Doppler energy: a new acquisition technique for the transthoracic detection of myocardial perfusion defects with the use of a venous contrast agent

Doppler tissue energy and stress echocardiography in the diagnosis of myocardial contusion in canines

We sought to evaluate the significance of Doppler tissue energy (DTE) and stressed echocardiography for diagnosing myocardial contusion (MC) in canines. Ten adult healthy dogs were anesthetized (3% pentobarbital sodium/i.v.) and impacted by BIM-II biological impact machine to induce MC. Conventional and stressed echocardiographies were used for segmental abnormal ventricular wall motions; DTE was also used to detect the abnormal ventricular wall motions and areas of injured myocardial fibers after MC, and the results were compared with those of triphenyl tetrazolium chloride (TTC) staining. The data show that both conventional and stressed echocardiographies identified ventricular wall segmental abnormal motions or even aneurysms. These segments were mainly distributed over the front and middle interventricular walls and anterolateral ventricular wall. The ventricular wall motion scoring and wall motion segment index (WMSI) increased remarkably after MC induction. Compared with TTC staining, the conventional echocardiography showed 100% sensitivity and 66.67% specificity, whereas the stressed echocardiography displayed 100% sensitivity and 88.89% specificity. DTE showed both the sensitivity and specificity of 100% for MC diagnosis. Thus, DTE has higher specificity than conventional and stressed echocardiographies. In conclusion, both DTE and stress echocardiography have higher clinical value for MC diagnosis in canines.

Noninvasive cardiac imaging in the evaluation of suspected acute coronary syndromes

Optimal management of patients presenting with chest pain to the emergency department is a major challenge, both in terms of a diagnostic dilemma and consumption of resources. The triage of such patients can be aided vastly by the appropriate use of noninvasive imaging. Noninvasive imaging modalities such as echocardiogram, radionuclide perfusion studies, positron emission tomography, cardiac magnetic resonance imaging and computed tomography have all been demonstrated to have favorable diagnostic and prognostic value, with an enhanced sensitivity to detect acute ischemia. A normal noninvasive evaluation in the appropriate clinical setting presents a strong argument against acute ischemia as an etiology of the chest pain. Randomized trials of both rest and stress imaging in the emergency department have confirmed a reduction in unnecessary hospitalizations and cost savings without compromising the safety of the patient. Cardiac magnetic resonance and computed tomography would provide an insight into subendocardial ischemia, the detection of which has previously been difficult, using single-photon emission tomography and echocardiography. In this review, novel hot-spot imaging modalities are discussed including infarct-avid imaging agents and ischemia-avid imaging agents, thus elucidating the pathophysiology of reperfusion-induced cell death. These agents represent work in evolution and are likely to be used routinely in the future as understanding of coronary syndromes and coronary artery disease becomes clearer.

Quantitative Echocardiographic Evaluation of Myocardial Perfusion Using Interrupted Contrast Infusion Technique: In Vivo Validation Studies and Feasibility in Human Beings

BACKGROUND

We recently developed a new approach for contrast echocardiographic quantification of myocardial perfusion, based on brief interruptions of contrast infusion, which was designed to overcome the limitations of existing techniques. In this study, our technique was initially validated in a series of animal experiments designed to detect regional perfusion variations in vivo. Subsequently, clinical feasibility of perfusion measurements was tested.

METHODS

Regional perfusion was measured transthoracically in 6 anesthetized pigs during baseline, partial left anterior descending coronary artery occlusion, and reperfusion, and validated with fluorescent microspheres. Adenosine-induced changes in perfusion were measured in 8 healthy volunteers. In both protocols, imaging was optimized during contrast infusion (Definity). Infusion was interrupted to allow contrast clearance and images were acquired during subsequent contrast inflow. Myocardial videointensity was measured over time and peak contrast inflow rate was calculated.

RESULTS

In pigs, partial coronary occlusion resulted in a 47 +/- 23% decrease in peak contrast inflow rate in the left anterior descending coronary artery perfusion territory (P < .05), which was reversed during reperfusion, without concomitant decrease in other perfusion territories. These changes were in agreement with microspheres. In human beings, adenosine increased peak contrast inflow rate to 278 +/- 123% of baseline (P < .05).

CONCLUSION

The interruption of contrast infusion technique is a sensitive tool for accurate quantification of myocardial perfusion, which may constitute an alternative to currently used techniques.

Quantitative Analysis of Myocardial Perfusion in Rats by Contrast Echocardiography

BACKGROUND

The ability to assess myocardial perfusion in small animals is important, especially to investigate models of myocardial ischemia. Myocardial perfusion is usually assessed by postmortem techniques, eliminating the possibility of follow-up. We sought to evaluate whether contrast echocardiography was able to quantify myocardial perfusion in rats.

METHODS

Twenty-four rats divided in 3 groups (sham-operated, and 8 and 21 days after left anterior descending coronary artery stenosis) underwent myocardial contrast echocardiography using intermittent triggered imaging. Peak plateau intensity and slope of refilling were compared with myocardial blood flow achieved with fluorescent microspheres.

RESULTS

High-quality images were easily obtained for each experiment. Close correlation was found between myocardial contrast echocardiography and myocardial blood flow, especially for measurements of peak plateau intensity x slope of refilling relative to the control area (y = 1.15 x -0.14, r = 0.86).

CONCLUSION

Quantification of myocardial perfusion in rats is feasible by myocardial contrast echocardiography using intermittent triggered imaging.

Combined assessment of myocardial perfusion and regional left ventricular function by analysis of contrast-enhanced power modulation images

BACKGROUND

Echocardiographic contrast media have been used to assess myocardial perfusion and to enhance endocardial definition for improved assessment of left ventricular (LV) function. These methodologies, however, have been qualitative or have required extensive offline image analysis. Power modulation is a recently developed imaging technique that provides selective enhancement of microbubble-generated reflections. Our goal was to test the feasibility of using power modulation for combined quantitative assessment of myocardial perfusion and regional LV function in an animal model of acute ischemia.

METHODS AND RESULTS

Coronary balloon occlusions were performed in 18 anesthetized pigs. Transthoracic power modulation images (Agilent 5500) were obtained during continuous intravenous infusion of the contrast agent Definity (DuPont) at baseline and during brief coronary occlusion and reperfusion and were analyzed with custom software. At each phase, myocardial perfusion was assessed by calculation, in 6 myocardial regions of interest, of mean pixel intensity and the rate of contrast replenishment after high-power ultrasound impulses. LV function was assessed by calculation of regional fractional area change from semiautomatically detected endocardial borders. All ischemic episodes caused detectable and reversible changes in perfusion and function. Perfusion defects, validated with fluorescent microspheres, were visualized in real time and confirmed by a significant decrease in pixel intensity in the left anterior descending coronary artery territory after balloon inflation and reduced rate of contrast replenishment. Fractional area change decreased significantly in ischemic segments and was restored with reperfusion.

CONCLUSIONS

Power modulation allows simultaneous online assessment of myocardial perfusion and regional LV wall motion, which may improve the echocardiographic diagnosis of myocardial ischemia.

In vitro quantification of flow using continuous infusion of Levovist and pairs of harmonic power Doppler images

To evaluate the potential of harmonic power Doppler to quantify perfusion using a continuous infusion of contrast, two dialysis cartridges were perfused with different flow rates adjusted between 0 to 300 mL/min, corresponding to flow ratios comprised between 300:0 and 150:150. The contrast agent (Levovist, Schering) was injected at constant rates (0.6 to 5 g/h). Sequential pairs of images showing simultaneously the cross-sections of the two filters were acquired with a HDI 5000 (ATL) and the Doppler data were processed with HDI lab software (ATL). The absolute values of the signal in the different regions-of-interest (ROI) were not closely related to flow rate. At the opposite, the rapid signal decrease between the first and the second image of each pair was inversely proportional to the flow rate. An index of perfusion [PerI = image 1/(image 1 -- image 2)] was defined. It correlated closely with the absolute and relative flow rates. For the latter, the slopes of regression were found to be independent of the infusion rate of Levovist. Thus, the use of pairs of images combined with a continuous infusion of Levovist provide a quantification of perfusion.

Assessment of myocardial perfusion by myocardial contrast echocardiography using harmonic power and the transvenous contrast agent SHU 563A in acute coronary occlusion and after reperfusion

BACKGROUND

Harmonic power Doppler imaging is a novel technique for the assessment of myocardial perfusion by contrast echocardiography. In this study, we examined whether myocardial contrast echocardiography using harmonic power Doppler and the new transvenous contrast agent SHU 563A can identify myocardial perfusion defects during coronary occlusion and reperfusion.

METHODS

To assess the potential of this technique, we occluded either the left anterior descending coronary artery or the circumflex coronary artery for 2 to 3 h followed by 1 h reperfusion in 10 dogs in an open chest model. After transvenous administration of SHU 563A, an air-filled, polymeric contrast agent, myocardial contrast echocardiography was performed in short and long axis views with triggered harmonic power Doppler imaging after coronary occlusion and reperfusion. Post-mortem triphenyl tetrazolium chloride staining was performed to verify infarction. Harmonic power Doppler and anatomic data were analyzed by independent observers.

RESULTS

During coronary occlusion, harmonic power Doppler showed perfusion defects in all 10 dogs. The defect size in the short axis view at papillary muscle level ranged 4-51% (14+/-13%) and 3-43% (16+/-10%) in the long axis view (% total LV slice area). After reperfusion (1 h) and infusion of dipyridamole (0.56 mg/kg), power Doppler demonstrated perfusion defects in seven dogs: 0-20% (9+/-8%) (short axis view) and 0-48% (13+/-14%) (long axis view). Five dogs showed anatomic infarction. The anatomic infarct area was 0-18% (6+/-8%) (slices corresponding to the echocardiographic short axis images). Perfusion defect size by harmonic power Doppler correlated well with residual infarct size (r=0.82, P<0.01).

CONCLUSIONS

Myocardial contrast echocardiography using harmonic power Doppler and the new contrast agent SHU 563A accurately displays perfusion defects during acute coronary occlusion and after reperfusion. The site and size of residual myocardial infarction is reliably identified on line, in color. This approach has excellent potential for clinical application.

Contrast Echocardiography in Clinical Practice

After FDA approval of the new-generation contrast agent Optison (Mallinckrodt Medical, St. Louis, MO) January 1998, the use of contrast in echocardiograhy has become an invaluable tool. A review of 100 patients revealed contrast to be useful for endocardial border definition and wall segment analysis, enhancement of pulsed Doppler, and chamber opacification for the detection of thrombi. Evaluation of wall segments by two observers before and after injection of the contrast agent revealed an increase in the number of wall segments visualized by 4.8. Postinjection readings were consistent between the two observers. Routine contrast echocardiography may provide a more diagnostic study.

Stereolithographic biomodeling to create tangible hard copies of cardiac structures from echocardiographic data: in vitro and in vivo validation

OBJECTIVES

This study investigated the feasibility, accuracy and clinical potential of creating polymer hard copies of echocardiographic data using stereolithography.

BACKGROUND

Three-dimensional (3D) echocardiography has so far been limited by the need to display reconstructed 3D objects on a two-dimensional screen. Thus, tangible stereolithographic polymer models created from echocardiographic data could enhance our spatial perception of cardiac anatomy and pathology.

METHODS

Hard-copy replicas of water-filled latex balloon phantoms (n = 7) and porcine liver specimens (n = 12) were generated from echocardiographic images using stereolithography (computerized laser polymerization). In addition, we created 24 models of the mitral valve from 12 transesophageal studies (normal = 6, mitral stenosis n = 4, prolapse/flail leaflet n = 8, annular dilation n = 2, leaflet restriction n = 2 and following mitral valve repair n = 2).

RESULTS

Excellent agreement was found for comparison of volumes (r = 0.98, SEE = 3.46 mm3, mean difference = 0.25 +/- 3.33 mm3) and maximal dimensions (r = 0.99, SEE = 0.16 cm, mean difference = 0.03 +/- 0.16 cm) between phantoms and their corresponding replicas. Visual and tactile examination of mitral valve models by two blinded observers allowed correct depiction of mitral valve anatomy and pathology in all cases.

CONCLUSIONS

Stereolithographic modeling of echocardiographic images is feasible and provides tangible polyacrylic models that are true to scale, shape and volume. Such models offer accurate depiction of mitral valve anatomy and pathology in patients studied with transesophageal echocardiography. This technique could have substantial impact on diagnosis, management and preoperative planning in complex cardiovascular disorders.

Enhancement of ultrasound-accelerated thrombolysis by echo contrast agents: dependence on microbubble structure

The combination of ultrasound (US) exposure and ultrasound contrast agent (UCA) further increases the amount of drug-mediated thrombolysis. The aim of this study was to examine the efficacy of the combination of US and UCA on tissue plasminogen activator (tPA) thrombolysis, and the dependence on the microbubble structure. A catheter-type transducer capable of US emission (10 MHz, spatial peak temporal average intensity = 1.02 W/cm2 and peak negative pressure = 0.33 MPa) in the continuous-wave mode was employed. In 28 artificial white thrombi, serial changes in acoustic properties monitored by echography and histopathology during the tPA-mediated thrombolysis were analyzed. The thrombi were assorted to 4 groups; UCA nontreated (Control), sonicated albumin (A)-, SH-U508A (SH)- and dodecafluoropentane emulsion (DDFP)-treated groups. Persistence of microbubble opacification and thrombus weight were also measured. After the sample was suspended in a beaker with tPA (8000U) and 100 mL of saline, the UCA was administered and the mixture exposed to US for 10 min. Weight reduction of the thrombus was greatest in the DDFP Group (-49 +/- 8%), and that in the A Group (-8 +/- 5%) was not significantly different from that in the Control Group (-5 +/- 1%). The persistence of the microbubbles expressed as the decay of the time-intensity curve, was longest in the DDFP Group. The echo intensity of the superficial layer of the thrombus exposed to US was high and weight loss was marked. Multiple cavity formation was observed histopathologically. The stability of the microbubbles was an important factor of the US and UCA enhancement effect on tPA-mediated thrombolysis. This combination therapy has potential for clinical application in patients with thrombotic arterial and venous occlusion and left arterial thrombus.

Can power motion imaging mode combined with contrast agent assess myocardial contraction and perfusion? In vitro and in vivo studies

Power motion imaging (PMI) is a new Doppler imaging technology that can enhance contrast agent visualization. We hypothesized that PMI combined with Albunex injection (CPMI) could provide new insights into myocardial contractility and perfusion. In a first step, PMI signal was studied with an in vitro phantom. In a second step, PMI signal was studied in 10 rabbits at different workloads. In a third step, 10 rabbits were studied before and after ischemia produced by coronary ligation and finally during reperfusion. During the latter protocol, epicardial echocardiography analyzed by PMI and fundamental mode was performed before and after Albunex injection. PMI signal was well correlated with phantom velocity. PMI signal and myocardial wall thickening were well correlated, particularly in the septal area. On the coronary occlusion model, ischemia was associated with a significant decrease in PMI and CPMI signals, whereas reperfusion was associated with a significant decrease in PMI signals only, indicating stunning. We conclude that PMI combined with CPMI is a powerful tool to assess myocardial contractility and perfusion.

Interaction of Microbubbles with Ultrasound

The clinical need for bedside myocardial perfusion studies is obvious in the present era of revascularization. Animal and first clinical studies suggest that microbubbles can be used as intravascular tracers of perfusion in conjunction with echocardiography as an imaging modality. In order to fully appreciate the potential and limitations of this approach, the complex interactions of microbubbles within an acoustic field need to be elucidated. Most importantly, there is a strong dependence of bubble effects on the acoustic pressure. At low pressures, linear backscatter yields low signal intensities; at medium range of pressures, bubble resonance causes the reflection of nonlinear signals with harmonic frequencies; and at high pressures, spontaneous acoustic emission with high signal intensity occurs as a final signal of the bubble in its process of disintegration. Thus, in order to allow sufficient replenishment of bubbles to the imaging plane, triggered imaging should be used with one frame every second to eighth cardiac cycle. Traditional gray scale echocardiography was not successful as an imaging modality because of the similarity of gray shades between the myocardium and the contrast effect. Subsequently, second harmonic imaging was developed and was fairly successful in contrast detection, although inherent problems persisted due to the overlap of fundamental and harmonic frequencies in the filtered signals. Harmonic power Doppler imaging turned out as the most sensitive acquisition method, however, with an early dropout at medium range attenuation. In theory, the new technique of pulse inversion may be most promising as this bubble specific imaging modality should combine high sensitivity of detection with great tolerance for attenuation effects in humans. First in vitro studies have confirmed its superiority over harmonic power Doppler in combination with stabilized microbubbles such as SonoVuetrade mark. Thus, we will have to accomplish a lot more work and comparative studies in humans before myocardial contrast echocardiography can emerge as a reproducible technique for evaluating myocardial perfusion with high diagnostic accuracy.

Estimation of blood perfusion using ultrasound

Ways to measure blood perfusion using ultrasound techniques such as continuous-wave Doppler, pulsed Doppler, colour Doppler and power Doppler will be reviewed. From a certain standpoint, blood perfusion may be defined as the difference between arterial inflow and arterial outflow from a considered volume, i.e. capillary flow. The low velocities and small blood volumes involved make the signal-to-noise ratio, dynamic range and frequency resolution critical factors in the detection system. Another limiting factor is tissue motion which obscures the blood signal. Perfusion may still under certain conditions be estimated with the first moment of the Doppler power spectrum, as obtained with any Doppler ultrasound method. Modern flow mapping techniques also make it possible to estimate perfusion by counting the number of pixels that indicate flow, but low flow velocities cannot be included in the estimate. Future high-frequency systems may, however, provide very detailed images of minute flow distributions in superficial layers. Contrast agents are widely used today to enhance the blood signal, and a technique named harmonic imaging can suppress movement artefacts from surrounding tissue. Transient signals from disrupting contrast agent particles in an ultrasound field can potentially be used for perfusion quantification. Future developments to extract the blood flow signal from its noisy environment, aside from contrast agents, may be multiple sample volumes, frequency compounding and/or improved signal processing. The lack of an adequate perfusion phantom for verification of measurements of microcirculatory flow becomes, however, more apparent with improved detectability of slow flows.