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BRL Abstracts Database |
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Your search for ultrasound produced 3296 results. Page 276 out of 330
| Title |
Thresholds for cavitation produced in water by pulsed ultrasound. |
| Author |
Atchley AA, Frizzell LA, Apfel RE, Holland CK, Madanshetty S, Roy RA. |
| Journal |
Ultrasonics |
| Volume |
|
| Year |
1988 |
| Abstract |
The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles. |
| Title |
Thresholds for fetal hemorrhages produced by a piezoelectric lithotripter. |
| Author |
Dalecki D, Child SZ, Raeman CH, Penney DP, Mayer R, Cox C, Carstensen EL. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1997 |
| Abstract |
Hemorrhage to fetal tissues occurred when late-term pregnant mice were exposed to lithotripter fields of relatively low amplitude. These hemorrhages were always observed in tissues near developing bone or cartilaginous structures such as the head, limbs and ribs, while soft tissues distant from bone were relatively free of hemorrhage. Thresholds for hemorrhage in the fetus were determined for exposures of pregnant mice on the 18th day of gestation to 200 pulses from a piezoelectric lithotripter. Animals were exposed to axial peak positive pressures of either 0 (sham), 1, 2, 3, 5 or 10 MPa. Thresholds for hemorrhage to the head, limbs, ribs and lung were all < 1 MPa. |
| Title |
Thresholds for focal ultrasonic lesions in rabbit kidney, liver, and testicle. |
| Author |
Frizzell LA, Linke CA, Carstensen EL, Fridd CW. |
| Journal |
IEEE Trans Biomed Eng |
| Volume |
|
| Year |
1977 |
| Abstract |
Thresholds are reported for productio of focal ultrasonic lesions in rabbit liver, kidney, and testicle for 2 and 6 MHz, single pulse exposures ranging in length from 1 to 60 s. Intensity-exposure time thresholds are found to be nearly independent of frequency as shown in earlier work for brain tissue. The results compare favorably with a thermal model for prediction of thresholds which considers the activation energy for thermal destruction of the tissues, their thermal diffusivity, absorption coefficients, time and intensity of exposure, and baseline temperature. All thresholds are of the same magnitude as those previously reported for brain. Those differences which are observed can be explained by the thermal model for the action of ultrasound on tissue. |
| Title |
Thresholds for hemorrhages in mouse skin and intestine induced by lithotripter shock waves. |
| Author |
Miller DL, Thomas RM. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1995 |
| Abstract |
In vivo biological effects of ultrasound should be characterized as thermal or cavitational to understand their etiology and significance. A spark-gap shock-wave lithotripter was built and used to compare cavitation-induced hemorrhages to the heat-induced petechial hemorrhages caused by continuous-wave ultrasound in mouse intestine. Intestinal hemorrhages induced in anesthetized hairless mice by the lithotripter pulses involved tissue destruction with bleeding into the lumen of the intestine, and were associated with intestinal gas bubbles. Skin hemorrhages were also observed, which appeared to be contusions, with no actual breakage of the skin. Administration of 100 shock waves with peak positive amplitude of 18.5 MPa produced an average of 7.6 (standard error [SE] 3.1, n = 6) intestinal hemorrhages and 45 (SE 11) skin hemorrhages. The counts and severity of hemorrhages increased with increasing numbers (3 to 300) of shock waves. Absorbers of varying thickness were used to reduce the pressure amplitude of the shock waves, which were thereby modified into low frequency ultrasound pulses. For 100 pulse exposures, apparent thresholds for effects occurred between 1.6 and 4.0 MPa for the intestinal hemorrhages and between 0.6 and 1.6 MPa for the skin hemorrhages. The low 1-Hz pulse repetition frequency precluded significant heating, and so these effects were the result of cavitation, which probably occurred inside the intestines or in the surrounding water. Compared to the previously observed thermal petechia, the cavitation-induced hemorrhages could be distinguished on the basis of their appearance upon histological examination, and also by the relative values of the thermal and mechanical exposure indices associated with the two different exposure modes. |
| Title |
Thresholds for inertial cavitation in albunex suspensions under pulsed ultrasound conditions. |
| Author |
Chang PP, Chen WS, Mourad PD, Poliachik SL, Crum LA. |
| Journal |
IEEE Trans UFFC |
| Volume |
|
| Year |
2001 |
| Abstract |
Stabilized microbubbles used as echo-contrast agents can be destroyed by ultrasonic irradiation. We have identified two pressure thresholds at which these microbubbles undergo inertial cavitation (here, defined as the collapse of gas bubbles followed by emission of an acoustic broadband noise). The first threshold (P1) corresponds to the pressure at which all the microbubbles in a cavitation field lose their property as an effective scatterer because of fragmentation or deflation. The second threshold (P2) is associated with the acoustic reactivation of the remnants of the contrast agents and is related to the onset of more violent inertial cavitation. P1 and P2 were measured as a function of the concentration of Albunex(R) (Molecular Biosystems Inc., San Diego, CA) contrast agent, the number of transmitting acoustic cycles, and the pulse repetition frequency (PRF). The ultrasound frequency used was 1.1 MHz, and the peak negative acoustic pressures ranged from 0 to 8 MPa. Our results, measured in Isoton(R) II (Coulter Diagnostics, Miami, FL) and whole blood solutions, showed that P1 increased with increasing Albunex(R) concentration and decreased with increasing PRF, whereas P2 decreased with increasing Albunex(R) concentration and was independent of the PRF. Both P1 and P2 decreased with increasing number of acoustic cycles N for N<10 and were independent of the number of cycles for N>10. Ultrasound images of Albunex(R) acquired by a commercial scanner showed echo enhancement not only at pressure levels below P1 but also at levels above P2. The threshold P2 was achieved at ultrasound energies above the diagnostic level. Inertial cavitation produced at P2 was associated with a higher level of hemolysis compared with P1. The results of this investigation have potential significance for both diagnostic and therapeutic ultrasound applications. |
| Title |
Thresholds for intestinal hemorrhage in mice exposed to a piezoelectric lithotripter. |
| Author |
Dalecki D, Raeman CH, Child SZ, Carstensen EL. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1995 |
| Abstract |
The threshold for hemorrhage in mouse intestine was determined using the fields of a piezoelectric lithotripter. Exposures were controlled by variation of the position of the animal relative to the focus and by variation of the voltage used to charge the lithotripter. The range of peak positive pressure for exposures was approximately 50 MPa to 1 MPa. Each exposure consisted of 200 pulses at a repetition rate of approximately 1 Hz. Depending upon the exposure level, intestinal lesions ranged in size from small petechiae to hemorrhagic regions extending 5 cm or more along the intestine. Threshold for intestinal hemorrhage with this exposure protocol was in the range of 1 to 3 MPa. At threshold, the lithotripter waveform was nearly sinusoidal. |
| Title |
Thresholds for premature contractions in murine hearts exposed to pulsed ultrasound. |
| Author |
MacRobbie AG, Raeman CH, Child SZ, Dalecki D. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1997 |
| Abstract |
A single pulse of high intensity ultrasound can produce either a premature.ventricular contraction or a reduction in the aortic pressure in frog hearts. The.objective of this study was to determine whether similar ultrasound exposures can.produce premature contractions in the mammalian heart. The cardiac activity of.murine hearts in vivo was monitored noninvasively using electrocardiography.and plethysmography. Each ultrasound exposure was a single pulse of.ultrasound, several milliseconds in duration, delivered to the murine heart during.diastole. The thresholds for producing a premature contraction with a 5-ms.ultrasound pulse at 1.2 MHz was approximately 2 MPa peak positive pressure..The occurrence of premature contractions decreased as the duration of the.ultrasound pulse decreased. These results found with the mammalian heart are.similar to those reported earlier for the frog heart. No damage to cardiac tissue.was observed grossly, although significant hemorrhage occurred to adjacent lung.tissue. . |
| Title |
Thresholds for premature contractions in murine hearts exposed to pulsed ultrasound. |
| Author |
MacRobbie AG, Raeman CH, Child SZ, Dalecki D. |
| Journal |
Ultrasound Med BIol |
| Volume |
|
| Year |
1997 |
| Abstract |
A single pulse of high intensity ultrasound can produce either a premature ventricular contraction or a reduction in the aortic pressure in frog hearts. The objective of this study was to determine whether similar ultrasound exposures can produce premature contractions in the mammalian heart. The cardiac activity of murine hearts in vivo was monitored noninvasively using electrocardiography and plethysmography. Each ultrasound exposure was a single pulse of ultrasound, several milliseconds in duration, delivered to the murine heart during diastole. The threshold for producing a premature contraction with a 5-ms ultrasound pulse at 1.2 MHz was not, vert, similar 2 MPa peak positive pressure. The occurrence of premature contractions decreased as the duration of the ultrasound pulse decreased. These results found with the mammalian heart are similar to those reported earlier for the frog heart. No damage to cardiac tissue was observed grossly, although significant hemorrhage occurred to adjacent lung tissue. |
| Title |
Thresholds for premature ventricular contractions in frog hearts exposed to lithotripter fields. |
| Author |
Dalecki D, Keller BB, Carstensen EL, Neel DS, Palladino JL,.Noordergraaf A. |
| Journal |
Ultrasound Med Biol |
| Volume |
|
| Year |
1991 |
| Abstract |
Piezoelectrically generated lithotripter shocks were shown to produce premature.ventricular contractions of the frog heart. Anesthetized grass frogs, Rana pipiens,.were studied following implantation of an aortic catheter and EKG leads. The.most sensitive phase of the heart cycle for the generation of premature ventricular.contractions with lithotripter shocks at 30 MPa peak pressure was found to be the.T-P segment. During this phase of the heart cycle, the minimum peak-positive.pressure shock wave necessary to produce a premature ventricular contraction in.a frog heart was between 5 MPa and 10 MPa. |
| Title |
Thresholds for transient cavitation produced by pulsed ultrasound in a controlled nuclei environment. |
| Author |
Holland CK, Apfel RE. |
| Journal |
J Acoust Soc Am |
| Volume |
|
| Year |
1990 |
| Abstract |
Transient cavitation is a discrete phenomenon that relies on the existence of stabilized nuclei, or pockets of gas within a host fluid, for its genesis. A convenient descriptor for assessing the likelihood of transient cavitation is the threshold pressure, or the minimum acoustic pressure necessary to initiate bubble growth and subsequent collapse. An automated experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz. A fluid jet was used to convect potential cavitation nuclei through the focal region of the insonifying transducer. Potential nuclei tested include 1-microns polystyrene spheres, microbubbles in the 1- to 10-microns range that are stabilized with human serum albumin, and whole blood constituents. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Measurements of the transient cavitation threshold in water, in a fluid of higher viscosity, and in diluted whole blood are presented. These experimental measurements of cavitation thresholds elucidate the importance of ultrasound, host fluid, and nuclei parameters in determining these thresholds. These results are interpreted in the context of an approximate analytical theory for the prediction of the onset of cavitation. |
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