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At least 559 records · Page 31Linked to original sources
PubMed1975

Evaluation of ultrasonic bubble detectors in vitro using calibrated microbubbles at selected velocities.

Two standard Doppler ultrasonic devices, currently used for detecting bubbles in vivo, have been evaluated and compared in vitro using carefully calibrated uniform micro-bubbles rising at terminal velocity through a static aqueous medium. Two unexpected findings were observed: (a) the focal length of the transducer apparently decreases for smaller bubble sizes, and (b) a significant horizontal convection current was produced by one of the instruments. When the medium was in motion, it was found that the sensitivity varied markedly with bubble velocity, varying from a minimum detectable diameter of 40 mum at 55 cm/sec to 170 mum at 20 cm/sec. These findings are discussed with regard to the limitations of the Doppler technique for monitoring gas emboli in vivo and as an early warning for decompression sickness in divers.

Decompression Sickness↗
PubMed1990

Measurements of frequency spectra of transmission coefficients to ultrasound through trapped microbubbles.

Techniques which use hydrophobic polycarbonate thin sheets containing randomly spaced, fairly uniform small pores immersed in water to trap air bubbles have been found to be useful in biophysical experiments. The utilization of broadband polyvinylidene fluoride transducers in this work made it possible to measure a continuous frequency spectrum of the transmission coefficient of the trapped bubbles. The results of the measurements show: (1) the frequency response curve of the bubble ensemble is much broader than that of a single bubble predicted by theory; and (2) as the incident sound pressure at a micropore membrane increases from 110 to 660 Pa the resonance frequency of bubbles shifts to lower values by as much as 7%.

Air↗
PubMed1991

Imaging microbubbles and tissues using a linear focussed scanner operating at 20 MHz: possible implications for the detection of cavitation thresholds.

B-Scan images have been used as a means of detecting the onset of cavitation in tissues. New echoes appearing during simultaneous sonication with therapeutic ultrasound have often been attributed to bubble formation. In this study temperature rises up to 15 degrees C W-1 cm2 in guinea pig leg (post-mortem) were found, which would itself increase the rate of growth of gas bubbles. It is also hypothesized that a temperature dependent attenuation coefficient predicted by Bamber and Hill may, at least in part, account for new echoes appearing in tissues. Possible artefactual echoes in the B-scan images arising from acoustically induced gas bubbles in the coupling medium are also discussed.

Animals↗
PubMed1994

Nitrogen microbubbles induce a disappearance of single platelets (aggregation) with porcine platelets: a comparative study of the effects of anticoagulants and blood collection methods.

The pathogenesis of decompression illness (DCI) is uncertain. DCI involves all parts of the organism where gas bubbles are produced. They have both primary and secondary effects and have been classified as an agonist aggregating human platelets. In vitro effects of N2 bubbles on porcine platelets were investigated. Comparative studies using two different anticoagulants and three different sampling methods were performed. A disappearance of single platelets interpreted as platelet aggregation was observed in the presence of N2 bubbles in all studied groups. Aggregatory responses were more profound with platelets in heparinized plasma than in citrated plasma. In citrated plasma the aggregatory responses were more profound when blood was obtained from nonanaesthetized (awake) animals than from slaugtherhouse animals. Adrenaline (1 microM) had an inhibitory effect on N2 bubble induced platelet aggregation in vitro. The pig could be useful to investigate possible gas bubble effects in vivo.

Animals↗
PubMed1990

Contrast agents for diagnostic ultrasound: development and evaluation of polymer-coated microbubbles.

Although the concept of an ultrasound contrast agent dates from Gramiak's work in 1968 in which indocyanine green was injected into the ascending aorta and heart, no universally accepted contrast agent for ultrasound now exists. This is primarily due to problems with stability, size and/or toxicity of the agents which have been investigated. Development of an effective ultrasound contrast agent would be highly significant for the health care industry, since it would greatly expand the scope of ultrasound (a noninvasive and safe procedure) as a diagnostic technique. While encapsulated gas bubbles offer particular advantages in stability over hand-agitated systems, they frequently present problems with size. Capsules larger than 10 microns in diameter become entrapped in the capillary bed of the lung. This paper describes the use of ionotropic gelation of the naturally occurring polysaccharide, alginate, for microencapsulation of air. Two procedures have been investigated. A novel jet head has been developed which allows co-extrusion of a solution of sodium alginate and air to produce nascent microencapsulated air bubbles which fall into a hardening solution of calcium ions. A second method employs ultrasound to introduce cavitation-induced bubbles into the alginate before capsule formation by spraying. Power spectra of these preparations demonstrate echogenicity (that is strong scatter of the incident ultrasound wave back to the emitting transducer, which also acts as a receiver), with resonant peaks that are a function of capsule size and wall characteristics.

Alginates↗
PubMed2004

Noninvasive microbubble-based pressure measurements: a simulation study.

This paper describes a noninvasive method to measure local hydrostatic pressures in fluid filled cavities. The method is based on the disappearance time of a gas bubble, as the disappearance time is related to the hydrostatic pressure. When a bubble shrinks, its response to ultrasound changes. From this response, the disappearance time, and with it the hydrostatic pressure, can be determined. We investigated the applicability of the gases Ar, C(3)F(8), Kr, N(2), Ne, and SF(6), based on their diffusive properties. For pressure measurements with a limited duration, e.g. 150 ms, Kr and Ar bubbles are most suitable, since they are most sensitive to pressure change. If there is also a limitation to bubble size, e.g. a maximum diameter of 6 microm, SF(6) is most suitable. We present improvements of a method that correlates the duration of the decay of the fundamental ultrasound response to the hydrostatic overpressure. We propose to correlate the duration until subharmonic occurrence in combination with its decay, to hydrostatic overpressure, since the subharmonic decays more rapidly than the fundamental response. For a dissolving Ar gas bubble with an initial diameter of 14 microm, the overpressure can be determined 4 times as precise from the decay of the subharmonic response as from the decay of the fundamental response. Overpressures as small as 11 mmHg may be discriminated with this method.

Contrast Media↗
PubMed2002

Implementing microbubble contrast in the echocardiography laboratory: a sonographer's perspective.

The sonographer is instrumental in incorporating ultrasound contrast into routine clinical use. Optimization of the ultrasound settings and image capture is the primary responsibility of the sonographer, who is the point person of the main contrast team. The responsibilities of the sonographer are varied and include designing a guideline-compliant laboratory environment and being familiar with contrast agents and echocardiographic equipment settings.

Artifacts↗
PubMed2002

[Microbubbles-ultrasound interactions].

Echographic contrast agents have a dynamic behaviour in the ultrasound field which is the base of their detection as well as that of myocardial perfusion quantification.

Contrast Media↗
PubMed2000

Attenuation of ultrasound in porous media with dispersed microbubbles

The dispersion relation for a granular bed with a small amount of fine bubbles is formulated and analyzed. It is assumed that the grain size is much larger than the bubble's radius and that their volume concentration is small. The study is motivated by the problem of acoustic diagnostics of fixed bed chemical reactors operating in multiphase flow regime.

Journal Article↗