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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 268 out of 330
| Title |
The thermal pulse-decay method for determining ultrasound absorption coefficients. |
| Author |
Parker KJ, Lele PP. |
| Journal |
Proc Ultrason Symp IEEE |
| Volume |
|
| Year |
1982 |
| Abstract |
The classic method for determining the absorption coefficient of a tissue requires measuring the material rate of heating during ultrasonic insonation. Errors result primarily from viscous heating around the temperature probe, and from the effects of heat transfer to cooler surroundings. A temperature pulse-decay technique, which circumvents the major errors associated with the rate-of-heating method, is proposed and evaluated in liver tissue samples. The pulse-decay technique allows for the use of significantly smaller beamwidths than the rate-of-heating method. Absorption measurements using each technique are shown for comparison. |
| Title |
The threshold for brain damage in rabbits induced by bursts of ultrasound in the presence of an ultrasound contrast agent (Optison). |
| Author |
Hynynen K, McDannold N, Martin H, Jolesz FA, Vykhodtseva N. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
2003 |
| Abstract |
The purpose of this study was to test the hypothesis that burst ultrasound (US) in the presence of a US contrast agent using parameters similar to those used in brain blood flow measurements causes tissue damage. The brains of 10 rabbits were sonicated in 3-8 locations with 1.5-MHz, 10- micro s bursts repeated at a frequency of 1 kHz at temporal peak acoustic pressure amplitudes ranging from 2 to 12.7 MPa. The total sonication time for each location was 20 s. Before each sonication, a bolus of US contrast agent was injected IV. Contrast-enhanced magnetic resonance (MR) images were obtained after the sonications to detect local enhancement in the brain. Whole brain histological evaluation was performed, and the sections were stained with hematoxylin and eosin (H and E), TUNEL, and vanadium acid fuchsin (VAF) staining to evaluate tissue effects, including apoptosis and ischemia. Both the magnetic resonance imaging (MRI) contrast enhancement and histology findings indicated that brain tissue damage was induced at a pressure amplitude level of 6.3 MPa. The damage included vascular wall damage, hemorrhage and, eventually, necrosis. Mild vascular damage was observed localized in a few microscopic tissue volumes in about half of the sonicated locations at all pressure values tested (down to 2 MPa). However, these sonications did not induce any detectable tissue effects, including ischemia or apoptosis. As a conclusion, the study showed that the US exposure levels currently used for blood flow measurements in brain are below the threshold of blood-brain barrier opening or brain tissue damage. However, one should be aware that brain damage can be induced if the exposure level is increased. |
| Title |
The threshold for thermally significant cavitation in dog's thigh muscle in vivo. |
| Author |
Hynynen K. |
| Journal |
ULtrasound Med Biol |
| Volume |
|
| Year |
1991 |
| Abstract |
In this study the threshold of thermally significant transient cavitation in vivo in dog's thigh muscle was investigated as a function of frequency from 0.246 MHz to 1.68 MHz. Cavitation, evidenced by strong emission of wide band noise monitored by a hydrophone, appeared to increase the energy absorption in tissue at the focal zone of a focused ultrasound beam as measured with an embedded thermocouple. This was indicated by a significant increase in the temperature, a loss of smooth temperature rise during the 1 s sound pulse and a significant reduction in the acoustic power transmitted through the thigh. This thermal phenomenon was associated with a strong emission of wide band noise which was monitored by a hydrophone. In addition, strong echoes appeared in ultrasound images during the pulses that caused the noise emission and the thermal effect. These echoes appeared preferentially at locations where there was acoustic heterogeneity. The measured cavitation pressure amplitude threshold was found to depend almost linearly on frequency with a slope of about 5.3 MPa MHz−1. (The extrapolated static pressure threshold was 0.6 MPa.) When these measured levels are compared to those typical of clinical application, it appears that the transient cavitation can be avoided when perfusion independent high temperature hyperthermia is induced with focused and pulsed ultrasound fields. However, intensities required during scanned focused ultrasound hyperthermia, where sharply focused transducers are used to heat large tumors at low frequencies (1 MHz or below), could rise above the threshold. Thus, care should be taken when focused ultrasound systems are designed so that the maximum peak pressure is below the threshold in order to avoid unpredictable biological effects induced by transient cavitation. Finally, it is unlikely that the present diagnostic ultrasound units which operate at higher frequencies and in pulsed mode could cause transient cavitation in vivo |
| Title |
The threshold for thermally significant cavitation in dog's thigh muscle in vivo. |
| Author |
Hynynen K. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1991 |
| Abstract |
In this study the threshold of thermally significant transient cavitation in vivo in dog's thigh muscle was investigated as a function of frequency from 0.246 MHz to 1.68 MHz. Cavitation, evidenced by strong emission of wide band noise monitored by a hydrophone, appeared to increase the energy absorption in tissue at the focal zone of a focused ultrasound beam as measured with an embedded thermocouple. This was indicated by a significant increase in the temperature, a loss of smooth temperature rise during the 1 s sound pulse and a significant reduction in the acoustic power transmitted through the thigh. This thermal phenomenon was associated with a strong emission of wide band noise which was monitored by a hydrophone. In addition, strong echoes appeared in ultrasound images during the pulses that caused the noise emission and the thermal effect. These echoes appeared preferentially at locations where there was acoustic heterogeneity. The measured cavitation pressure amplitude threshold was found to depend almost linearly on frequency with a slope of about 5.3 MPa MHz-1. (The extrapolated static pressure threshold was 0.6 MPa). When these measured levels are compared to those typical of clinical application, it appears that the transient cavitation can be avoided when perfusion independent high temperature hyperthermia is induced with focused and pulsed ultrasound fields. However, intensities required during scanned focused ultrasound hyperthermia, where sharply focused transducers are used to heat large tumors at low frequencies (1 MHz or below), could rise above the threshold. Thus, care should be taken when focused ultrasound systems are designed so that the maximum peak pressure is below the threshold in order to avoid unpredictable biological effects induced by transient cavitation. Finally it is unlikely that the present diagnostic ultrasound units which operate at higher frequencies and in pulsed mode could cause transient cavitation in vivo. |
| Title |
The ultrasonic assessment of discordant growth in twin pregnancies. |
| Author |
MacLean M Mathers A Walker JJ Cameron AD Howat R. |
| Journal |
Ultrasound Obstet Gynecol |
| Volume |
|
| Year |
1992 |
| Abstract |
Discordant fetal growth, as defined by an intertwin weight difference (calculated as a percentage of the weight of the larger twin) of 20% or more, is associated with an increased perinatal mortality and morbidity. In this study, the accuracy of ultrasound estimation of fetal weight by abdominal circumference and biparietal diameter measurements in 107 twin pregnancies was assessed. Discordant growth greater than 20% as measured by ultrasound was found to have a sensitivity of 5-25%, a specificity of 87-95%, a positive predictive value of 12-43% and a negative predictive value of 80-85% at gestations ranging from 28 weeks to term. ROC calculations showed that changes in the discordance level used did not improve the value of the test. These results indicate that ultrasound measurement of fetal weight alone cannot be used accurately to assess discordant growth in twin pregnancies. |
| Title |
The ultrasonic characterization of breast malignancies. |
| Author |
Jellins J, Reeve TS, Kossoff G, Griffiths K. |
| Journal |
Proc First Int Symp Ultrasound Cancer - Brussels |
| Volume |
|
| Year |
1982 |
| Abstract |
No abstract available. |
| Title |
The ultrasonic degradation of biological macromolecules under conditions of stable cavitation. I. Theory, methods, and application to deoxyribonucleic acid. |
| Author |
Pritchard NJ Hughes DE Peacocke AR. |
| Journal |
Biopolymers |
| Volume |
|
| Year |
1966 |
| Abstract |
Solutions of calf thymus DNA have been degraded in the presence of vibrating air bubbles in ultrasonic fields of low power which would not normally induce ultrasonic cavitation. The DNA was degraded to a limiting intrinsic viscosity, after which further irradiation by ultrasound had little or no effect. This limiting intrinsic viscosity decreased with increase in the ultrasonic intensity. Previously developed theories have been adapted to calculate the maximum velocity gradient associated with the streaming of the solution around such vibrating air bubbles. The tensile force which is induced and which acts on the DNA has been calculated on the basis of current theories of degradation by hydrodynamic shear. These calculations indicate that the degradation of the DNA by ultrasound under conditions of ?stable cavitation? is mainly the result of the shearing forces engendered in the solution around the oscillating bubbles. |
| Title |
The ultrasonic nonlinearity parameter of biological media and its dependence on tissue structure. |
| Author |
Dunn F. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1986 |
| Abstract |
The magnitude of the nonlinearity parameter depends on the macromolecular content of tissue and B/A is greater for an intact organ, which has structural integrity, than for the same organ which has been macerated.
The nonlinearity parameter has not been measured in vivo.
Finite amplitude distortion of ultrasonic waves from medical equipment has been observed in human calf muscle (Starritt et al., 1985).
A principal effect of the nonlinearity of tissues is the generation of higher harmonics, which can increase the heating effect. |
| Title |
The ultrasonic properties of gastric cancer tissues obtained with a scanning acoustic.microscope system. |
| Author |
Saijo Y, Tanaka M, Okawai H, Dunn F. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1991 |
| Abstract |
A newly developed scanning acoustic microscope (SAM) system operating in the frequency range of 100-200 MHz has been employed to measure the attenuation and the sound speed of formalin-fixed specimens of five different types of gastric cancer. Signet-ring cell carcinoma specimens exhibit attenuation constant and sound speed values significantly lower than other types of gastric cancer tissues. Tubular adenocarcinoma specimens exhibit a trend toward higher attenuation and sound speed values as the cell type became differentiated. Our measurements and observations suggest that the ultrasonic properties are influenced by cellular arrangement, intercellular junction and intracellular chemical components. |
| Title |
The unknowns of ultrasound. |
| Author |
Thompson RC. |
| Journal |
FDA Consum |
| Volume |
|
| Year |
1983 |
| Abstract |
No abstract available. |
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