Microbubbles concentration

The response echo signal is not always proportional to the microbubble concentration. When it is low, the increase in the concentration of microbubbles (several thousand particles per milHHter) does result in the respective increase... 

Fig. 7. Concentration dependence of ultrasoimd backscatter signal by dispersions with increasing microbubble concentrations (from left to right). Imaging performed using a fimdamental frequency scheme. Samples placed on top of an ultrasoimd tissue phantom (bottom)...

The actual distributions of microbubble diameters in sea water, measured by Weitendorf in the optically defined control volume, ranged between 20 and 117 pm. Within this diameter range, the usual number of measured microbubbles per cm3 was of the order of 10 to 100, yielding an approximate microbubble concentration of 104-105/liter for these ocean experiments (ref. 

Although backscatter is highly dependent on bubble size, microbubbles must have a limited size range. If the bubble is too small, it will be short-lived (i.e., it will collapse under cardiac and systemic pressures). If the bubble is too large (> 10 pm), the pulmonary capillaries will trap it or the bubbles may transiently obstruct tbe capillaries and act as gas emboli therefore, microbubble size is limited from 1 to 5 pm in diameter. At present, all microbubble contrast agents which have been developed for ultrasound are small, gas-filled microbubbles, about 3 pm in size, and all commercial ultrasound contrast media for human applications are encapsulated microbubbles. So, microbubbles are too large to escape the capillaries and, contrary to most X-ray and MRI contrast media, which are rapidly distributed to the extra-vascular, extracellular space, most microbubbles are confined to the vascular bed. The increase in Doppler signal intensity they produce is linearly proportional to relative microbubble concentration. 

Li et al. [76] confirmed that efficacy of phenol degradation depends on microbubble formation. In their experiments, they observed no change in phenol concentration if micro-bubble formation was stopped. The phenol decomposition rate was found maximum in the case when O2 was passed in the solution due to highest micro-bubble formation followed by air and N2 respectively. 

Fig. 8. Concentration dependence of ultrasound backscatter signal by plates coated with a layer of targeted microbubbles. Surface concentrations of microbubbles (as observed by bright-field optical microscopy, bottom) increase from left to right. Imaging performed using a fundamental frequency scheme. Samples placed on top of an ultrasound tissue phantom. Reprinted from Advanced Drug Delivery Reviews v. 37, A.L. Klibanov, Targeted delivery of gas-filled microspheres, contrast agents for ultrasound imaging, p. 145. Copyright, 1999, with permission from Elsevier Science...

Experiments to distinguish between these two possibilities have often involved measurements of ultrasonic attenuation (ref. 5,9,31,32). The popularity of this approach derives in part from the fact that small impurities in liquids, such as suspended particles, have negligible influence on attenuation in comparison with even a very small concentration of microbubbles (ref. 9). (Microbubbles, in contrast to solid particles, appreciably increase the compressibility of a liquid, introducing forms of viscous losses and nonreversible energy exchanges that do not exist in the case of solid particles.) It is therefore of considerable interest that all fresh tap water samples measured by Turner (ref. 9) showed substantial and persistent abnormal (ultrasonic) attenuation, amounting to a minimum of 44% over that of distilled water it was concluded that this result stemmed from the presence of stabilized micron-sized bubbles. 

From all of their above-described studies, Johnson and Cooke conclude that since a significant proportion of the bubbles produced by breaking waves at sea are smaller than 200 pm in diameter (ref. 33,75), and because small bubbles dissolve in air-saturated sea water (when biological surfactant concentrations are low) as a result of surface tension alone, the number of film-stabilized marine microbubbles that are produced by breaking waves can be very large and show strong periodicity (ref. 42). 

Table 4.3). This same negative result for endoglycosidase H was found in a second test employing a higher concentration of both enzymes, and a much longer reaction period with the microbubble glycopeptide surfactant (Table 4.4). 

The trough itself measured 20 x 12 cm, was milled from a block of Teflon, and held approximately 750 ml of liquid. Monomolecular films were prepared on a subphase of (ultrapure) distilled water or on aqueous subsolutions containing varying concentrations of either NaF, HC1, NaOH, thiourea, or dimethyl sulfoxide (DMSO). Aliquots, between 50 and 250 pi, of the ethanol-solubilized microbubble-surfactant mixture were applied slowly to the surface of the subsolution from a Hamilton microsyringe. It was found unnecessary to allow the films to stand for more than 2 min after spreading before taking measurements. Furthermore, following compression or expansion, the surface pressure was observed to remain constant for periods of up to at least 10 min. All measurements were made at 20.0 0.5°C. 

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