff is the effective attenuation coefficient of the turbid medium and Z is the depth of the embedded sphere below the irradiated surface. The variation of E with depth within the absorber and biphasic acoustic pulse production have been verified experimentally. Further experiments demonstrate that a small (3-mm diameter), highly absorbing sphere can be detected and localized at a depth of 37.5 mm within a 0.5% solution of Liposyn with a spatial resolution of 1 x 6 mm2, using a biologically safe level of infrared irradiation (lambda = 1064 nm) and a conventional ultrasound transducer (frequency = 2.25 MHz). These results suggest that photoacoustic ultrasound imaging may have application to biologic systems such as the human breast. |
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