Integrated backscatter and digital acquisition during myocardial contrast echocardiography: is there an advantage over conventional echocardiography for intracoronary injections?

Myocardial blood volume and the amount of viable myocardium early after mechanical reperfusion of acute myocardial infarction: prospective study using venous contrast echocardiography

BACKGROUND

Myocardial capillary perfusion is a prerequisite of myocellular viability after reperfusion of acute myocardial infarction. It was hypothesised that the magnitude of myocardial capillary perfusion, assessed by transmural signal intensity in venous contrast echocardiography as a corollary of the blood volume of myocardial capillaries, and the amount of viable myocardium, represented by differential levels of contractile function two weeks after reperfusion, are correlated.

OBJECTIVES

To evaluate the role of venous contrast echocardiography for the identification of viable myocardium in patients with acute myocardial infarction early after successful mechanical reperfusion.

METHODS

60 patients with a first acute myocardial infarction underwent venous contrast echocardiography several hours after successful mechanical reperfusion (median time interval 190 min.). The relative transmural videointensity (median (25th, 75th percentiles)) of akinetic segments was determined. After two weeks, contractile function was re-evaluated at rest and during dobutamine infusion if segments without functional recovery were present.

RESULTS

Relative videointensity early after reperfusion differed significantly between functional groups after two weeks: normokinesia (88% (77%, 100%)), hypokinesia (74% (54%, 99%)), and akinesia with (61% (48%, 76%)) and without contractile reserve (31% (22%, 46%)). Relative videointensity and contractile function were significantly correlated (r = -0.67). The diagnostic accuracy of relative videointensity > 50% for prediction of contractility of initially akinetic segments at rest or during dobutamine was 82% (chi2 = 76.2, p < 0.001).

CONCLUSIONS

Early after successful mechanical reperfusion of acute myocardial infarction, the magnitude of capillary perfusion in the perfusion territory of an infarct related artery is correlated with the amount of viable myocardium. Quantitative venous contrast echocardiography can be used for accurate identification of viable myocardium.

Effect of pressure on intracardiac backscatter from microbubbles

The backscatter from sonicated albumin microbubbles (Albunex) was analyzed using acoustic densitometry in an in vitro pulsatile heart model to evaluate the effects of pressure on the backscatter from Albunex, and the cardiac cyclic changes of intracardiac backscatter from sonicated albumin microbubbles in 16 healthy persons were analyzed. It was found that the Albunex microbubbles were compressed in systole and decompressed in diastole, causing corresponding changes of backscatter in cardiac cycle. Although the intensities of backscatter in diastole and systole were related to the concentration of microbubbles, the concentration of microbubbles had no effect on the difference of end-diastolic and end-systolic backscatter. The difference of the backscatter was highly correlated with end-systolic pressure (r = 0.96, P = 0.001). In human studies, we also observed same intracardiac cyclic changes of backscatter from sonicated albumin microbubbles. Our study indicates that it is possible to evaluate the intracardiac pressure non-invasively by analyzing the intracardiac backscatter from the microbubbles with acoustic densitometry.

Myocardial perfusion and wall motion in infarction border zone: assessment by myocardial contrast echocardiography

Several mechanisms have been proposed to explain the decreased wall motion (WM) at the borders of myocardial infarction (MI). We used myocardial contrast echocardiography (MCE) to investigate the relation of perfusion to WM in infarcted border zones (BZs) 6 weeks after MI in 5 sheep. After quantifying the extent of WM abnormality and the perfusion defect, normal (NL), infarcted, and BZs were defined. Peak intensity after contrast was measured in acoustic units (AU). Radiolabeled microspheres were injected to measure regional blood flow. The heart was stained with 2,3, 5-triphenyltetrazolium chloride (TTC). The perfusion defect on MCE was 33% +/- 7% of the total myocardial area and correlated well with TTC (r = 0.92, P <.03). The BZ was 8% +/- 5% of the total myocardial area. Peak intensity after contrast was decreased in MI compared with BZ and NL (MI: 2.5 +/- 1.9 AU, BZ: 8.0 +/- 3.8 AU, P <.005; NL: 10.2 +/- 6.9 AU, P <.02) and comparable in NL and BZ. The blood flow measured by microspheres was not different in NL and BZ but was decreased in MI (NL: 1.6 mL/g/min, BZ: 1.5 +/- 0.5 mL/g/min, MI: 0.7 +/- 0.5 mL/g/min; P <.0001). In this model of chronic ovine MI, the BZ was small and its perfusion was preserved. These findings support the hypothesis that tethering of normal myocardial segments explains the abnormal wall motion noted at the borders of MI.

Persistent opacification of the left ventricle and myocardium with a new echo contrast agent

Echo contrast agents with long survival times open up new fields of application in the investigation of tissue perfusion and cardiovascular function. The purpose of this study was to characterize the time-course of the opacification of the heart cavities and myocardium with a new long-lasting second-generation, phospholipid-based echo contrast agent containing perfluoropentane (BY963-C5F12), and to compare its contrast potency with that of air-filled phospholipid monolayer (BY963-air). Doses of 0.03 mL/kg, 0.08 mL/kg and 0.16 mL/kg of BY963-air and BY963-C5F12 were administered intravenously to six conscious dogs weighing 25-36 kg. A transthoracic echocardiography was performed to evaluate peak intensity and area under the curve (AUC) from regions-of-interest placed in the right ventricle, left ventricle and left ventricular (LV) myocardium using acoustic densitometry. All injections were well tolerated, without wall-motion abnormalities or ECG changes. The LV cavity and myocardium were uniformly and well opacified for both echo contrast agents. However, at all administered doses, the contrast efficacy and duration were much more pronounced using BY963-C5F12 than with BY963-air. For the myocardium, the average peak intensity increased from 11.9+/-2.8 to 15.0+/-2.7 (not significant) following injection of BY963-air and from 12.8+/-3.2 to 18.7+/-2.8 (p < 0.01) following IV administration of BY963-C5F12; the latter corresponding to an increase in myocardial opacification of 46%. In conclusion, these results show the high myocardial opacification of BY963-C5F12 as compared to BY963-air. The simple incorporation of a perfluorocarbon gas into the phopholipid monolayer BY963 instead of air alters the acoustic properties of this contrast agent, resulting in qualitatively different application potentials for tissue opacification.

Myocardial Perfusion Abnormalities by Intravenous Administration of the Contrast Agent NC100100 in an Experimental Model of Coronary Artery Thrombosis and Reperfusion

The aim of this study was to evaluate a second-generation echo contrast agent (NC100100) for the study of myocardial perfusion. In eight anesthetized open-chest dogs, this agent was injected intravenously under baseline conditions, during acute coronary thrombosis, and after reperfusion, using both fundamental (FI) and harmonic (HI) imaging, both continuous and intermittent imaging, and both ultrasound (US) and integrated backscatter (IBS) imaging. Contrast injections did not modify the hemodynamic parameters. With all imaging modalities, myocardial contrast enhancement (MCE) was higher with intermittent than with continuous imaging (134 vs 82 gray level/pixel using FI, P = 0.02; 62 vs 32 acoustic units using US HI, P = 0.02; and 52 vs 12 dB using IBS, P = 0.05). MCE equally increased using either US or IBS imaging. The accuracy of MCE in detecting perfusion defects during coronary occlusion and myocardial reperfusion after thrombolysis was very good (sensitivity and specificity = 93% and 95% and 89% and 93%, respectively). The extent of myocardial perfusion defects by echo contrast showed a closer correlation with microspheres using HI (r = 0.82) than FI (r = 0.53). Thus, the intravenous administration of NC100100 during intermittent HI allows myocardial perfusion abnormalities to be accurately detected during acute myocardial infarction.

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.

[New developments in parameter-oriented roentgen densitometry perfusion analysis within the scope of heart catheter studies]

X-ray densitometric evaluation of digital subtraction coronary arteriograms allows a qualitative and quantitative detection of contrast medium propagation through the epicardial coronary arteries, the capillary system and the coronary venous system. So-called "time-density-curves" (TDCs) can be generated following Lambert-Beer's law similar to indicator dilution curves by using contrast medium as the indicator. Several time and density parameters can be derived from these TDCs, which are related to local myocardial perfusion. Different animal validation studies have shown the applicability of this concept for in-vivo evaluation of coronary blood flow and myocardial perfusion. Nevertheless, absolute measurement of volumetric coronary blood flow or myocardial perfusion failed. Therefore, relative changes in coronary blood flow or myocardial perfusion in response to pharmacologically induced maximum hyperemia were measured and coronary or myocardial perfusion reserve was calculated as the ratio of hyperemic flow or perfusion divided by baseline values. Despite theoretical attractions for an application during routine cardiac catheterization, this densitometric approach did not get a wide acceptance. Primary reason for this limited use in specialized centers was the time consuming process of densitometric evaluation of the subtraction coronary arteriograms, which require digital cine angiography and necessitates enormous computer hard ware. This main limitation has been overcome since more powerful computer hard ware (processor speed, hard disk space, digitization boards) has become rapidly available during the last years at more moderate pricing and digital techniques today are state of the art in cardiac catheterization laboratories. In addition, soft ware program packages allowed an automatization of the digitization and densitometric evaluation process. These programs include ECG triggered cine image digitization with improved temporal resolution, semiautomatic definition of regions-of-interest including definition of reference regions-of-interest for the detection of background density changes and quality-controlled densitometric parameter analysis. This progress made an application during routine cardiac catheterization feasible. In animal validation studies this improved X-ray densitometric approach for evaluation of local myocardial perfusion was validated versus colour-coded microsphere techniques. The time parameter "rise time", defined as the time from the start of local contrast medium induced density change to its maximum revealed a close correlation (r2 = 0.965) to the results of the microsphere technique over a wide range of perfusion. We have applied this technique before and after coronary interventions such as balloon angioplasty and stenting. Results documented an improvement of poststenotic myocardial perfusion reserve immediately after coronary balloon angioplasty and an additional improvement after adjunct coronary stenting. Only after stenting but usually not after coronary balloon angioplasty alone poststenotic myocardial perfusion reserve gained the intraindividual reference level, measured in a perfusion bed supplied by an epicardial coronary artery without stenoses. These results documented the functional benefit of coronary stenting on poststenotic myocardial perfusion in addition to the well known morphologic benefit with the creation of a larger and more circular conduit.

Contrast ultrasonography for 2-D opacification of heart cavities, peripheral vessels, kidney and muscle

Contrast ultrasonography of peripheral vessels and peripheral organs has been only sparsely used to evaluate peripheral tissue blood flow. The purpose of the study was to characterize intraluminal opacification of renal and femoral arteries and veins, of skeletal muscle and renal parenchyma after intraarterial (IA) injection of BY963, a newly developed ultrasound contrast agent being evaluated in Phase II and III trials, and to compare it with opacification of heart cavities after intravenous injection (IV) in dogs. A further purpose was to quantitate possible opacification losses during the first transcapillary passage of BY963 through pulmonary and peripheral microcirculation. BY963 was administered at the dose of 5 mL/animal/vascular territory (0.2 mL/kg). The peak intensity (intensity units = IU) and the area-under-the-curve (AUC, IU x heart cycles) were estimated from regions-of-interest placed in the right ventricle (RV), left ventricle (LV), main renal artery and vein, kidney, femoral artery and vein and adductor muscle. Following single IV injection, the average peak intensity and AUC values were 33 +/- 3 (mean +/- SE) and 674 +/- 109 for the RV, and 27 +/- 2 and 870 +/- 74 for the LV (p < 0.05), respectively. Following single IA injection in the descending aorta, the average peak intensities and AUC values were 35 +/- 2 and 613 +/- 139 in the renal artery and 26 +/- 4 (p < 0.05) and 639 +/- 151 in the renal vein (nonsignificant), respectively. For the femoral vessels, the average peak intensities and AUC values were 30 +/- 3 and 469 +/- 63 in the femoral artery, and 21 +/- 2 (p < 0.05) and 517 +/- 44 in the femoral vein (nonsignificant), respectively. The values for the output-to-input intensity ratios for peak intensity and AUC were 0.82 +/- 0.06 and 1.36 +/- 0.12 for the LV/RV ratio, 0.73 +/- 0.08 and 1.02 +/- 0.05 for the renal vein/renal artery ratio, and 0.71 +/- 0.09 and 1.16 +/- 0.13 for the femoral vein/femoral artery ratio, respectively (nonsignificant). In conclusion, these results demonstrate the high opacification potency of BY963 in the LV, renal and femoral veins, being of the same order of magnitude as that in the RV, renal and femoral arteries, respectively. Finally, the loss of opacification properties of BY963 during the first transcapillary (pulmonary or peripheral-capillary) passage is minimal.