Intravenous Albunex during transesophageal echocardiography: quantitative assessment by videodensitometry and integrated backscatter analysis from unprocessed radiofrequency signals

Impact of contrast echocardiography on diagnostic algorithms: pharmacoeconomic implications

The major goal of medicine in the era of managed care is to control escalating costs and to attain a high level of quality health care. Capitation has limited access to expensive and unnecessary testing, placing an emphasis on the prudent use of available technology. A vast armamentarium of available diagnostic screening tests are available within cardiology. Routine two-dimensional (2-D) echocardiography is a high-quality, low-cost test that provides enhanced portability and real-time test interpretation over other noninvasive test modalities. The echocardiogram may cost up to 50% less than competitive nuclear single-photon emission computed tomography (SPECT) imaging. However, on average 10% of routine and 33% of stress echocardiograms are suboptimal (disproportionately affecting obese patients and those with lung disease). Myocardial contrast echocardiography has been shown to provide enhanced endocardial border delineation and left ventricular opacification, to enhance Doppler signal, and to provide information on myocardial perfusion. In several recent phase II and III studies, the use of a contrast agent has been shown to improve the diagnostic accuracy of echocardiography substantially. Improvements in the diagnostic capabilities of echocardiography have been shown to (1) impact upon downstream repetitive testing in patients with an initially nondiagnostic echocardiogram, (2) potentially increase laboratory throughput, and (3) reduce the rate of false-positive and negative tests as a result of improved image quality. As clinical and cost-effectiveness parallel one another, the use of myocardial contrast echocardiography in selected patient cohorts will result in improved diagnostic accuracy and a cost-effective pattern of care.

Comparison of Acoustic Densitometry and Dobutamine Echocardiography for an Assessment of Myocardial Viability

AIM

The aim of this study has been to compare acoustic densitometry and dobutamine echocardiography for an assessment of myocardial viability.

METHODS AND RESULTS

Thirty-four patients with coronary artery disease and dysfunctional myocardial segments, who were referred for myocardial revascularization, underwent a viability assessment using low-dose dobutamine echocardiography and acoustic densitometry. Results of the two techniques were compared to follow-up resting echocardiography. This follow-up examination was performed at a mean of 3 months after successful revascularization in order to assess the recovery of function in revascularized, initially dysfunctional segments. Echocardiography was performed in standard views using 16-segment model of the left ventricle. Viable myocardium was identified by the augmentation of systolic thickening of an abnormal segment by at least one grade during dobutamine infusion and by the value of the maximal amplitude of cyclic variation of integrated backscatter. Acoustic densitometry had the sensitivity and specificity to predict functional recovery 90% and 77%, respectively. Dobutamine echocardiography had the sensitivity and specificity to predict contractile reserve 83% and 81%, respectively. The results were statistically comparable. Concordance between these methods was 80%.

CONCLUSION

Acoustic densitometry and dobutamine echocardiography did not statistically differ in the prediction of functional recovery dysfunctional myocardial segments after revascularization.

Rapid quantitative assessment of myocardial perfusion: spectral analysis of myocardial contrast echocardiographic images

We described a novel rapid spectral analysis technique performed on raw digital in-phase quadrature (IQ) data that quantitatively differentiated perfused from nonperfused myocardium based on the simultaneous comparison of local fundamental and harmonic frequency band intensity levels. In open-chest pigs after ligation of the left anterior descending coronary artery (LAD) and continuous venous contrast infusion, the fundamental-to-harmonic intensity ratio (FHIR) for samples placed within the left ventricular (LV) cavity (10.8 +/- 1.7 dB) and perfused myocardium (13.7 +/- 1.6 dB) were significantly (P <.001) lower than for nonperfused myocardium (27.1 +/- 2.9 dB). In attenuated images, the FHIR for the LV cavity and perfused myocardium were also significantly (P <.05) lower than for the nonperfused myocardium (21.4 +/- 3.0 dB, 34.4 +/- 3.2 dB, and 40.2 +/- 4.4 dB, respectively). Spectral properties of contrast microbubbles, as characterized by the FHIR, allow for rapid quantitative assessment of myocardial perfusion from data contained in a single-image frame, without requiring background image subtraction and image averaging.

Transmural variation of myocardial attenuation measured with a clinical imager

The objective of this study was to quantify the transmural variation in attenuation for the septal and lateral walls of the heart. Our approach was to utilize a commercially available ultrasonic imaging system to acquire images of excised sections of eight sheep hearts with an orientation similar to that encountered in the apical four-chamber view. The measured values (mean +/-SE) of the slope of attenuation for the transmural regions of the septum are: 1.40 +/-0.11, 0.99 +/-0.09, and 1.85 +/-0.16 (dB/cm/MHz) for the left subendocardial, midmyocardial, and right subendocardial zones, respectively. The analogous data from the lateral wall are: 1.42 +/-0.11, 0.83 +/-0.07, and 1.20 +/- 0.16 (dB/cm/MHz) for the subendocardial, midmyocardial, and subepicardial zones, respectively. These data demonstrate that ultrasonic attenuation associated with the septum and the lateral wall, when imaged in a manner similar to that of the apical four-chamber view, is anisotropic.

Radiofrequency spectral analysis of attenuated ultrasound signals in experiments with echo contrast microbubbles

Conventional gray-scale myocardial contrast echocardiography cannot distinguish perfused but attenuated from nonperfused myocardium because both may appear similar at low image intensity. We hypothesized that with radiofrequency spectral analysis of attenuated ultrasound signals, the harmonic-to-fundamental frequency ratio of the peak power spectrum (HFR(P)) could determine the presence of contrast microbubbles. We measured frequency responses of Optison microbubbles at defined degrees of ultrasound signal attenuation with different formulations of silicone (55D, 80A, and 3M); gray-scale intensities of Optison plus water compared with degassed water were analyzed at different attenuation settings (-25, -32, and -44 dB, respectively). HFR(P) values of Optison plus water were significantly higher than reference values of degassed water at each attenuation setting (55D, -14 +/- 2 dB versus -30 +/- 2 dB, P <.001; 80A, -19 +/- 2 dB versus -30 +/- 3 dB, P <.01; 3M, -22 +/- 2 dB versus -30 +/- 3 dB, P <.05), even though conventional videodensitometric analysis could not distinguish them. HFR(P) analysis objectively detects microbubbles in clinically relevant conditions of attenuation.

Transmural variation of myocardial attenuation and its potential effect on contrast-mediated estimates of regional myocardial perfusion

Promising technical developments suggest that it may be feasible to use contrast echocardiography to estimate regional myocardial perfusion. Although the optimal approach has not yet been determined, the use of a nonlinear (harmonic) response of the contrast agent is common to several recent advances. The purpose of this article is to delineate the relation between the anisotropic (angle-dependent) ultrasonic attenuation of the myocardium through which the sound wave has propagated and the regional, nonlinear response of the contrast agent. Apparent perfusion will be modulated by this regionally varying, path-dependent attenuation, which is determined by the local angle between the propagating sound wave and the myofiber orientation. We illustrate the potential magnitude of the effect of myocardial anisotropy for the apical 4-chamber view by examining propagation along the septum and the lateral wall. We present experimentally measured values of the attenuation of excised sheep myocardium, showing statistically significant differences in the attenuation in the mid wall compared with that in symmetrical zones to the left and right of the mid wall, reflecting the well-known myofiber orientations in these 3 regions. The nonlinear (harmonic) response of a contrast agent depends on the local pressure amplitude, which for a given mechanical index is determined by the attenuation accumulated along the path to the point where the regional perfusion is estimated.

Effects of tissue anisotropy and contrast acoustic properties on myocardial scattering in contrast echocardiography

In this study we explored the potential effects that tissue anisotropy, in conjunction with the acoustic properties of contrast, may have on quantitative measurements of myocardial perfusion with the use of ultrasonic contrast agents. We used a computer simulation of the parasternal short-axis view, based on previously measured values for the anisotropy of backscatter and attenuation of myocardium, to predict the backscattered energy from 18 specific regions within the heart before and after myocardial contrast perfusion. Results demonstrated a regional variation of contrast enhancement in the short-axis view and variations caused by incremental increases in contrast level for specific myocardial regions. Thus quantitative assessment of myocardial perfusion with contrast echocardiography is influenced by the anisotropic properties of the myocardium, and the resulting postcontrast image will depend on the interaction between tissue properties and contrast acoustic properties. The degree of myocardial enhancement caused by the presence of contrast may depend on the spatial position of the specific region investigated with respect to the transducer and the amount of contrast in the myocardium.

Assessment of Left Ventricular Area at Risk by Myocardial Contrast Two-Dimensional Echocardiography: An Evaluation of a New Animal Model

BACKGROUND: New echocardiographic contrast agents are commonly tested in the dog model. However, this species has a number of drawbacks, including difficulties in experimentation, cost, and ethical considerations. The rabbit has a number of advantages due to its relative simple coronary circulation. The present study was designed to evaluate the rabbit model for determination of areas of risk (ARs) by contrast echocardiography. METHODS: Eight rabbits were intubated and mechanically ventilated after occlusion of the left coronary artery with a ligature. The transducer (operating at 7.5 MHz) was positioned on the right ventricle through a right thoracotomy. The images were obtained after intra-aortic injection of 1 ml of Albunex, followed by 3 ml of dye (Blue Uniperse) for histological analysis postmortem. The ARs were obtained after circumscription of the various echocardiographic and histological images. RESULTS: Excellent echocardiographic images were obtained, largely due to the hemodynamic stability of the rabbit to ischemia. Echocardiographic ventricular areas, absolute AR, and relative AR correlated closely with postmortem data (r = 0.86, 0.94, and 0.94, respectively). The measurements were highly reproducible with low variability. CONCLUSIONS: The rabbit model shows promise for study by contrast echocardiography of myocardium subjected to ischemia. This method for determination of ARs was validated against postmortem findings. The method also should be of value in the evaluation of reperfusion.

Detection of perfusion defects during coronary occlusion and myocardial reperfusion after thrombolysis by intravenous administration of the echo-enhancing agent BR1

The purpose of this study was to detect myocardial perfusion defects as a result of coronary occlusion and myocardial reperfusion after thrombolysis with intravenous (i.v.) administration of the echo contrast agent BR1 (Bracco Research, Switzerland), which consists of microbubbles (median diameter 2.5 microm) containing sulfur exafluoride in a phospholipidic shell. To generate a coronary thrombosis, a copper coil was advanced into the left circumflex coronary artery in eight anesthetized dogs with opened chest cavities. Coronary occlusion occurred 18 +/- 10 minutes after the insertion of the coil and was documented both by an electromagnetic flow meter (as zero blood flow) and by radiolabeled microspheres (as myocardial perfusion defect). After 2 hours of occlusion, streptokinase was infused i.v.; reperfusion was documented by both the flow-meter and microspheres. Left ventricular cavity enhancement was apparent after all contrast injections. Peak cavity intensity did not increase with dose and was not affected by signal processing (suggesting signal saturation), whereas the duration of contrast effect significantly increased with the dose (from 26 +/- 16 to 147 +/- 74 seconds). Myocardial contrast intensity also increased after contrast (from 15 +/- 12 to 21 +/- 18 gray level/pixel, p < 0.001). Contrast echo detected myocardial perfusion defects (corresponding to 17% +/- 11% of LV cross-sectional area) in all the injections performed during coronary occlusion and detected myocardial reperfusion with a sensitivity of 50% versus microspheres. The extent of perfusion defects by contrast echo showed a good correlation with microspheres (r = 0.73). Myocardial reperfusion was not detected by changes in heart rate, aortic pressure, pulmonary arterial pressure, cardiac output, left ventricular fractional area change, or wall-motion score index. Hemodynamic parameters were not affected by contrast injections. Thus, the i.v. administration of BR1 allows us to accurately detect myocardial perfusion defects during coronary occlusion and, to a lesser extent, myocardial reperfusion after thrombolysis.

The emerging role of contrast agents in echocardiography

Because of an outstanding track record for diagnostic accuracy, noninvasive properties, ease of use, and relatively low expense, echocardiography has become a leading technique in the evaluation of cardiac disorders. In the three decades since echocardiography entered the ranks of standard cardiac diagnostic tools, refinements and technological advances have progressively increased its usefulness. One of the most noteworthy advancements has been the development of ultrasound contrast agents, which investigators are avidly seeking to apply to a broad spectrum of clinical settings and issues.

Recognition of acute cardiac allograft rejection from serial integrated backscatter analyses in human orthotopic heart transplant recipients. Comparison with conventional echocardiography

BACKGROUND

Previous studies showed that moderate and severe acute cardiac rejection (AR) but not mild AR is associated with significant myocardial acoustic changes. This study examines whether serial measurements of end-diastolic two-dimensional integrated backscatter (2D-IB) enhance the diagnostic potential of ultrasonic tissue analysis in AR.

METHODS AND RESULTS

Serial endomyocardial biopsies, conventional echocardiograms, and parasternal long-axis radiofrequency signals for determination of posterior wall and septal 2D-IB were performed in 52 transplant patients. Histology showed no AR in 155 biopsy samples, AR grade 1A in 25, AR grade 1B/2 in 27, and AR grade 3A/3B in 13. Whereas no significant 2D-IB changes occurred between AR-free studies and during AR grade 1A, posterior wall and septal 2D-IB increased during AR grade 1B/2 from -47.80 +/- 4.36 to -42.97 +/- 5.11 dB and from -36.72 +/- 7.45 to -32.52 +/- 7.98 dB (P < .001 and P < .05, respectively) and during AR grade 3A/3B from -47.96 +/- 4.74 to -38.25 +/- 5.32 dB and from -37.92 +/- 5.87 to -31.01 +/- 4.62 dB (P < .001 and P < .01, respectively). Changes in posterior wall and septal 2D-IB were greater during AR grade 3A/3B than during AR grade 1B/2 (P < .01 and P < .05). Increases of 1.5 dB in posterior wall or septal 2D-IB indicated AR grades > or = 1B with sensitivities of 88% and 83% and specificities of 89% and 85%; posterior wall and septal 2D-IB increases of 5.5 and 3.8 dB identified AR grades > or = 3A with sensitivities of 92% and 79% and specificities of 90% and 84%. Although a weak inverse correlation between posterior wall and septal 2D-IB changes and posterior wall and septal thickening (r = .41 and r = .39, both P < .001) and fractional diameter shortening (r = .35, P < .001) was found, significant 2D-IB increases also occurred in some rejecting patients with unaltered contraction.

CONCLUSIONS

Increases in end-diastolic posterior wall and septal 2D-IB in serial studies permit reliable identification not only of moderate and severe AR but also of mild AR. Because 2D-IB increase significantly more in AR with myocyte damage than without such damage, an estimate of AR severity appears feasible. Significant myocardial acoustic changes during AR may occur independently of changes in contractile performance.

Myocardial ultrasonic tissue characterization in patients with different types of left ventricular hypertrophy: a videodensitometric approach

Although analysis of the radio frequency signal is the most accurate approach to myocardial tissue characterization, clinical diffusion has been limited because of the complex technology required. Much easier to perform, videodensitometric analysis could represent a valuable alternative. Previous works carried out on radio frequency data have shown that the absolute value of ultrasonic back scatter increases while its diastole-to-systole variation decreases in the hypertrophied myocardium. This study was aimed at clarifying whether alterations in characterization indexes of ultrasonic tissue can be detected by means of a videodensitometric approach, whether a specific type of left ventricular (LV) hypertrophy can be identified with this method, and finally what possible relationships exist between parameters of contractile function and tissue characterization indexes. Myocardial echo intensity (MEI), its cyclic variation (CV), and the dynamic relationship between myocardial signal and wall thickness variations during the cardiac cycle were assessed in 20 healthy subjects, 11 patients with essential hypertension and LV hypertrophy, 15 patients with hypertrophic cardiomyopathy, and 4 patients with primary amyloidosis. The CV was lower in the interventricular septum of patients with cardiac hypertrophy as a group, compared with that of control subjects (13.0% +/- 5.6% versus 18.8% +/- 5.5%, p < 0.001), but it was similar among patients with different types of hypertrophy. In control subjects, a significant inverse correlation was found between the progressive decrease of the myocardial signal and the parallel increase in wall thickness during systole; this correlation was lost in 60% of patients with hypertrophic cardiomyopathy and 50% of those with amyloidosis, but only in 9% of patients with essential hypertension (chi square analysis 12.68, p < 0.01). The CV was associated with systolic wall thickening (r = 0.53, p = 0.0001) and fractional shortening (r = 0.44, p = 0.0014). MEI and its CV per se cannot distinguish among different types of LV hypertrophy; however, the loss of an inverse relationship between the myocardial signal and wall thickness may suggest abnormal myocardial conditions in individual patients with the same disease or comparable wall thickness.