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At least 595 records · Page 33Linked to original sources
PubMed2002

Basic acoustic properties of microbubbles.

Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.

Acoustics↗
PubMed2001

Assessment of left atrial appendage filling pattern by using intravenous administration of microbubbles: comparison between mitral stenosis and mitral regurgitation.

Mitral stenosis (MS) and mitral regurgitation (MR) are the most frequent conditions that cause a dilation and dysfunction of the left atrial appendage (LAA). Despite similarly dilated LAA in patients with MS and MR, the incidence of LAA thrombi and the risk of thromboembolism is different between these patients. The purpose of this study was to characterize the filling pattern of LAA by using intravenous administration of perfluorocarbon-exposed dextrose albumin (PESDA) during transesophageal echocardiographic examination in patients with MS and MR. Twenty-four patients with moderate to severe MS, 12 patients with severe MR, and a control group including 30 patients with conditions other than mitral valve disease underwent transesophageal echocardiographic examination with an intravenous bolus injection of PESDA. LAA emptying and filling velocities and maximal and minimal areas of LAA and LAA ejection fraction were measured. Digital gray-scale intensity (GSI) of the left atrial (LA) and LAA cavity after PESDA injection was measured by off-line analysis. Compared with control patients, patients with MS or MR had larger maximal and minimal areas of LAA and reduced LAA ejection fraction. LAA peak emptying flow velocity was significantly lower in patients with MS compared with those of MR or control patients. LAA peak filling velocity was significantly lower in patients with MS compared with that of control patients. However, there was no significant difference of LAA peak filling velocity between the patients with MS and MR. There was no significant difference of GSI ratio of LAA and LA between patients with MR and control patients; however, GSI ratio of LAA and LA was significantly lower in patients with MS compared with that of MR. The incidence of LAA spontaneous echo contrast and LAA thrombi in patients with MS was significantly higher than that of the patients with MR and control subjects (P <.005). Despite similarly dilated LAA area and depressed contractile function of LAA in patients with MS and MR compared with control patients, profoundly impaired LAA filling with resultant flow stasis was demonstrated by contrast echocardiography in patients with MS. These findings may explain the higher incidence of LAA spontaneous echo contrast and thrombus in patients with MS.

Aged↗
PubMed1997

Gray-scale second harmonic imaging of the liver with galactose-based microbubbles.

RATIONALE AND OBJECTIVES: The authors evaluate the efficacy of SHU 508A, a galactose-based contrast agent used in gray-scale second harmonic imaging in vitro and in vivo experiments. METHODS: A Toshiba prototype harmonic imaging system (2.5/5.0 MHz) was used with SHU 508A in a phantom experiment and to image the liver in five healthy rabbits and one rabbit that had VX-2 tumors in the liver. RESULTS: In the second harmonic imaging, most of the fundamental components of the backscattered echo were eliminated, and good images with high contrast were obtained in the phantom experiment. Liver parenchyma was enhanced clearly in all rabbits at 0.3 mL/kg, an effect that lasted for 90 seconds. The tumor, which was mostly necrotic, was depicted clearly as a negative enhanced area surrounded by enhanced healthy liver. CONCLUSIONS: Gray-scale second harmonic imaging is a promising new method for visualization of perfusion of organs.

Animals↗
PubMed2001

Perfectly monodisperse microbubbling by capillary flow focusing.

Here we report a simple microfluidics phenomenon which allows the efficient mass production of micron size gas bubbles with a perfectly monodisperse and controllable diameter. It resorts on a self-excited breakup phenomenon (which locks at a certain frequency) of a short gas microligament coflowing in a focused liquid stream. In this work, we describe the physics of the phenomenon and obtain closed expressions for the bubble diameter as a function of the liquid and gas properties, geometry, and flow parameters, from a large set of experimental results.

Biochemistry↗
PubMed2006

Director-configurational transitions around microbubbles of hydrostatically regulated size in liquid crystals.

A high-pressure technique is introduced which allows a continuous variation of the inclusion size in liquid crystal colloids. We use a nematic liquid crystal host into which micrometer-sized gas bubbles are injected. By applying hydrostatic pressures, the diameter of these gas bubbles can be continuously decreased via compression and absorption of gas into the host liquid crystal, so that the director configurations around a single bubble can be investigated as a function of the bubble size. The theoretically predicted transition from a hyperbolic hedgehog to a Saturn-ring configuration, on reduction of the particle size below a certain threshold, is confirmed to occur at the radius of a few micrometers.

Journal Article↗
PubMed2001

Techniques for perfusion imaging with microbubble contrast agents.

The acoustic properties of ultrasound contrast agents vary widely with agent composition and insonation conditions. For contrast imaging, methods are required to match RF and Doppler processing to each combination of transmission parameters and agent and tissue properties. We propose a method that uses the measured or modeled echoes from agent and tissue to specify directly the characteristics of RF and Doppler filters for contrast imaging. The proposed method is sufficiently general to cover most common imaging techniques including harmonic greyscale, Doppler, and pulse inversion imaging. Using this method, sample filters were designed to detect myocardial perfusion with the contrast agent Optison (Mallinckrodt Medical, St. Louis, MO) under selected imaging conditions. Simplified power Doppler filtering, using a weighted sum of the Doppler samples, matched the performance of more complicated matrix filters. By coordinating the selection of RF and Doppler filters rather than designing these filters sequentially, agent-to-tissue contrast was increased by up to 3.9 dB. Under some conditions, fundamental RF filtering outperformed harmonic filtering for intermittent Doppler imaging.

Contrast Media↗
PubMed1991

The effects of the microbubble suspension SH U 454 (Echovist) on ultrasound-induced cell lysis in a rotating tube exposure system.

Human erythrocytes resuspended at different hematocrits in autologous plasma at 37 degrees C were exposed to the therapeutic intensities of continuous-wave 0.75-MHz ultrasound in vitro in a rotating tube exposure apparatus designed to maximize the destructive effects of cavitational activity. Provided that large numbers of additional gas bubbles had not been introduced during the various preparative and manipulatory procedures, the addition of Echovist at final concentrations comparable with those currently being used for clinical investigations resulted in a statistically significant increase in the amount of cell lysis in vitro in those samples having hematocrits less than 2%. The amount of cell lysis produced at any given ultrasound intensity decreased with increasing hematocrit in both the controls and the cell suspensions containing Echovist until it was virtually zero in both cases at hematocrits of 5.5% or greater. The addition of Echovist to samples that already contained large numbers of stabilized gas bubbles and/or had hematocrits greater than 5.5% produced no detectable cell lysis even at ultrasonic intensities as high as 3 W/cm 2 spatial average, temporal average (SATA). It is therefore unlikely that Echovist would cause appreciable amounts of cell lysis when the gas bubbles were being exposed to ultrasound under the conditions used for clinical investigations in vivo.

Adult↗
PubMed1996

Spontaneous Microbubbles in Transit in the Right Cardiac Chambers.

We present two patients with echocardiographically detected small echoes passing through the right heart cavities, consistent with the clinical picture of thromboemboli. After anticoagulant treatment, pulmonary artery pressure fell and the echocardiographic picture of small echoes in transit disappeared. In the authors' opinion, the detected echoes passing through the right heart represented microthromboemboli in transit. (ECHOCARDIOGRAPHY, Volume 13, July 1996)

Journal Article↗
PubMed2000

Oscillations of polymeric microbubbles: effect of the encapsulating shell

A model for the oscillation of gas bubbles encapsulated in a thin shell has been developed. The model depends on viscous and elastic properties of the shell, described by thickness, shear modulus, and shear viscosity. This theory was used to describe an experimental ultrasound contrast agent from Nycomed, composed of air bubbles encapsulated in a polymer shell. Theoretical calculations were compared with measurements of acoustic attenuation at amplitudes where bubble oscillations are linear. A good fit between measured and calculated results was obtained. The results were used to estimate the viscoelastic properties of the shell material. The shell shear modulus was estimated to between 10.6 and 12.9 MPa, the shell viscosity was estimated to between 0.39 and 0.49 Pas. The shell thickness was 5% of the particle radius. These results imply that the particles are around 20 times more rigid than free air bubbles, and that the oscillations are heavily damped, corresponding to Q-values around 1. We conclude that the shell strongly alters the acoustic behavior of the bubbles: The stiffness and viscosity of the particles are mainly determined by the encapsulating shell, not by the air inside.

Journal Article↗