Interactions between microbubbles and ultrasound: in vitro and in vivo observations

Stimulated acoustic emission detected by transcranial color doppler ultrasound : a contrast-specific phenomenon useful for the detection of cerebral tissue perfusion

BACKGROUND AND PURPOSE

Experimental and clinical data suggest that insonation of echo-contrast agents with high acoustical power produces disintegration of microbubbles, resulting in a pseudo-Doppler phenomenon called stimulated acoustic emission (SAE). The purpose of this study was to investigate whether SAE might be detected by transcranial color Doppler imaging and whether these signals might be used for cerebral tissue perfusion measurements.

METHODS

Nonmoving microbubbles (SHU 563 A) were insonated in vitro through the temporal parts of a human cadaver skull, and contrast signals were detected by velocity-coded color Doppler and power Doppler recordings. Transcranial color as well as power Doppler investigations were performed in 10 healthy volunteers with the echo-contrast agent Levovist (SHU 508 A).

RESULTS

Color Doppler signals indicating SAE were observed in vitro and in transcranial human investigations. These signals were characterized by a mosaic of color Doppler pixels ranging over the full color scale. Apparent velocity information and spatial distribution of SAE signals changed from image frame to image frame. In the experimental model, the intensity of SAE signals decreased exponentially over time. With an increase of acoustic power, there was a significant increase of the maximum signal intensity (P<0.01) and a significantly shortened signal duration (P<0.01), consistent with stronger and more rapid disintegration. In humans, SAE signals were clearly detected in cerebral tissue regions. The intensity of SAE signals in those regions (eg, temporal cortex, 3.7+/-1.2 dB) was approximately 8 times lower than the signal enhancement in the major cerebral arteries (eg, in the MCA, 29.5+/-5.6).

CONCLUSIONS

Echo-contrast specific color Doppler signals known as SAE are detectable by transcranial color and power Doppler sonography. Signals due to SAE might represent tissue perfusion, thereby providing a method for imaging flow with transcranial ultrasound.

Echocardiographic destruction of albumin microbubbles directs gene delivery to the myocardium

BACKGROUND

The noninvasive, tissue-specific delivery of therapeutic agents to the heart would be a valuable clinical tool. This study addressed the hypothesis that albumin-coated microbubbles could be used to effectively deliver an adenoviral transgene to rat myocardium by ultrasound-mediated microbubble destruction.

METHODS AND RESULTS

Recombinant adenovirus containing beta-galactosidase and driven by a constitutive promoter was attached to the surface of albumin-coated, perfluoropropane-filled microbubbles. These bubbles were infused into the jugular vein of rats with or without simultaneous echocardiography. Additional controls included ultrasound of microbubbles that did not contain virus, virus alone, and virus plus ultrasound. One group underwent ultrasound-mediated destruction of microbubbles followed by adenovirus infusion. Rats were killed after 4 days and examined for beta-galactosidase expression. The hearts of all rats that underwent ultrasound-mediated destruction of microbubbles containing virus showed nuclear staining with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside substrate, indicating expression of the transgene. None of the control animals showed myocardial expression of the beta-galactosidase transgene. By quantitative analysis, beta-galactosidase activity was 10-fold higher in the treated group than in controls (P<0.0001).

CONCLUSIONS

Ultrasound-mediated destruction of albumin-coated microbubbles is a promising method for the delivery of bioactive agents to the heart.

Quantification of mitral regurgitation in the cardiac catheterization laboratory with contrast echocardiography

BACKGROUND

There is no method of quantifying the severity of mitral regurgitation (MR) from injection of tracer directly into the left ventricular (LV) cavity, a method commonly used in the cardiac catheterization laboratory.

METHODS AND RESULTS

We used a previously validated mathematical model that derives regurgitant fraction (RF) from the relative tracer washout from the left atrial (LA) and LV cavities. Thirty-nine patients referred for diagnostic cardiac catheterization with clinical evidence of possible MR were included in the study. Five milliliters of a microbubble mixture was power-injected into the LV during simultaneously performed contrast echocardiography. Relative changes in background-subtracted video intensity were measured from the LV and LA, and the resultant model-derived RF was correlated with the severity of MR on cineangiography. The severity of MR ranged from 0 to 4+ on cineangiography with corresponding model-derived RF of 0 to 0.69 on contrast echocardiography. A close linear relation was noted between angiographic severity of MR and model-derived RF on contrast echocardiography (y = 0.1x + 0.03, r = 0.89, P <.001). Contrast echocardiography was more sensitive than cineangiography for detecting mild MR.

CONCLUSIONS

We describe a new method of measuring the severity of MR in the cardiac catheterization laboratory. Apart from being quantitative, this method can be safely used during cardiac catheterization in patients in whom iodinated contrast agents may be potentially harmful.

Feasibility of mitral flow assessment by echo-contrast ultrasound, part I: determination of the properties of echo-contrast agents

Data on the ultrasonic properties of commercially available contrast agents are limited by being instrument-dependent, especially with regard to their backscattering properties. The present work describes methods of measurements that provide instrument-independent estimations of a contrast agent's attenuation coefficient and integrated backscatter index and provide them as functions of its concentration. The two studied commercially available contrast agents were Albunex and Levovist SHU 508-A, both representative of agents in common use for echocardiography. The attenuation coefficients and integrated backscatter indices of both agents were found to be a linear function of their concentrations. Proportionality coefficients +/- their standard deviations are provided. Actually, square root values of the averaged backscatter indices normalized with respect to the rms of the reference signal were determined. The coefficients of proportionality were found to be: C(A) = 3.11+/-0.1813 dB/mm; C(L) = 0.07+/-0.005 dB/mm for attenuation coefficients of the Albunex and Levovist contrast agents, respectively, and the corresponding values for backscattering were: D(A) = 0.07+/-0.0054; D(L) = 0.02+/-0.0012. Being apparatus-independent, the findings of the study are important prerequisites for the use of these echo-contrast agents as an indicator in research for a quantitative assessment of blood flow.

Quantitative 3-dimensional contrast echocardiographic determination of myocardial mass at risk and residual infarct mass after reperfusion: experimental canine studies with intravenous contrast agent NC100100

Two-dimensional contrast echocardiography has been shown to enable the evaluation of myocardial perfusion abnormalities. However, its ability to quantify a regional myocardial mass is limited. The goal of this study was to examine the quantitative value of 3-dimensional echocardiography (3DE) in the estimation of myocardial mass at risk, salvaged mass, and residual infarct mass after intravenous injection of contrast. We created acute coronary occlusion, followed by reperfusion in 10 dogs. Three-dimensional echocardiographic data were acquired at the end of each stage, and the perfusion defect mass and dysfunctional mass were measured. The true mass at risk and infarct mass were determined by anatomic methods. The anatomic mass at risk (x) (27.1+/-14.6 g or 23.8%+/-9.7% of the left ventricle [%LV]) correlated well with the 3DE-determined perfusion defect mass (y) during coronary occlusion (y = 0.5x+8.9; r = 0.90; P<.001; mean difference -4.8+/-8.1 g; or y = 0.7x + 6.5; r = 0.83, P<.01; mean difference -0.1+/-5.4 %LV). Good correlation was also found between the anatomic infarct mass (x) (9.3+/-8.1 g or 9.1+/-8.8 %LV) and the 3DE perfusion defect mass after reperfusion (y) (y = 1.2x+1.2; r = 0.93; P<.001; mean difference 2.3+/-4.0 g; or y = 1. 3x, r = 0.98, P <.0001; mean difference 2.7+/-3.7 %LV). The salvaged mass was 13.6 +/-11.0 %LV from anatomic methods and 14.2+/-13.0 %LV by 3DE. To conclude, with the use of intravenous contrast, 3DE could quantify the actual mass at risk during acute ischemia, and in the setting of reperfusion, the residual infarct mass and salvaged mass.

Comparison of myocardial contrast echocardiography with NC100100 and (99m)Tc sestamibi SPECT for detection of resting myocardial perfusion abnormalities in patients with previous myocardial infarction

OBJECTIVE

To determine whether myocardial contrast echocardiography (MCE) following intravenous injection of perfluorocarbon microbubbles permits identification of resting myocardial perfusion abnormalities in patients who have had a previous myocardial infarction.

PATIENTS AND INTERVENTIONS

22 patients (mean (SD) age 66 (11) years) underwent MCE after intravenous injection of NC100100, a novel perfluorocarbon containing contrast agent, and resting (99m)Tc sestamibi single photon emission computed tomography (SPECT). With both methods, myocardial perfusion was graded semiquantitatively as 1 = normal, 0.5 = mild defect, and 0 = severe defect.

RESULTS

Among the 203 normally contracting segments, 151 (74%) were normally perfused by SPECT and 145 (71%) by MCE. With SPECT, abnormal tracer uptake was mainly found among normally contracting segments from the inferior wall. By contrast, with MCE poor myocardial opacification was noted essentially among the normally contracting segments from the anterior and lateral walls. Of the 142 dysfunctional segments, 87 (61%) showed perfusion defects by SPECT, and 94 (66%) by MCE. With both methods, perfusion abnormalities were seen more frequently among akinetic than hypokinetic segments. MCE correctly identified 81/139 segments that exhibited a perfusion defect by SPECT (58%), and 135/206 segments that were normally perfused by SPECT (66%). Exclusion of segments with attenuation artefacts (defined as abnormal myocardial opacification or sestamibi uptake but normal contraction) by either MCE or SPECT improved both the sensitivity (76%) and the specificity (83%) of the detection of SPECT perfusion defects by MCE.

CONCLUSIONS

The data suggest that MCE allows identification of myocardial perfusion abnormalities in patients who have had a previous myocardial infarction, provided that regional wall motion is simultaneously taken into account.

New imaging technology: measurement of myocardial perfusion by contrast echocardiography

Myocardial perfusion imaging has long been a goal for the non-invasive echocardiographic assessment of the heart. However, many factors at play in perfusion imaging have made this goal elusive. Harmonic imaging and triggered imaging with newer contrast agents have made myocardial perfusion imaging potentially practical in the very near future. The application of indicator dilution theory to the coronary circulation and bubble contrast agents is fraught with complexities and sources of error. Therefore, quantification of myocardial perfusion by non-invasive echocardiographic imaging requires further investigation in order to make this technique clinically viable.

Myocardial perfusion by contrast echocardiography: diagnosis of coronary artery disease using contrast-enhanced stress echocardiography and assessment of coronary anatomy and flow reserve

The advent of intravenous contrast agents, and newer ultrasound technology to enhance their detection, promises to improve and augment our conventional stress echocardiographic practice by improving diagnostic accuracy and providing novel information regarding myocardial perfusion and functional assessment of the coronary vasculature. The combination of intravenous contrast and harmonic stress echocardiography is a powerful tool for improved wall motion analysis through enhanced image quality, routinely permitting the evaluation of patients with suboptimal images. In this era of cost containment, we await studies in large populations addressing resource utilization and cost-effectiveness to determine if, indeed, all patients presenting with stress echocardiography should receive contrast. Myocardial perfusion can be observed using the technique, but the complex interactions of microbubbles and ultrasound in patients must be understood more fully before its implementation becomes routine practice. Non-invasive imaging of coronary arteries using contrast-enhanced transthoracic harmonic echo/Doppler promises to expand the field of diagnostic and experimental echocardiography, bringing new insight into the pathophysiology of ischemic and non-ischemic heart disease. The continued development of newer contrast agents and refinement of ultrasound imaging equipment ensures that the applications of contrast echocardiography in the assessment of CAD will continue to increase.

Quantification of microbubble destruction of three fluorocarbon-filled ultrasonic contrast agents

The assessment of myocardial blood velocity using ultrasonic contrast agents is based on the premise that the vast majority of contrast microbubbles within a myocardial region can be destroyed by an acoustic pulse of sufficient magnitude. Determination of the period of time after destruction that a region of myocardium needs to reperfuse may be used to assess myocardial blood velocity. In this study, we investigated the acoustic pressure sensitivity of three solutions of intravenous fluorocarbon-filled contrast agents and the magnitude of acoustic pulse required to destroy the contrast agent microbubbles. A novel tissue-mimicking phantom was designed and manufactured to investigate the relationships between mean integrated backscatter, incident acoustic pressure and number of frames of insonation for three fluorocarbon-filled contrast agents (Definity(R), Optison(R), and Sonazoid(R), formerly NC100100). Using a routine clinical ultrasound (US) scanner (Acuson XP-10), modified to allow access to the unprocessed US data, the contrast agents were scanned at the four acoustic output powers. All three agents initially demonstrated a linear relationship between mean integrated backscatter and number of frames of insonation. For all three agents, mean integrated backscatter decreased more rapidly at higher acoustic pressures, suggesting a more rapid destruction of the microbubbles. In spite of the fact that there was no movement of microbubbles into or out of the beam, only the results from Definity(R) suggested that a complete destruction of the contrast agent microbubbles had occurred within the total duration of insonation in this study.

Contrast harmonic color Doppler left ventriculography: machine-interpreted left ventricular ejection fraction compared with equilibrium-gated radionuclide ventriculography

BACKGROUND

Multi-gated acquisition (equilibrium-gated radionuclide ventriculography) (MUGA) is considered the gold standard for measuring left ventricular ejection fraction (LVEF) because it is accurate, machine interpreted, and reproducible. Echocardiographic LVEF measurements are subject to variability in image acquisition and interpretation and to the limitations of 2-dimensional (2D) versus 3-dimensional imaging.

GOAL

The shortcomings of traditional echocardiography may be addressed by combining multiplane 2D harmonic imaging, echocardiographic contrast, color Doppler ultrasonography, and digital image processing to create a new imaging modality: contrast harmonic color Doppler left ventriculography.

METHODS

We compared the accuracy of a new method for measuring LVEF that allows for machine interpretation and uses contrast-enhanced intermittent harmonic color Doppler ultrasonography (CHCD). Quantitative LVEF measurements by hand-traced harmonic 2D echocardiography, contrast-enhanced harmonic 2D echocardiography, CHCD, and machine-interpreted CHCD were compared with MUGA in 35 patients.

RESULTS

Contrast-enhanced intermittent harmonic color Doppler provided images with vivid endocardial definition in all patients, but hand-traced harmonic 2D echocardiography and contrast-enhanced harmonic 2D echocardiography had inadequate images in 9% of patients. The MUGA LVEF range was 0. 09 to 0.70. All echocardiographic methods showed excellent correlation with the MUGA LVEF (R (2) > 0.96), but the CHCD method had the best limits of agreement.

CONCLUSIONS

Contrast-enhanced intermittent harmonic color Doppler LVEF correlates with MUGA at least as well as traditional noncontrasted echocardiography, but it provides diagnostic images in a greater proportion of patients. The CHCD images have vivid endocardial delineation and can be machine interpreted.

Quantification of the physiological relevance of a coronary stenosis using myocardial contrast echocardiography

MCE can be used in the catheterization laboratory or in the operating room to provide rapid assessments of the functional significance of a coronary stenosis from direct arterial injections of microbubbles. In the past few years, the development of more stable microbubble contrast agents, and a better understanding of the interactions between ultrasound and microbubbles have led to the development of a truly non-invasive approach to quantify MBF using venous infusions. Furthermore, additional insights into the physiology of coronary stenosis, particularly as it affects MBV, have been obtained using MCE.

Contrast echocardiography: current and future applications

Recent updates in the field of echocardiography have resulted in improvements in image quality, especially in those patients whose ultrasonographic (ultrasound) evaluation was previously suboptimal. Intravenous contrast agents are now available in the United States and Europe for the indication of left ventricular opacification and enhanced endocardial border delineation. The use of contrast enables acquisition of ultrasound images of improved quality. The technique is especially useful in obese patients and those with lung disease. Patients in these categories comprise approximately 10% to 20% of routine echocardiographic examinations. Stress echocardiography examinations can be even more challenging, as the image acquisition time factor is critically important for accurate detection of coronary disease. Improvements in image quality with intravenous contrast agents can facilitate image acquisition and enhance delineation of regional wall motion abnormalities at the peak level of exercise. Recent phase III clinical trial data on the use of Optison and several other agents (currently under evaluation) have revealed that for approximately half of patients, image quality substantively improves, which enables the examination to be salvaged and/or increases diagnostic accuracy. For the "difficult-to-image" patient, this added information results in (1) enhanced laboratory efficiency, (2) a reduction in downstream testing, and (3) possible improvements in patient outcome. In addition, substantial research efforts are underway to use ultrasound contrast agents for assessment of myocardial perfusion. The detection of myocardial perfusion during echocardiographic examinations will permit the simultaneous assessment of global and regional myocardial structure, function, and perfusion-all of the indicators necessary to enable the optimal noninvasive assessment of coronary artery disease. Despite the added benefit in improved efficacy of testing, few data exist regarding the long-term effectiveness of these agents. Currently under evaluation are the clinical and economic outcome implications of intravenous contrast agent use for daily clinical decision making in a variety of patient subsets. Until these data are known, this document offers a preliminary synthesis of available evidence on the value of intravenous contrast agents for use in rest and stress echocardiography. At present, it is the position of this guideline committee that intravenous contrast agents demonstrate substantial value in the difficult-to-image patient with comorbid conditions limiting an ultrasound evaluation of the heart. For such patients, the use of intravenous contrast agents should be encouraged as a means to provide added diagnostic information and to streamline early detection and treatment of underlying cardiac pathophysiology. As with all new technology, this document will require updates and revisions as additional data become available.

Developments in cardiac ultrasound

This article gives an overview of recent developments in cardiac ultrasound for the general hospital physician. It discusses contrast echocardiography, harmonic imaging, three-dimensional echocardiography, Doppler tissue imaging and perfusion imaging and give an outlook on future perspectives.

Defining left ventricular segmental and global function by echocardiographic intraventricular contrast flow patterns

Contrast echocardiography improves left ventricular (LV) endocardial border delineation by enhancement of the blood-tissue interface. In particular, the contrast appearing within the LV chamber exhibits characteristic flow patterns over the cardiac cycle, which may be related to the surrounding myocardial wall motion. To determine the relation between the LV intracavitary contrast flow pattern and surrounding wall motion, we reviewed the contrast-enhanced images of 348 consecutive patients studied at rest. We defined 2 different patterns of intracavitary contrast flow as visualized from apical views: a swift, vertical, and homogeneous flow towards the apex (pattern A), and a distinctly protracted, swirling, and heterogeneous flow (pattern B). Images recorded on videotapes were reviewed and the type of pattern (A or B) was determined within the initial 30 to 45 seconds of contrast appearance in the left ventricle. Contrast flow patterns interpreted by independent reviewer were then compared with the interpretation of the LV segmental and global function in each patient. Results demonstrate that 224 of 245 (91%) patients exhibiting pattern A had normal LV segmental function. Furthermore, all but 1 patient (102 of 103) with pattern B had > or =1 wall motion abnormality (p <0.0001). Contrast flow pattern B was observed irrespective of the location of LV wall motion abnormality. Global LV function was normal in 93% of patients exhibiting pattern A, whereas varying degrees of LV dysfunction were noted in 83% of patients with pattern B (p <0.0001). The presence of mitral regurgitation (p = 0.46), aortic insufficiency (p = 0.066), or mitral inflow Doppler abnormality (p = 0.102) was not significantly associated with either pattern. Thus, during contrast echocardiography, the LV intracavitary contrast flow pattern complements the assessment of global and segmental LV function.

An Easily Implemented Method to Improve Detection of Ultrasound Contrast in Body Tissues

The recent commercial availability of ultrasound contrast agents poses new challenges to ultrasound lprofessionals. There is a need to identify the most:relevant clinical applications for which the agents are indicated, as well as ways jin which to use existing ultrasound technology with contrast. Herein is described la new imaging tehnique frame-one maging that: imp rovsthe detection of contrast in bo0d:tissues. Frame-one imaging was attempted in six examin attons of suspected renal abnormalities performed as part of the FDA Phase IcliIcIal trials of the contrast agent Definity® (DuPont Pharmaceuticals, N. Billerica,: M, A). Conventional and harmonic gray-scale imaging was perfrmed at baseline and postcntrat addm inistration. US imaging results were compared to surgical findings. U1, sing gray-scalle harm: o:nicimagg, improved depiction: of:contrast enhancement was achieved in all:cases where FOI was employed. Contrast enhancement was better demonstrated using the FOI tchnique than with continuous real-time imaging. FOI improved the display of contrast enhancement from within normal tissues and vessels as wel as:frombi tumor vessels and tumor tissue. FOI may permit improved tumor characterization using contrasenhanced sonogphy. Additiol vestigations are neede to determine if FOL can imp0rovethe diagnostic potential of contrast-enhanced sonography.

Feasibility of right ventricular myocardial opacification by contrast echocardiography and comparison with left ventricular intensity

To demonstrate the feasibility and quantify the intensity of right ventricular (RV) myocardial opacification by myocardial contrast echocardiography (MCE), we analyzed MCE produced by intravenous injection of 0.15 ml/kg of QW7437 in 8 closed-chest dogs. MCE could produce visual opacification of the RV wall similar in time course to that of the left ventricular wall, and the data supported the potential role of MCE in evaluating RV hypertrophy, contraction, and perfusion abnormalities.

Real-Time Contrast Echo Assessment of Myocardial Perfusion at Low Emission Power: First Experimental and Clinical Results Using Power Pulse Inversion Imaging

Power pulse inversion (PPI) has been developed for echocontrast specific imaging in order to reduce destruction of microbubbles. The purpose of this study was to evaluate PPI for real-time contrast echocardiography. Therefore, in vitro studies in a physiological flow-phantom and clinical examinations in patients with coronary artery disease were performed. The in vitro rersults of this study indicate that PPI allows real-time imaging at low emission power and is almost nondestructive to contrast microbubbles of Definity. At this low emission power a strong linear relationship between the dosage of the contrast agent and the resulting PPI signal intensity was found (R = 0.998, p < 0.001). In the clinical examinations real-time imaging using low mechanical index PPI resulted in strong myocardial signals and a complete filling of the cavities indicating absence of bubble destruction. Most striking was the ability of PPI to display myocardial thickening and wall motion simultaneously with the assessment of myocardial contrast replenishment following ultrasound induced bubble destruction by high power frames. We conclude that PPI allows nondestructive contrast imaging both in experimental and clinical settings. Therefore, real-time imaging of myocardial perfusion and real-time assessment of contrast replenishment following ultrasound induced destruction of microbubbles is feasible. Moreover, PPI allows simultaneous assessment of perfusion and myocardial function.

In vivo kinetics of microbubbles of SH U 508 A (Levovist): comparison with indocyanine green in rabbits

The aim of this study was to evaluate in vivo kinetics of microbubbles of SH U 508 A, in comparison with Indocyanine Green, a dye used as an indicator of blood flow. Microbubble kinetics were evaluated in various vessels (i.e., vena cava, aorta, renal artery, renal vein and portal vein) in rabbits after injection of SH U 508 A by measuring Doppler signals (n = 5). The kinetics of Indocyanine Green were evaluated by measuring absorbance using a photodiode (n = 5). Test substances (SH U 508 A 300 mg/mL and Indocyanine Green 1.25 mg/mL) were injected IV at a dose of 0.1 mL/kg B.W. Peak signal intensity was observed immediately after injection of SH U 508 A, followed by biphasic decay. The rates of biphasic decay were similar in all vessels. A second peak of the signal, which indicated recirculation of the microbubbles, was observed in the vena cava. The circulation and recirculation times of the microbubbles after injection of SH U 508 A were similar to that of Indocyanine Green. These findings suggest that the majority of SH U 508 A microbubbles circulate through the body similarly to blood flow, without retention, in the vasculature.

Echocardiographic determination of risk area size in a murine model of myocardial ischemia

Genetically altered mice are useful to understand cardiac physiology. Myocardial contrast echocardiography (MCE) assesses myocardial perfusion in humans. We hypothesized it could evaluate murine myocardial perfusion before and after acute coronary ligation. MCE was performed before and after this experimental myocardial infarction (MI) in anesthetized mice by intravenous injection of contrast microbubbles and transthoracic echo imaging. Time-video intensity curves were obtained for the anterior, lateral, and septal myocardial walls. After MI, MCE defects were compared with the area of no perfusion measured by Evans blue staining. In healthy animals, intramyocardial contrast was visualized in all the cardiac walls. The anterior wall had a higher baseline video intensity (53 +/- 17 arbitrary units) than the lateral (34 +/- 13) and septal (27 +/- 13) walls (P < 0.001) and a lower increase in video intensity after contrast injection [50 +/- 17 vs. 60 +/- 24 (lateral) and 65 +/- 29 (septum), P < 0.01]. After MI, left ventricular (LV) dimensions were enlarged, and the shortening fraction was decreased. A perfusion defect was imaged with MCE in every mouse, with a correlation between MCE perfusion defect size (35 +/- 13%) and the nonperfused area by Evans blue (37 +/- 16%, y = 0.77x + 6.1, r = 0.93, P < 0. 001). Transthoracic MCE is feasible in the mouse and can accurately detect coronary occlusions and quantitate nonperfused myocardium.

Acoustically stimulated transient power scattering explains enhanced detection of the very low velocities in myocardial capillaries by power Doppler imaging: an in vitro study

BACKGROUND

Although enhanced detection of myocardial perfusion signals by power Doppler imaging during contrast echocardiography has been noted, flow velocities in the coronary microvasculature should generally be below the threshold for Doppler motion detection. It has been suggested that in this situation nonlinear scattering related to acoustically stimulated microsphere oscillation or destruction may be responsible for the detected Doppler shift.

METHODS AND RESULTS

This study examined the behavior of MRX 115 (ImaRx Pharmaceuticals) microbubbles during harmonic and nonharmonic power Doppler imaging at varying power outputs (mechanical indexes 0. 3, 0.5, 0.7, and 0.9) in a perfusion tube model under zero-flow conditions. Boluses of MRX 115 0.5-mL suspension were introduced into the model, and flow was halted during each imaging period. Once power Doppler imaging was implemented, a signal was detected as unique sparkling color pixels corresponding to individual bubble destruction events, even in the absence of contrast movement. This phenomenon continued until all contrast bubbles disappeared from the region subjected to power Doppler imaging, usually within 35 to 40 seconds. Off-line videointensity measurements showed that initial power Doppler signal intensity and maximum signal decay rates increased parallel to increasing power output and were substantially greater for nonharmonic than for harmonic imaging modes.

CONCLUSION

This relationship between signal intensity and decay rate and acoustic power output suggests that transient scattering related to bubble destruction is responsible for generation of the power Doppler signal in the absence of flow. This would explain the enhanced detection of the very low velocity flows in the myocardial capillaries by power Doppler contrast imaging.

Evaluation of myocardial, hepatic, and renal perfusion in a variety of clinical conditions using an intravenous ultrasound contrast agent (Optison) and second harmonic imaging

OBJECTIVE

To assess the potential of intravenous Optison, a second generation ultrasound contrast agent, and various ultrasound imaging modes to determine myocardial, kidney, and liver perfusion in normal subjects and patients with left ventricular dysfunction or chronic pulmonary disease together with renal or hepatic dysfunction.

METHODS

Five normal subjects and 20 patients underwent grey scale echocardiographic imaging of myocardium, kidney, and liver during 505 intravenous injections of Optison. Images were assessed qualitatively by two independent observers and quantitatively using video densitometry to determine the peak contrast enhancement effect.

RESULTS

Qualitative analysis showed that intermittent harmonic imaging was superior to either conventional fundamental or continuous harmonic imaging for all organs. Quantitative analysis showed that the peak change in echocardiographic intensity v baseline during continuous harmonic imaging was 11 units for myocardium (p < 0.03), 7 units for kidney (NS), and 14 units for liver (p < 0.05). During intermittent harmonic imaging the peak change was significantly greater, being 33 units for myocardium (p < 0.0001), 24 units for kidney (p < 0.0002), and 16 units for liver (p < 0.001).

CONCLUSIONS

Organ tissue perfusion can be demonstrated following intravenous injection of Optison, particularly when used in combination with intermittent harmonic imaging techniques. This contrast agent is effective in a variety of clinical conditions.

Rapid Three-Dimensional Myocardial Contrast Echocardiography: Volumetric Quantitation of Nonperfused Myocardium After Intravenous Contrast Administration

Current acquisition methods for quantitative three-dimensional myocardial contrast echocardiography require long acquisition times and therefore require the invasive administration of deposit contrast agents administered intra-arterially or into the left atrium. This study addressed the feasibility of obtaining accurate and precise quantitative volumetric measurements of nonperfused myocardium after an intravenous bolus of echocardiographic contrast agent using a rapid three-dimensional myocardial contrast echocardiographic acquisition technique. An open-chest pig model of acute left anterior descending coronary artery (LAD) occlusion was used. After LAD ligature, an intravenous bolus of contrast agent was given and images were obtained over a 12-second period using a continuously rotating transducer placed at the apical position. There was no significant microbubble destruction during the rotational acquisition period as measured by differences in mean gray scale values of apical, mid, and basal myocardial regions between the first and last image frames of acquisition. Calculated volumes of nonperfused myocardium demonstrated significant agreement and correlation (mean difference +/- SD = -0.30 +/- 1.71 cm(3); r = 0.89; P < 0.01; y = 1.06x - 1.08) with anatomic specimens. When expressed as percent of total LV volume being nonperfused, the mean difference +/- SD was 2.1 +/- 3.6%, r = 0.94, P < 0.01, and y = 1.33x - 4.08. We conclude that accurate and precise measurements of nonperfused myocardium after an acute LAD coronary artery occlusion can be obtained after the intravenous bolus administration of a contrast material when a rapid 12-second acquisition with a continuously rotating transducer is used.

The influence of Doppler system settings on the clearance kinetics of different ultrasound contrast agents

OBJECTIVE

To evaluate the influence of different Doppler system settings on time-intensity curves after ultrasound contrast agent (UCA) bolus injection. This is important for the comparison of different UCAs.

METHODS

Six sedated dogs were investigated with a transcranial Doppler system and Doppler power, sample volume size and high pass filter settings were modified during the procedure. Mean time intensity curves were determined and peak values of mean intensity as well as the decrease in Doppler intensity were compared for the different system settings. Three different UCAs were used (SonoVue(TM), BY963 and Levovist(TM)).

RESULTS

The Doppler time intensity curves showed a typical two phase decrease with a distribution phase alpha and an elimination phase beta with all three UCAs. Altering the system settings had a significant effect on the mean peak Doppler intensity for SonoVue(TM) (P=0.02) but not for BY963 or Levovist(TM) (P=0.07 and P=0.39, respectively), due to high variation of the Levovist(TM) and BY963 intensity values. There were no significant differences between the alpha slopes of BY963 and Levovist(TM) (P=0.96), or the beta slope of Levovist(TM) and SonoVue(TM) (P=0.62), when the results of all system settings were combined.

CONCLUSION

Different Doppler system settings show no significant influence on the decrease of mean Doppler intensity, but have a significant effect on peak intensity.

Real-time visualization of myocardial perfusion and wall thickening in human beings with intravenous ultrasonographic contrast and accelerated intermittent harmonic imaging

Previous work has demonstrated that at higher peak negative pressures, microbubbles are destroyed by diagnostic ultrasonography. At lower pressures (lower mechanical index), less destruction occurs but enhanced contrast persists. In animals, this lower mechanical index has resulted in enhanced contrast after administration of intravenous microbubbles with intermittent imaging at faster frame rates. We tested whether this accelerated intermittent imaging technique could produce myocardial contrast and detect myocardial perfusion abnormalities in 25 patients (10 with normal wall motion, 15 after myocardial infarction). Three independent reviewers detected persistent myocardial contrast defects within the infarct zone throughout the cardiac cycle in 9 of the 15 patients after acute myocardial infarction; the presence of such defects was predictive of a persistent regional wall motion abnormality at 4-week follow-up. Interobserver agreement on regional contrast enhancement ranged from 88% to 90%. We conclude that accelerated intermittent imaging permits real-time visualization of myocardial blood flow and wall thickening.

Analysis of flash echo from contrast agent for designing optimal ultrasound diagnostic systems

Microbubble-based contrast agents can enhance echoes in areas of low blood flow, but the bubbles are extremely sensitive and collapse easily when exposed to ultrasound (US) irradiation. An experimental study of bubble collapse was carried out to design new functions for US diagnostic systems to detect echoes from microbubbles more efficiently. For contrast agent (Levovist) solution, a high-intensity, but momentary, echo (flash echo), was observed in the first frame image after a several-second suspension of transmission, but was not seen in the second frame image. These "flash echo" signals were analyzed and categorized based on microscopic observation, and the results showed that the longevity of the microbubbles was reduced by conditions such as B-mode imaging. Next, a numerical simulation of the bubbles in liquid was performed under the same conditions as in the in vitro experiment. The results showed that even bubbles less than 1 microm in diameter expand and collapse within one pulse drive, which would generate flash echoes. The flash echo imaging system described here permits flexible intermittent scanning with variable intervals, with a variable number of frames at the trigger, and with simultaneous monitoring at low power output. Animal experiments were also conducted to evaluate the system. As the interval between frames was increased, the flash echoes gradually increased, and perfusion in the parenchyma was clearly observed with an interval of 4 s.

Potential advantage of flash echocardiography for digital subtraction of B-mode images acquired during myocardial contrast echocardiography

Optimal assessment of myocardial perfusion with contrast echocardiography by using B-mode imaging often requires image alignment and background subtraction, which are time consuming and need extensive expertise. Flash echocardiography is a new technique in which primary images are gated to the electrocardiogram and secondary images are obtained by transmitting ultrasound pulses in rapid succession after each primary image. Myocardial opacification is seen in the primary image and not in the secondary images because of ultrasound-induced bubble destruction. Because the interval between the primary and first few secondary images is very short, cardiac motion between these images should be minimal. Therefore we hypothesized that 1 or more secondary images could be subtracted from the primary image without the need for image alignment. The ability of ultrasound to destroy microbubbles was assessed by varying the sampling rate, line density, and mechanical index in 6 open-chest dogs. The degree of translation between images was quantified in the x and y directions with the use of computer cross-correlation. At sampling rates of 158 Hz or less and a mechanical index of more than 0.6, videointensity rapidly declined to baseline levels by 25 ms. Significant translation between images was noted only at intervals of more than 112 ms. It is concluded that flash echocardiography can be used for digital subtraction of baseline from contrast-enhanced B-mode images without image alignment. Background subtraction is therefore feasible on-line, potentially eliminating the need for off-line image processing in the future.

Direct video-microscopic observation of the dynamic effects of medical ultrasound on ultrasound contrast microspheres

RATIONALE AND OBJECTIVES

Ultrasound can cause destruction of microbubble contrast agents used to enhance medical ultrasound imaging. This study sought to characterize the dynamics of this interaction by direct visual observation of microbubbles during insonification in vitro by a medical ultrasound imaging system.

METHODS

Video microscopy was used to observe air-filled sonicated albumin microspheres adsorbed to a solid support during insonation.

RESULTS

Deflation was not observed at lowest transmit power settings. At higher intensities, gas left the microparticle gradually, apparently dissolving into the surrounding medium. Deflation was slower for higher microsphere surface densities. Intermittent ultrasound imaging (0.5 Hz refresh rate) caused slower deflation than continuous imaging (33 Hz).

CONCLUSIONS

Higher concentrations of microbubbles, lower ultrasound transmit power settings, and intermittent imaging each can reduce the rate of destruction of microspheres resulting from medical ultrasound insonation.

New ultrasound contrast agents for left ventricular and myocardial opacification

Until recently, the use of contrast agents with 2-dimensional echocardiography has been limited to the detection of intracardiac shunts or abnormal venous connections. The advent of commercially available transpulmonary contrast agents and progress in imaging technology changed this situation. New indications for contrast echocardiography include improved assessment of ventricular function by endocardial border enhancement and the assessment of myocardial perfusion. The major advantage of novel contrast agents is their persistence in circulation, due to the content of a gas that is poorly soluble in plasma or a specific microcapsule wall composition. These features, in conjunction with advanced imaging techniques (intermittent harmonic imaging, harmonic power Doppler, pulse inversion Doppler) allow the detection of minute amounts of the agents in myocardium. There are more than 10 echocardiographic contrast agents undergoing clinical or late preclinical tests. Apart from commercially available Albunex, Levovist, Optison, such agents as EchoGen, Quantison, NC100100 and PESDA have been successfully used in humans. Initial clinical data demonstrating the feasibility of myocardial perfusion studies in patients have been presented for PESDA, Optison, Quantison and NC100100. Early attempts are being made for therapeutic applications of microbubbles, including ultrasound-intensified thrombolysis, tissue targeting and drug delivery. Rapid progress in microbubble technology and imaging techniques has raised a wide interest of the clinicians for contrast echocardiography, which may soon become an established technique for the evaluation of myocardial perfusion, competitive for radionuclide imaging.

Quantification and time course of microvascular obstruction by contrast-enhanced echocardiography and magnetic resonance imaging following acute myocardial infarction and reperfusion

OBJECTIVES

We aimed to validate contrast-enhanced echocardiography (CE) in the quantification of microvascular obstruction (MO) against magnetic resonance imaging (MRI) and the histopathologic standards of radioactive microspheres and thioflavin-S staining. We also determined the time course of MO at days 2 and 9 after infarction and reperfusion.

BACKGROUND

Postinfarction MO occurs because prolonged ischemia produces microvessel occlusion at the infarct core, preventing adequate reperfusion. Microvascular obstruction expands up to 48 h after reperfusion; the time course beyond 2 days is unknown. Though used to study MO, CE has not been compared with MRI and thioflavin-S, which yield precise visual maps of MO.

METHODS

Ten closed-chest dogs underwent 90-min coronary artery occlusion and reperfusion. Both CE and MRI were performed at 2 and 9 days after reperfusion. The MO regions by both methods were quantified as percent left ventricular (% LV) mass. Radioactive microspheres were injected for blood flow determination. Postmortem, the myocardium was stained with thioflavin-S and 2,3,5-triphenyltetrazolium chloride.

RESULTS

Expressed as % total LV, MO by MRI matched in size MO by microspheres using a flow threshold of <40% remote (4.96+/-3.52% vs. 5.32+/-3.98%, p=NS). For matched LV cross sections, MO by CE matched in size MO by microspheres using a flow threshold of <60% remote (13.27+/-4.31% vs. 13.5+/-4.94%, p=NS). Both noninvasive techniques correlated well with microspheres (MRI vs. CE, r=0.87 vs. 0.74; p=NS). Microvascular obstruction by CE corresponded spatially to MRI-hypoenhanced regions and thioflavin-negative regions. For matched LV slices at 9 days after reperfusion, MO measured 12.94+/-4.51% by CE, 7.11+/-3.68% by MRI and 9.18+/-4.32% by thioflavin-S. Compared to thioflavin-S, both noninvasive techniques correlated well (CE vs. MRI, r=0.79 vs. 0.91; p=NS). Microvascular obstruction size was unchanged at 2 and 9 days (CE: 13.23+/-4.11% vs. 12.69+/-4.97%; MRI: 5.53+/-4.94% vs. 4.68+/-3.44%; p=NS for both).

CONCLUSIONS

Both CE and MRI can quantify MO. Both correlate well with the histopathologic standards. While MRI can detect regions of MO with blood flow <40% of remote, the threshold for MO by CE is <60% remote. The extent of MO is unchanged at 2 and 9 days after reperfusion.

Update on myocardial contrast echocardiography: a surgeon's perspective

The ability to evaluate myocardial perfusion and microvascular structural integrity can help surgeons predict the necessity for surgical intervention, the sequence of intraoperative interventions, the risk of perioperative infarction, the likelihood of successful surgical recovery, and the degree of long-term clinical benefit. The ability to directly assess perfusion intraoperatively may allow surgeons to reliably evaluate a patient's myocardial perfusion at any time during the operative procedure. As this article will discuss, surgeons may use myocardial contrast echocardiography intraoperatively to evaluate myocardial function and integrity, to determine the order of graft placement, to determine the success of bypass graft patency, and to help predict those patients who will experience successful cardiac function after recovering from surgery.

Delivery of colloidal particles and red blood cells to tissue through microvessel ruptures created by targeted microbubble destruction with ultrasound

BACKGROUND

We have previously shown that the application of ultrasound to thin-shelled microbubbles flowing through small microvessels (<7 microm in diameter) produces vessel wall ruptures in vivo. Because many intravascular drug- and gene-delivery vehicles are limited by the endothelial barrier, we hypothesized that this phenomenon could be used to deliver drug-bearing vehicles to tissue.

METHODS AND RESULTS

An exteriorized rat spinotrapezius muscle preparation was used. Intravascular fluorescent red blood cells and polymer microspheres (PM) (205 and 503 nm in diameter) were delivered to the interstitium of rat skeletal muscle through microvessel ruptures created by insonifying microbubbles in vivo. On intravital microscopy, mean dispersion areas per rupture for red blood cells, 503-nm PM, and 205-nm PM were 14.5x10(3) microm2, 24. 2x10(3) microm2, and 27.2x10(3) microm2, respectively. PM dispersion areas were significantly larger than the mean dispersion area for red blood cells (P<0.05).

CONCLUSIONS

Microvessel ruptures caused by insonification of microbubbles in vivo may provide a minimally invasive means for delivering colloidal particles and engineered red blood cells across the endothelial lining of a targeted tissue region.

Improved left ventricular endocardial border delineation and opacification with OPTISON (FS069), a new echocardiographic contrast agent. Results of a phase III Multicenter Trial

OBJECTIVES

The echocardiographic contrast-enhancing effects and safety profile of ALBUNEX (a suspension of air-filled albumin microspheres) were compared with the new contrast agent OPTISON (formerly FS069: a suspension of albumin microspheres containing the gas perfluoropropane) in 203 patients with inadequate noncontrast echocardiograms.

BACKGROUND

The efficacy of ALBUNEX has been limited by its short duration of action. By using perfluoropropane instead of air within the microsphere, its duration of action is increased.

METHODS

Each patient received ALBUNEX (0.8 and 0.22 mL/kg) and OPTISON (0.2, 0.5, 3.0, and 5.0 mL) on separate days a minimum of 48 hours apart. Echocardiograms were evaluated for increase in left ventricular (LV) endocardial border length, degree of LV opacification, number of LV endocardial border segments visualized, conversion from a nondiagnostic to a diagnostic echocardiogram, and duration of contrast enhancement. A thorough safety evaluation was conducted.

RESULTS

Compared with ALBUNEX, OPTISON more significantly improved every measure of contrast enhancement. OPTISON increased well-visualized LV endocardial border length by 6.0+/-5.1, 6.9+/-5.4, 7.5+/-4.7, and 7.6+/-4.8 cm, respectively, for each of the four doses, compared with only 2.2+/-4.5 and 3.4+/-4.6 cm, respectively, for the two ALBUNEX doses (p < 0.001). 100% LV opacification was achieved in 61%, 73%, 87%, and 87% of the patients with the four doses of OPTISON, but in only 16% and 36% of the patients with the two ALBUNEX doses (p < 0.001). Conversion of nondiagnostic to diagnostic echocardiograms with contrast occurred in 74% of patients with the optimal dose of OPTISON (3.0 mL) compared with only 26% with the optimal dose of ALBUNEX (0.22 mL/kg) (p < 0.001). The duration of contrast effect was also significantly greater with OPTISON than with ALBUNEX. In a subset of patients with potentially poor transpulmonary transit of contrast (patients with chronic lung disease or dilated cardiomyopathy), OPTISON more significantly improved the same measures of contrast enhancement compared with ALBUNEX and did so to the same extent as in the overall population. Side effects were similar and transient with the two agents.

CONCLUSION

OPTISON appears to be a safe, well-tolerated echocardiographic contrast agent that is superior to ALBUNEX.

Direct in vivo visualization of intravascular destruction of microbubbles by ultrasound and its local effects on tissue

BACKGROUND

Our aim was to observe ultrasound-induced intravascular microbubble destruction in vivo and to characterize any resultant bioeffects.

METHODS AND RESULTS

Intravital microscopy was used to visualize the spinotrapezius muscle in 15 rats during ultrasound delivery. Microbubble destruction during ultrasound exposure caused rupture of < or = 7-microm microvessels (mostly capillaries) and the production of nonviable cells in adjacent tissue. The number of microvessels ruptured and cells damaged correlated linearly (P<0.001) with the amount of ultrasound energy delivered.

CONCLUSIONS

Microbubbles can be destroyed by ultrasound, resulting in a bioeffect that could be used for local drug delivery, angiogenesis, and vascular remodeling, or for tumor destruction.

Phase III multicenter trial comparing the efficacy of 2% dodecafluoropentane emulsion (EchoGen) and sonicated 5% human albumin (Albunex) as ultrasound contrast agents in patients with suboptimal echocardiograms

OBJECTIVES

This study was performed to compare the safety and efficacy of intravenous 2% dodecafluoropentane (DDFP) emulsion (EchoGen) with that of active control (sonicated human albumin [Albunex]) for left ventricular (LV) cavity opacification in adult patients with a suboptimal echocardiogram.

BACKGROUND

The development of new fluorocarbon-based echocardiographic contrast agents such as DDFP has allowed opacification of the left ventricle after peripheral venous injection. We hypothesized that DDFP was clinically superior to the Food and Drug Administration-approved active control.

METHODS

This was a Phase III, multicenter, single-blind, active controlled trial. Sequential intravenous injections of active control and DDFP were given 30 min apart to 254 patients with a suboptimal echocardiogram, defined as one in which the endocardial borders were not visible in at least two segments in either the apical two- or four-chamber views. Studies were interpreted in blinded manner by two readers and the investigators.

RESULTS

Full or intermediate LV cavity opacification was more frequently observed after DDFP than after active control (78% vs. 31% for reader A; 69% vs. 34% for reader B; 83% vs. 55% for the investigators, p < 0.0001). LV cavity opacification scores were higher with DDFP (2.0 to 2.5 vs. 1.1 to 1.5, p < 0.0001). Endocardial border delineation was improved by DDFP in 88% of patients versus 45% with active control (p < 0.001). Similar improvement was seen for duration of contrast effect, salvage of suboptimal echocardiograms, diagnostic confidence and potential to affect patient management. There was no difference between agents in the number of patients with adverse events attributed to the test agent (9% for DDFP vs. 6% for active control, p = 0.92).

CONCLUSIONS

This Phase III multicenter trial demonstrates that DDFP is superior to sonicated human albumin for LV cavity opacification, endocardial border definition, duration of effect, salvage of suboptimal echocardiograms, diagnostic confidence and potential to influence patient management. The two agents had similar safety profiles.

Contrast echocardiography: review and future directions

Recent developments and advances in contrast echocardiography have been made to improve the diagnosis and evaluation of cardiac structures and function. By coupling new developments in acoustic instrumentation with new contrast agents, information that was previously difficult or impossible to gather by standard 2-dimensional echocardiography can now be obtained. Numerous studies have been published confirming the advantages of using contrast during echocardiographic studies, particularly with stress testing and myocardial perfusion. This review aims to summarize (1) the various contrast agents that are available or being developed; (2) factors that have been found to affect the strength of enhanced signals; (3) the new developments in instrumentation that improve the ability of scanners to differentiate echo contrast from cardiac tissue; and (4) the documented and possible future uses of contrast echocardiography.

Increasing the dose and rate of Albunex infusion leads to superior left ventricular contrast effect

In routine clinical use, the efficacy of Albunex in producing clinically useful opacification may be lower than in initial clinical studies. We hypothesized that increasing either the rate of injection or amount of Albunex administered would increase left ventricular opacification. Fifty adult volunteers were each injected with Albunex in five volume/rate combinations. Blinded reviewers evaluated left ventricular opacification and endocardial border delineation compared with the baseline (noncontrast) echocardiogram. In addition, captured digitized images were analyzed with video-densitometric techniques. Injected at the highest volume/rate tested (20 ml at 3.0 ml/sec), Albunex provided the greatest improvement in left ventricular opacification, endocardial border delineation, and quality of the echocardiogram. The administration of Albunex caused no serious adverse events at any volume/rate regimen tested. Our data indicate that faster injection rates and larger dose volumes than those currently recommended by the package insert significantly improve Albunex ultrasound contrast without compromising safety.

Quantification of myocardial blood flow with ultrasound-induced destruction of microbubbles administered as a constant venous infusion

BACKGROUND

Ultrasound can cause microbubble destruction. If microbubbles are administered as a continuous infusion, then their destruction within the myocardium and measurement of their myocardial reappearance rate at steady state will provide a measure of mean myocardial microbubble velocity. Conversely, measurement of their myocardial concentration at steady state will provide an assessment of microvascular cross-sectional area. Myocardial blood flow (MBF) can then be calculated from the product of the two.

METHODS AND RESULTS

Ex vivo and in vitro experiments were performed in which either flow was held constant and pulsing interval (interval between microbubble destruction and replenishment) was altered, or vice versa. In vivo experiments were performed in 21 dogs. In group 1 dogs (n=7), MBF was mechanically altered in a model in which coronary blood volume was constant. In group 2 dogs (n=5), MBF was altered by direct coronary infusions of vasodilators. In group 3 dogs (n=9), non-flow-limiting coronary stenoses were created, and MBF was measured before and after the venous administration of a coronary vasodilator. In all experiments, microbubbles were delivered as a constant infusion, and myocardial contrast echocardiography was performed using different pulsing intervals. The myocardial video intensity versus pulsing interval plots were fitted to an exponential function: y=A(1-e[-betat]), where A is the plateau video intensity reflecting the microvascular cross-sectional area, and beta reflects the rate of rise of video intensity and, hence, microbubble velocity. Excellent correlations were found between flow and beta, as well as flow and the product of A and beta.

CONCLUSIONS

MBF can be quantified with myocardial contrast echocardiography during a venous infusion of microbubbles. This novel approach has potential for measuring tissue perfusion in any organ accessible to ultrasound.

Myocardial perfusion characteristics and hemodynamic profile of MRX-115, a venous echocardiographic contrast agent, during acute myocardial infarction

We sought to determine whether MRX-115, a new venous echocardiographic contrast agent, could accurately assess risk area during coronary occlusion and infarct size after reperfusion by using novel imaging modalities meant to selectively enhance contrast signals. In 12 open-chest dogs, venous injections of 0.5 ml of MRX-115 were performed during baseline and coronary occlusion and after reperfusion in the presence of exogenous hyperemia. Ultrasound was transmitted at 2 MHz and received at both 2 MHz (fundamental) and 4 MHz (harmonic) frequencies during continuous and intermittent (end-systolic only) imaging. The risk area during coronary occlusion was compared with technetium autoradiography, and the infarct size after reperfusion was compared with postmortem tissue staining. MRX-115 produced no alterations in hemodynamic or pulmonary gas exchange at any stage. During continuous (both fundamental and harmonic) and intermittent fundamental imaging, measurements of perfusion defects were precluded in many dogs by either poor signal enhancement or posterior wall attenuation. By comparison, these measurements were possible during intermittent harmonic imaging in all dogs except one, which had a very small infarction during reflow. Correlation analysis between perfusion defect size on intermittent harmonic imaging and either autoradiographic risk area or postmortem infarct size gave r values of 0.83 and 0.92, respectively. We conclude that MRX-115 is hemodynamically well tolerated and, when imaging is performed after venous injection, can accurately assess regions of hypoperfusion when combined with intermittent harmonic imaging. These results are promising for the use of this approach in patients with acute myocardial infarction.

Myocardial viability in patients with chronic coronary artery disease and previous myocardial infarction: comparison of myocardial contrast echocardiography and myocardial perfusion scintigraphy

The aim of this study was to compare perfusion patterns on myocardial contrast echocardiography with those on myocardial perfusion scintigraphy for the assessment of myocardial viability in patients with previous myocardial infarction. Accordingly, perfusion scores with the two techniques were compared in 91 ventricular regions in 21 patients with previous (>6 weeks old) myocardial infarction. Complete concordance between the two techniques was found in 63 (69%) regions; 25 (27%) regions were discordant by only 1 grade, and complete discordance (2 grades) was found in only 3 (3%) regions. A kappa statistic of 0.65 indicated good concordance between the two techniques. Although the scores on both techniques demonstrated a relation with the wall motion score, the correlation between the myocardial contrast echocardiography and wall motion scores was closer (r = -0.63 vs r = -0.50, p = 0.05). It is concluded that myocardial contrast echocardiography provides similar information regarding myocardial viability as myocardial perfusion scintigraphy in patients with coronary artery disease and previous myocardial infarction.

Modeling the myocardial dilution curve of a pure intravascular indicator

The dispersion and dilution of contrast medium through the myocardial vasculature is examined first with a serial model comprised of arterial, capillary, and venous components in series to determine their time-concentration curves (TCC) and the myocardial dilution curve (MDC). Analysis of general characteristics shows that the first moment of the MDC, adjusted for that of the aortic TCC and mean transit time (MTT) from the aorta to the first intramyocardial artery, is one-half the MTT of the myocardial vasculature and that the ratio of the area of the MDC and aortic TCC is the fractional myocardial blood volume (MBV). The use of known coronary vascular morphometry and a set of transport functions indicates that the temporal change in MDC is primarily controlled by the MTT. An analysis of several models with heterogeneous flow distributions justifies the procedures to calculate MTT and MBV from the measured MDC. Compared with previously described models, the present model is more general and provides a physical basis for the effects of flow dispersion and heterogeneity on the characteristics of the MDC.

New developments in ultrasound systems for contrast echocardiography

Contrast echocardiography (CE) has evolved significantly in the past decade. Contrast agents and the hardware and software used to detect them and display optimal images have developed in tandem. Not only are hardware and contrast agents available that allow left ventricular cavity enhancement, but recent research points to the usefulness of CE for the evaluation of myocardial perfusion in the cardiac catheterization laboratory and operating room. Advances in ultrasound technology, such as transient harmonic imaging and integrated backscatter, coupled with the development of newer contrast agents that contain smaller, more stable microbubbles capable of transpulmonary passage for intravenous injection, promise a vast increase in the applications of CE in clinical practice.

Detection of Coronary Artery Disease With Myocardial Contrast Echocardiography

Background The purpose of this study was to determine whether myocardial contrast echocardiography (MCE) can be used to detect coronary artery disease (CAD) during rest and pharmacological stress in humans through the use of venous injections of contrast.

Methods and Results Thirty patients with known or suspected CAD underwent MCE and 99m Tc-sestamibi single-photon emission computed tomography (SPECT) at baseline and after dipyridamole (0.56 mg · kg −1 ) infusion. Ten myocardial segments (5 each in the apical two- and four-chamber views) from the two sets of images using both methods were scored for myocardial perfusion as follows: 1=normal, 0.5=mildly reduced, and 0=severely reduced. The information from baseline and postdipyridamole images was then used to determine whether an abnormal segment was irreversible (similar abnormal perfusion at baseline and after dipyridamole) or reversible (perfusion better at baseline compared with after dipyridamole). Concordance between segmental scores was 92% (κ=.99) for both methods. Concordance between normal perfusion and reversible or irreversible segmental defects was 90% (κ=.80). Agreement between the two methods for each of the three vascular territories in each patient was 90% (κ=.77), while agreement for the presence or absence of CAD in each patient was 86% (κ=.86). In the 4 patients with disagreement, the perfusion scores were 0.5 for SPECT and 1.0 for MCE.

Conclusions This study shows that MCE, with venous injection of contrast, can define the presence of CAD during rest and pharmacological stress. The location of perfusion abnormalities and their physiologic relevance (reversible or irreversible) by MCE is similar to that provided by SPECT. MCE, therefore, holds promise for the noninvasive assessment of myocardial perfusion in humans.

Enhancement of left ventricular cavity opacification by harmonic imaging after venous injection of Albunex

Venous injection of Albunex does not consistently produce left ventricular (LV) cavity opacification during conventional echocardiography. We postulated that by increasing the signal-to-noise ratio, harmonic imaging will result in more successful LV cavity opacification and provide a better assessment of regional LV systolic function. Forty-two patients with poor baseline endocardial delineation were given 10 ml intravenous injections of Albunex during continuous fundamental and harmonic imaging. Change in segmental wall-thickening scores and the confidence levels for these scores were assessed for 3 observers with various levels of experience. Compared with fundamental imaging, harmonic imaging significantly improved the success of LV cavity opacification (83% vs 62%, p <0.05). The background-subtracted video intensity within the central two thirds of the LV cavity increased threefold (from 10 +/- 15 to 31 +/- 29, p <0.05) with harmonic imaging. The spatial extent of opacification increased from 40% of the LV cavity during fundamental imaging to 65% with harmonic imaging (p <0.001). The confidence level for assessing regional LV systolic function improved (p <0.05) after contrast administration, particularly when observer experience was limited. We conclude that in patients with poor endocardial definition, injection of intravenous Albunex should be combined with harmonic imaging to improve LV cavity opacification.