Intracoronary air-filled albumin microspheres for myocardial blood flow measurement

Confounding effects of myocardial background intensity and attenuation in contrast echocardiography: an in vivo study

It has been shown in vitro that the time-intensity data of echo contrast agents may be influenced by the background intensity of the myocardium and attenuation at high contrast agent concentrations. In the present study, these effects are evaluated from in vivo data. An effect of background intensity of the myocardium on the determination of the transit rate of the contrast agent could not be demonstrated unambiguously. A statistically significant relation between transit rate and background intensity was found only for intermediate flows in the transmural region. The magnitude of this relation was such that it does not provide a serious source of error. Attenuation and shadowing typically underestimate the transit rate of the contrast agent, which results in overestimation of flow. It is recommended that the lowest doses of contrast agent inducing myocardial opacification should be applied.

Methods for quantifying ultrasound backscatter and two-dimensional video intensity: implications for contrast-enhanced sonography

Quantification of acoustic backscatter energy is believed to be useful for assessing "tissue character" and for quantifying the regional concentration of echo contrast. Measurement of ultrasonic video intensity has been the traditional means of quantifying backscatter energy, with "integrated backscatter" considered the gold standard. The purpose of this work is to review the commonly used methods for quantifying ultrasonic backscatter and to describe the difference between detected backscatter energy and the intrinsic tissue backscatter coefficient. Many of the quantification pitfalls that can lead to erroneous conclusions will also be discussed. A set of eight rubber phantoms with backscatter coefficient from -6 dB to +15 dB relative to liver were imaged at 2.5, 3.5, and 5.0 MHz. Methods for calculating the acoustic backscatter intensity from calibrated video intensity measurements and for calculating the tissue backscatter coefficient are described and tested using equipment from two different manufacturers. A commercially available automatic "acoustic densitometry" system with on-board quantitative integrated backscatter is also evaluated. Ultrasound attenuation and ultrasound system factors were found to strongly influence the detected backscatter intensity using either calibrated video intensity or on-board integrated backscatter. Special system transfer functions and attenuation correction were found to be useful in converting video intensity and integrated backscatter to a measure of the intrinsic tissue backscatter coefficient. With these correction factors, the correlation between the measured tissue backscatter coefficient and the phantom backscatter coefficient was excellent (r = 0.99, intercept 0.0, regression slope essentially 1.0) at all three imaging frequencies with traditional video intensity or on-board integrated backscatter. Uncalibrated video intensity and on-board integrated backscatter have limitations when used in isolation for tissue characterization. Rigorous attention to the imaging parameters and the use of calibration functions are necessary before video intensity measurement or integrated backscatter can be used reliably to measure the tissue backscatter coefficient.

Ultrasound-mediated destruction of contrast agents. Effect of ultrasound intensity, exposure, and frequency

RATIONALE AND OBJECTIVES

Although ultrasound contrast microbubbles theoretically could serve as tracers for the noninvasive quantification of blood flow, results have been inconsistent. Accurate quantification may be limited by ultrasound energy-mediated microbubble destruction. This study examined the effect of different ultrasound delivery parameters on microbubble destruction.

METHODS

Experiments were performed in an in vitro hydraulic perfusion model consisting of a thin-walled rubber tube encased in agar. Ultrasonic parameters tested during different parts of the experiment were (1) intensity, (2) duration, and (3) frequency. Four ultrasound contrast agents: Aerosomes MRX115 (ImaRx Pharmaceuticals Corp., Tucson, AZ), Imagent AF0150 US (Alliance Pharmaceutical Corp., San Diego, CA), Levovist (Berlex Laboratories, Wayne, NJ), and Echogen (Sonus Pharmaceuticals, Bothel, WA) were imaged with three different ultrasound systems: ATL Ultramark AM-9 HDI, Vingmed 800 and Hewlett-Packard 2500.

RESULTS

Microbubble destruction and reductions in reflectivity were noted in all agents tested. Although no significant reductions in counts or reflectivity occurred at 0.3 W/cm2 with any agent, exposure to 25 W/cm2 produced more than 80% reductions in both microbubble counts (P < 0.0001) and reflectivity (P < 0.0001). Declines in reflectivity were increased by longer exposure to ultrasound (P < 0.0001); slower flow through an ultrasound beam (P < 0.0001); continuous, rather than intermittent, imaging (P = 0.0002); use of a higher pulse repetition rate (P < 0.0001); and exposure to 2.5 MHz, rather than 7.5 MHz, ultrasound (P < 0.0001).

CONCLUSIONS

Ultrasound energy-mediated destruction of contrast microbubbles is a function of many factors, including ultrasound intensity, duration, and frequency. Optimization of ultrasound delivery parameters may be used to maximize or minimize the destruction of ultrasound contrast agents.

Contrast echocardiography for assessment of myocardial perfusion

It has been suggested that the myocardial perfusion can be qualitatively and quantitatively assessed by different ultrasound contrast techniques. It has been reported that the intracoronary or intraaortic administration of the ultrasound contrast agents can be used to visualize perfusion defects or to analyze the coronary flow reserve. The perfusion analysis after intracoronary injection of ultrasound contrast agents seems to be established, but there are a lot of open questions. A topographic (qualitative) perfusion analysis with visualization of perfusion defects and perfusion areas or analysis of collaterals has been demonstrated. A quantitative analysis of myocardial blood flow has been described but the existing studies are inconsistent. It is not known which parameters of the contrast wash-out curves should be used for perfusion analysis and if the Stewart-Hamilton curve analysis can be transferred to all ultrasound contrast agents as a model for quantitative myocardial blood flow assessment. The development of the transpulmonary contrast agents for echocardiographic evaluation of left ventricular cavity has the impact for myocardial perfusion imaging. The increase of myocardial intensity does not mean that a qualitative or quantitative perfusion analysis can be clinically used. In this field we have to differentiate between the possibilities of qualitative discrimination of perfusion defects and quantitative perfusion (myocardial blood flow) analysis. The different scanning conditions, the poor transthoracic ultrasound window and insufficient enhancement of the myocardial intensity make it problematic to quantify the myocardial perfusion. At the moment myocardial intensity will be increased after intravenous injection of transpulmonary contrast agents, but the value for perfusion analysis has not been shown. New ultrasound technologies such as second harmonic imaging, power-mode and raw data analysis have to show the clinical importance of these techniques for perfusion analysis in daily clinical routine. The open questions of the perfusion analysis by contrast echocardiography will be discussed in this review article.

Contrast echocardiography: influence of ultrasonic machine settings, mixing conditions, and pressurization on pixel intensity and microsphere size of Albunex solutions in vitro

To use Albunex as a blood-flow tracer, the stability and consistency of microspheres under mixing conditions must be known. This study examined the effects of mixing conditions and machine settings on the size and echogenicity of Albunex solutions in vitro. Acoustic power, log compression, time-gain compensation, and transducer frequency were varied as Albunex solutions were imaged after mixing with magnetic stirring and pressurized. Higher acoustic power and lower transducer frequency decreased mean pixel intensity of Albunex solution images over time. Intensity, size, and number of Albunex microspheres were not significantly different between stirring speeds. The echogenicity of the Albunex solutions decreased with pressurization, and the critical pressure necessary to reduce the intensity to half its initial value increased with the logarithm of concentration (r = 0.91; p < 0.001). The microsphere size decreased with pressurization and remained smaller after pressure release (3.66 +/- 2.13 versus 1.47 +/- 0.95 microns; p < 0.01). These data indicate that acoustic power and transducer frequency may affect the physical properties of Albunex microspheres, decreasing mean videointensity. Pressure sensitivity of Albunex caused the decrease of videointensity and microsphere size.

Relation between air-filled albumin microbubble and red blood cell rheology in the human myocardium. Influence of echocardiographic systems and chest wall attenuation

BACKGROUND

We have previously shown that the intravascular rheology of sonicated air-filled albumin microbubbles is similar to that of red blood cells (RBCs) and that their myocardial transit rate is also similar to that of RBCs in the beating canine heart. In the present study, we tested the hypothesis that the myocardial transit rates of these microbubbles reflect those of RBCs in humans at different coronary flow rates.

METHODS AND RESULTS

RBC and microbubble transit rates were measured in 17 patients undergoing coronary angiography: in 8, measurements were made only at rest, whereas in 9, they were performed both at rest and during a pacing-induced increase in coronary blood flow. A gamma-variate function was used to derive mean RBC and microbubble transit rates from the time-activity and time-intensity plots after the left main injection of RBCs and microbubbles, respectively. There was linear correlation between the myocardial transit rates with both tracers with the slope of the correlation determined by the specific echocardiographic system that was used. Microbubble transit rate consistently overestimated RBC transit rate due to artificial narrowing of the time-intensity curves caused by chest wall attenuation of the echocardiographic signal, which was confirmed through in vitro experiments.

CONCLUSIONS

There is close correlation between air-filled albumin microbubbles and RBC rheology in the human myocardium. The use of these microbubbles in the cardiac catheterization laboratory could, therefore, provide further insights into myocardial blood flow/myocardial blood volume relations in humans.

Effects of shadowing on the time-intensity curves in contrast echocardiography: a phantom study

Literature states that the inability of myocardial contrast echocardiography to assess differences in flow between regions could be related to myocardial shadowing. In this study, we evaluate this effect on the time-intensity curves. In a perfusion phantom, shadowing was induced by high concentrations of a contrast agent (approximately 1 x 10(8) bubbles/mL), and evaluated for different flows (50-270 mL/min). The high concentration resulted in an increase of the video-intensity in regions of interest close to the transducer (2 mm) and a marked reduction in remote areas (20 mm). The peak intensity of the time-intensity curves did not correlate with flow (range r = 0.1, 0.37), while the inverse area under the curve correlated strongly (r = 0.98). The inverse curve width and the decay after peak intensity also correlated excellently with flow (r = 0.99 and range r = 0.97, 0.99). We opt for the decay as transmural flow indicator using contrast echocardiography, since this parameter is least affected by shadowing.

Myocardial contrast echocardiography in experimental coronary artery occlusion with a new intravenously administered contrast agent

A new intravenously administered ultrasound contrast agent was studied in eight dogs during intermittent coronary artery occlusion. The area of the myocardial contrast defect was compared with that of the acute wall motion abnormality induced by coronary occlusion. A close correlation was found between these two independent measures of acute myocardial ischemia. The peak change in myocardial intensity during coronary occlusion was significantly less than for the same segment before ischemia and for a remote nonischemic segment. This new, intravenously administered ultrasound contrast agent can be used to evaluate the spatial distribution of hypoperfused myocardium and should therefore prove valuable in the clinical evaluation of ischemic syndromes.

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.

Sonicated X-ray contrast agents for quantitative myocardial contrast echocardiography--a critical approach

Contrast echocardiography with sonicated radiographic contrast agents has been used for the qualitative and quantitative determination of myocardial blood flow. One major problem has been the size of the microbubbles since only bubbles smaller than 8 microns are expected to pass the capillary bed and larger bubbles may obstruct the capillaries and, thus, alter myocardial blood flow. These techniques have been used for several years, but their reliability has not yet been assessed accurately. Five different methods for the production of sonicated radiographic contrast agents (methods 1-3 from the literature, and 4 and 5 from our laboratory; M1-5) were evaluated for their use in quantitative contrast echocardiography. The sonication of non-ionic X-ray contrast media was performed with a standard titanium probe (20 kHz) for methods 1-4, with variation in the sonication time and the number of sonication jets used for each method. In M5, we used bubbles that were produced by the insufflation of oxygen in the X-ray contrast agent; large (> 8 microns) bubbles were destroyed by sonication at 380 kHz (resonance method). Mean bubble size was determined by computerized videomicroscopy. The effect of bubble size on the backscatter of the ultrasonic signal was calculated for each method. Mean bubble size (+/- 1 SD) ranged between 11.5 +/- 4 microns and 16.1 +/- 14 microns for M1-M5. The best values, i.e., the smallest bubbles, were found with M4 (prepressurized contrast medium). Assuming capillary passage for bubbles smaller than 8 microns, only 14%-48% of the bubbles were smaller than 8 microns (M1-M5). The best results with regard to bubble size (< or = 8 microns) were observed with M5 (48% < or = 8 microns). In regard to the influence of bubble size on the backscatter of the ultrasonic signal, 56%-98.5% of the signal was produced by bubbles larger than 15 microns (M1-5) but the best results were obtained with M4. It is concluded that capillary-passage of sonicated microbubbles (< or = 8 microns) can be expected in only 14%-48% of the bubbles for the five different sonication techniques. More than 50% of all microbubbles produced by these techniques are larger than the expected 8 microns. These large bubbles are responsible for the backscatter of the ultrasonic signal in the vast majority of cases. Thus, the sonication of radiographic contrast agents appears to be inappropriate for the production of uniformly small microbubbles and, thus, this method is not suitable for quantitative measurements of coronary blood flow.