In vitro study of the effects of volume changes on parameters of the radiofrequency amplitude versus time curve with sonicated albumin

The powerful microbubble: from bench to bedside, from intravascular indicator to therapeutic delivery system, and beyond

This review discusses the development, current applications, and therapeutic potential of ultrasound contrast agents. Microbubbles containing gases act as true, intravascular indicators, permitting a noninvasive, quantitative analysis of the spatial and temporal heterogeneity of blood flow and volumes within the microvasculature. These shelled microbubbles are near-perfect reflectors of acoustic ultrasound energy and, when injected intravenously into the bloodstream, reflect ultrasound waves within the capillaries without disrupting the local environment. Accordingly, microbubble ultrasound contrast agents are clinically useful in enhancing ultrasound images and improving the accuracy of diagnoses. More recently, ultrasound contrast agents have been used to directly visualize the vasa vasorum and neovascularization of atherosclerotic carotid artery plaques, thus suggesting a new paradigm for diagnosis and treatment of atherosclerosis. Future applications of these microscopic agents include the deliver of site-specific therapy to targeted organs in the body. Medical therapies may use these microbubbles as carriers for newer therapeutic options.

The effect of ultrasound frame rate on perfluorocarbon-exposed sonicated dextrose albumin microbubble size and concentration when insonifying at different flow rates, transducer frequencies, and acoustic outputs

The purpose of this article was to compare the effects of 1 and 30 Hz frame rates on perfluorocarbon-exposed sonicated dextrose albumin microbubble size and concentration in a flow cell containing either saline or blood at 37 degrees C. Microbubble size and concentration of perfluorocarbon-exposed sonicated dextrose albumin were measured after insonation at different acoustic outputs, transducer frequencies, and flow rates with the use of the two different frame rates and compared with no ultrasound exposure. At 2.0 MHz insonation frequency, microbubble concentration was significantly reduced with the use of a 30 Hz frame rate and peak negative pressures of 1.1 megaPascal (mPa). This destruction did not occur when using a lower acoustic output, a 1 Hz frame rate, or when flow rate was increased to 100 cc/min. One-hertz frame rates at 2.0 MHz resulted in a significantly larger mean microbubble size than 30 Hz or no ultrasound in both saline and blood, which was in part due to selective destruction of smaller microbubbles. These findings indicate that 30 Hz frame rates destroy perfluorocarbon-exposed sonicated dextrose albumin microbubbles only at higher diagnostic acoustic outputs. A 1 Hz frame rate prevents this destruction, especially destruction of larger (> 5.0 microns) microbubbles.

Increased ultrasound contrast and decreased microbubble destruction rates with triggered ultrasound imaging

Although transient myocardial contrast imaging has been able to produce visually evident myocardial contrast in animals and humans with very low intravenous doses of perfluorocarbon-exposed sonicated dextrose albumin (PESDA) microbubbles, the mechanism for improved contrast remains unclear. In this study we devised a flow chamber that measured the concentration of PESDA microbubbles that remained after exposure to diagnostic ultrasound pressures of 0.9 to 1.9 MPa and frequencies of 2.0, 2.5, and 3.5 MHz (first and second harmonic for 2.0 MHz), which were delivered at either 30 Hz (frames per second), 0.5 to 1.0 Hz, or without any ultrasound transmission. The videointensity within the flow chamber was also measured at 0, 20, 40, and 100 ml/min flow rates with the flow loop closed (i.e., constant microbubble concentration) with both triggered (0.5 to 1.0 Hz) and conventional (30 Hz) frame rates. The effluent microbubble concentration was significantly larger when PESDA was exposed to either no ultrasound or 0.5 to 1.0 Hz ultrasound. Furthermore, the videointensity of a constant number of microbubbles was significantly greater with 0.5 to 1.0 Hz (triggered) compared with 30 Hz (conventional) frame rates at each transmit frequency. The greatest difference was noted with the lower 2.0 MHz transmit frequency and the 20 ml/min flow rate, especially when a second harmonic receiving frequency was used. We conclude that the mechanism for improved contrast with triggered ultrasound imaging is because of both less microbubble destruction and increased videointensity from a constant number of microbubbles. Lower transducer frequencies and lower flow rates result in the greatest improvement in videointensity with triggered ultrasound transmission.

Evidence for a relation between inspired gas mixture and the left ventricular contrast achieved with Albunex in a canine model

BACKGROUND

In a previous experiment, a marked reduction in the right- and left-sided contrast effect of Albunex was noted in an intubated animal spontaneously breathing isoflurane in 100% oxygen. The theory suggests that the time course of echogenicity of microbubbles in liquid is dependent on the pressure and the gradients of dissolved gases. The present set of experiments tested whether the loss of contrast occurs at commonly used therapeutic concentrations of inspired oxygen.

HYPOTHESIS

This research tested the hypothesis that the left ventricular (LV) contrast effect achieved with intravenous injection of the ultrasound contrast agent Albunex is related to the inspired oxygen content.

METHODS

Intubated dogs were maintained in a spontaneously respiring anesthetic state on isoflurane and mixtures of oxygen (12-50%) in nitrogen. FIO2 was held steady for 15 min prior to injection of 0.08 ml/kg of Albunex. The contrast effects were recorded from a transthoracic short-axis view. Left and right ventricular brightness curves were generated from digitized sequences of end-diastolic frames. The minimum and maximum brightness and area under the time-brightness curves were determined.

RESULTS

The LV maximum brightness and area under the curve showed significant negative correlations (p = < 0.004) with the FIO2, while the minimum brightness showed a significant positive correlation (p = < 0.002). No significant correlations were found for the right ventricular brightness parameters.

CONCLUSIONS

These findings show an important relationship between the FIO2 and loss of the contrast effect of Albunex. This loss occurs at oxygen concentrations in the therapeutic range, but could be overcome by increasing the dose of Albunex. The mechanism is likely related to an outward nitrogen gradient causing a loss of echogenicity. The clinical implication is that patients on supplemental oxygen may require higher doses of Albunex to achieve optimal opacification.

Myocardial contrast echocardiography: relation of collateral perfusion to extent of injury and severity of contractile dysfunction in a canine model of coronary thrombosis and reperfusion

OBJECTIVES

This study sought to determine whether myocardial contrast echocardiography could be used to detect and quantitate collateral blood flow capable of limiting the effects of ischemia in an experimental model of coronary thrombosis and reperfusion.

BACKGROUND

Myocardial contrast echocardiography has been used to assess collateral blood flow in humans, but this technique has not been extensively validated in the experimental laboratory.

METHODS

Myocardial ischemia occurred after electrically induced left circumflex coronary artery thrombosis in a canine model. Ischemia was intensified by administration of vasodilators. Reperfusion was induced with recombinant tissue-type plasminogen activator. Myocardial perfusion was assessed with contrast echocardiography and radiolabeled microspheres. Infarct size was determined by histochemical staining methods. Myocardial samples were evaluated histologically.

RESULTS

The dogs were classified into two groups on the basis of contrast echocardiographic detection of perfusion in the ischemic region: those with (n = 13) and without collateral flow (n = 10). Collateral perfusion detected by contrast echocardiography paralleled changes detected by radiolabeled microspheres during thrombosis and vasodilator administration. A 91% agreement was observed between the two techniques in detecting collateral flow > 0.3 ml/min per g (p < 0.0001). Collateral perfusion correlated directly with radial shortening fractions of the ischemic myocardium (p < 0.01). Recovery of function after reperfusion was faster, infarct size was smaller (mean [+/- SD] 4 +/- 1% vs. 11 +/- 3%, p = 0.05), and histopathologic injury was less in dogs with than without collateral flow, respectively (p < 0.05).

CONCLUSIONS

Myocardial contrast echocardiography can identify physiologically significant collateral vessels capable of limiting the degree of ischemic damage during coronary thrombosis. The magnitude of collateral flow and the change in flow induced by vasodilators can be assessed and compares favorably with the microsphere standard.