Properties Of Microbubbles Research Articles

Zeitvarianzabbildung, ein Verfahren zur Detektion von Ultraschallkontrastmitteln

Ultrasound contrast agents which are injected into the blood stream mostly use encapsulated microbubbles as acoustical scatterers. The acoustic properties of microbubbles differ significantly from those of scatterers in tissue. When insonified, bubbles oscillate in a nonlinear mode, which is determined by a mechanical resonance frequency. The insonification can cause changes of the bubble diameters, complete or partial destruction of the shells as well as splitting or fusion of bubbles. Received echoes resulting from sequences of short pulses reflect the alterations of the acoustic properties of the bubbles and, therefore, allow the formation of high resolution images which show the distribution of the contrast agent within the tissue. Consequently, this new imaging technique has the potential to improve the visualization of small vessels and the assessment of perfusion conditions.

Nonlinear properties of microbubbles and applications to medical ultrasound imaging

Gas bubbles smaller than 10 μm dissolve within milliseconds even in gas-saturated liquids, but coatings, etc. can prevent dissolution. These stabilized microbubbles serve as transpulmonary contrast agents. When only their linear properties are employed, physical laws limit the diagnostic potential to detection in large diameter vessels. Nonlinear acoustic properties of contrast agents permit very important additional applications. Nonlinearity arises from bubble pulsations, shell properties, finite amplitude waves, and electronic hardware. The first two sources can dominate, allowing new diagnostic imaging modes sufficiently sensitive to detect isolated microbubbles in vivo. Harmonic imaging modes detect 2nd harmonics of the transmit frequency, mainly from microbubbles. Acoustic Emission modes destroy bubble shells by one pulse of moderate amplitude. Consequently, the free bubbles respond strongly to this and other pulses before dissolving. Observed lifetimes of 1–20 ms are enough for harmonic or conventional Doppler detection. Typically, bubbles are replenished too slowly to get similar signal intensity for more than one frame at normal rates. Optimal pulse timing should allow new microbubbles to move close to the beam axis before the next pulse arrives. Applications may be counting methods, flow tagging, and quantification of tissue perfusion, possibly combined with single-shot three-dimensional acquisition and HI modes.

Laboratory simulation of geogas microbubble flow

Preliminary laboratory tests provided first data on the behavior of gas microbubbles through porous media in the framework of the geogas theory. Under experimented conditions with laboratory equipment arranged for pressure controlled gas-tracer injection and sampling, gas microbubbles moved up to ten times faster than singlephase flow in dry media under the same injection pressure. Microbubbles were determined to be very sensitive to changes in injection pressure and their terminal velocity seems to be described with good approximation by the Stokes formula. The capability of microbubbles to pick up and transport upward for short distances solid ultra-small particles (metallic and radionuclide compounds) has been proved. Results are consistent with a time-dependent process linked to the transport properties of microbubbles (e.g., flotation), such as that reported by some authors.

Formation and coalescence properties of microbubbles

Fermentations that involve a gaseous substrate are inherently rate-limited by mass transfer from the gaseous to the liquid phase. Aerobic and synthesis-gas fermentations are primary examples of such mass-transfer-limited fermentations. The traditional approach to enhance gas-to-liquid mass transfer is to increase the impeller rate, thereby increasing the interfacial area available for mass transfer. However, this approach results in a dramatic increase in power consumption, especially for large-scale systems, because power consumption is proportional to the impeller rate to the third power and the impeller diameter to the fifth power. Microbubble dispersions offer an alternative, providing high interfacial area with the potential for low power input. In aerobic fermentations with yeast, the transport of oxygen from a gas to a liquid, reported in terms of k{sub 1}a, was found to be greater by an order of magnitude using microbubble sparging oven conventional air sparging. Microbubbles are surfactant-stabilized, spherical bubbles with diameters on the order of 100 {mu}m. By comparison, gas bubbles found in conventional sparging units range from 3-5 mm. The surfactant layer surrounding the microbubble generates a diffuse electric double layer that acts to repel other bubbles and prevent coalescence. A microbubble dispersion exhibits colloidal properties and is stable enoughmore » to be pumped. This article examines the effects of surfactant type and concentration on microbubble formation, drainage, and stability. Effects of salts on microbubbles dispersions are examined as well. This information is needed to evaluate the utility of microbubble dispersions for biotechnological applications.« less

Myocardial contrast echo effect: The dilemma of coronary blood flow and volume

Despite the useful information provided by myocardial contrast echocardiography, the meaning of myocardial contrast intensity remains elusive. This review is meant to define the contribution of physical and biologic factors in producing myocardial contrast and to elucidate the relative roles of coronary blood flow and intramyocardial blood volume in determining contrast effect. The main physical factors influencing the contrast echo effect include the properties of microbubbles as scattering elements (mainly their radius, compressibility, stability and concentration), electronic signal processing, instrument setting and contrast-induced signal attenuation. The effect of these factors can be limited by an appropriate experimental or clinical setup. Biologic factors are less easily controllable, and changes in coronary blood flow and alterations in myocardial blood volume appear to be the main determinants of myocardial contrast intensity. Moreover, these factors influence contrast intensity in opposite directions. Both the area under the time-intensity curve and the mean transit time of myocardial contrast are inversely related to coronary blood flow but directly related to myocardial vascularity and blood volume. Therefore, an increase in coronary flow not accompanied by an increase in myocardial vascularity and volume is accompanied by a decrease in the area under the curve and mean transit time of contrast. Conversely, an increase in coronary flow mediated by augmented myocardial vascularity and volume will produce an increase in the area under the curve and mean transit time. A better understanding of the physical and biologic determinants of contrast echo intensity will be fundamental in the clinical application of new agents and technologies.

703-3 Second Harmonic Mode Imaging Improves Contrast Agent [SHU 508-A] Visualization in an In Vitro Model

Non invasive myocardial contrast echocardiography requires an ultrasoundimaging mode capable of detecting small amounts of a crossing lungs contrastagent (CA) which reach the cardiac muscle. At present, commercialB-Mode imaging can not visualize CA in myocardium after intravenous injectionand transpulmonary passage. Second harmonic Mode imaging (SHM) uses resonance properties of microbubbles to produce a gray scale imageof CA. We tested an prototype ultrasound machine (Acuson) using SHM to detect CA SHU 508-A (Schering AG, Berlin) flow in an in vitro model at variousconcentrations, For each CA injection, videointensity in arbitrary Units(VU), in SHM and B-Mode was assessed:SHU 508-Ainjected volume (ml)VUB-ModeVUSHMVU% increase0.1251932680.253151670.55369301.06987262.082102244.010011515SHM vs. B-Mode p < 0.0001 SHM vs. B-Mode p < 0.0001 SHM is more sensitive than B-mode in detecting SHU 508-A, particularly at lower CA concentrations. SHM has a potential for the evaluation of low CA concentration in clinical setting and therefore in myocardial perfusion studies after CA intravenous injection.