Quantification of ultrasonic anisotropy in normal myocardium with lateral gain compensation of two-dimensional integrated backscatter images

Echocardiographic-based assessment of myocardial fiber structure in individual, excised hearts

The objective of this study was to assess the feasibility of using echocardiographic imaging as an approach for determining the myocardial fiber structure of intact, individual hearts. Seven formalin-fixed, ex vivo sheep hearts were imaged using a commercially available echocardiographic imaging system, and the intrinsic fiber structure for the reconstructed short-axis cross section was determined for a specific distance from the apex of each heart. Diffusion tensor magnetic resonance (DT-MR) images of each heart were acquired and fiber maps were created for comparison with the fiber structure obtained from the corresponding reconstructed echocardiographic images. These two methods of obtaining the fiber structure showed relatively good agreement, suggesting that measurements of fiber orientation for individual hearts can be derived from echocardiographic images. Further development of this method may provide a clinically useful approach for mapping the fiber orientation in individual patients over the heart cycle.

Improving the reproducibility of the cyclic variation of myocardial backscatter

The goal of myocardial tissue characterization is to augment information provided by two- and three-dimensional echocardiographic imaging, Doppler blood flow and speckle- or Doppler-derived tissue motion. Tissue characterization based on the systematic variation ofbackscattered ultrasound during the cardiac cycle ('cyclic variation') appears to be effective in characterizing both focal and diffuse myocardial pathologies. Unfortunately, comparison ofresults from different laboratories is difficult because of a lack of consistency among the several reported methods of analyzing the cyclic variation data. The goals of the present work are to present an improved method of analysis and to demonstrate that apparent disagreements are attributable primarily to the distinct approaches employed by different investigators. The improved automated method for determining the magnitude of cyclic variation utilizes binomial smoothing and an average deviation method and was validated using data acquired from 23 patients. This method illustrates a systematic means for resolving differences between laboratories. This resolution facilitates future comparisons between the cyclic variation of myocardial backscatter and measurements derived, for example, from strain-related approaches.

Intrinsic myoarchitectural differences between the left and right ventricles of fetal human hearts: an ultrasonic backscatter feasibility study

OBJECTIVE

Embryologically, cardiac chambers differ in their morphologic and contractile properties from the beginning. We hypothesized that a noninvasive ultrasonic backscatter investigation might illustrate the fundamental differences in myocardial morphologic properties of the 2 ventricles during heart development. The goals of this investigation were to 1) explore the feasibility of measuring the magnitude of cyclic variation of ultrasonic backscatter from the left and right ventricular free walls of fetal hearts; 2) compare measurements of the magnitude of cyclic variation from the left and right sides of the heart; and 3) determine if the observed results are consistent with predictions relating the overall backscatter level and the magnitude of cyclic variation.

METHODS

Cyclic variation data from the left and right ventricular free walls were generated from analyses of the backscatter from echocardiographic images of 16 structurally normal fetal hearts at mid-gestation.

RESULTS

The magnitude of cyclic variation was found to be greater for the left ventricular free wall than for the right ventricular free wall (4.5 +/- 1.1 dB vs 2.3 +/- 0.9 dB, respectively; mean +/- standard deviation; P < .0001, paired t test).

CONCLUSION

Measurements of the cyclic variation of backscatter can be obtained from both the left and right sides of fetal hearts demonstrating a significant difference between the measured magnitude of cyclic variation in the left and right ventricular myocardium. This observation is consistent with predictions relating the overall backscatter level and the magnitude of cyclic variation. The results of this study suggest cyclic variation measurements may offer a useful approach for characterizing intrinsic differences in myocardial properties of the 2 ventricles in assessing fetal heart development.

Regional variation in the measured apparent ultrasonic backscatter of mid-gestational fetal pig hearts

The goal of this study was to characterize and compare regional backscatter properties of fetal hearts through measurements of the apparent integrated backscatter. Sixteen excised, formalin-fixed fetal pig hearts, representing an estimated 53 to 63 days of gestation, were investigated. Spatially localized measurements of integrated backscatter from these specimens were acquired using a 50 MHz single-element transducer. The apparent integrated backscatter measurements demonstrate different patterns of backscatter from the myocardium of the right ventricle compared with that of the left ventricle. These backscatter measurements appear to be consistent with the anisotropy of the fiber orientation observed in histologic assessment of the same specimens. For each of the 16 hearts, the apparent integrated backscatter from the right ventricular myocardium was larger than that from the left ventricular myocardium, exhibiting mean apparent backscatter values of -35.9 +/- 2.0 dB and -40.1 +/- 1.9 dB (mean +/- standard deviation; n = 16; p < 0.001), respectively. This study suggests that the intrinsic ultrasonic properties of the left and right ventricular myocardium are distinct in fetal pig hearts at mid-gestation.

Left ventricular form and function revisited: applied translational science to cardiovascular ultrasound imaging

Doppler tissue imaging (DTI) and DTI-derived strain imaging are robust physiologic tools used for the noninvasive assessment of regional myocardial function. As a result of high temporal and spatial resolution, regional function can be assessed for each phase of the cardiac cycle and within the transmural layers of the myocardial wall. Newer techniques that measure myocardial motion by speckle tracking in gray-scale images have overcome the angle dependence of DTI strain, allowing for measurement of 2-dimensional strain and cardiac rotation. DTI, DTI strain, and speckle tracking may provide unique information that deciphers the deformation sequence of complexly oriented myofibers in the left ventricular wall. The data are, however, limited. This review examines the structure and function of the left ventricle relative to the potential clinical application of DTI and speckle tracking in assessing the global mechanical sequence of the left ventricle in vivo.

Measurements of the anisotropy of ultrasonic attenuation in freshly excised myocardium

Echocardiography requires imaging of the heart with sound propagating at varying angles relative to the predominant direction of the myofibers. The degree of anisotropy of attenuation can significantly influence ultrasonic imaging and tissue characterization measurements in vivo. This study quantifies the anisotropy of attenuation of freshly excised myocardium at frequencies typical of echocardiographic imaging. Results show a significantly larger anisotropy than previously reported in specimens of locally unidirectional myofibers. Through-transmission radio frequency-based measurements were performed on specimens from 12 ovine and 12 bovine hearts. Although ovine hearts are closer in size to human, the larger size of bovine hearts offers the potential for specimens in which myofibers are more nearly unidirectionally aligned. The attenuation coefficient increased approximately linearly with frequency. The mean slope of attenuation with frequency was 3-4 times larger for propagation parallel than for perpendicular to the myofibers. At perpendicular insonification, slopes between ovine and bovine myocardium were approximately equal. However, attenuation in bovine specimens was larger for angles approaching parallel. The difference in results for parallel appears consistent with what might be expected from increased myofiber curvature associated with smaller lamb hearts. Quantitative knowledge of anisotropy of attenuation may be useful in understanding mechanisms underlying the interaction of ultrasound with myocardium.

Anisotropy of apparent backscatter in the short-axis view of mouse hearts

The goals of this investigation were to measure the anisotropy of backscattered ultrasound observed in the short-axis view of mouse hearts in systole and diastole and to compare these measurements with predictions from a computer simulation. Measurements of midmyocardial apparent backscatter were obtained from analyses of the hearts of seven wild-type mice using a clinical imaging system utilizing a linear array with a nominal center frequency of 13 MHz. A computer model simulating the short-axis view was implemented based on previous measurements of the angle of insonification dependence of myocardial backscatter and attenuation. Results demonstrate that the measured backscatter was largest for those myocardial regions corresponding to approximately perpendicular insonification relative to the myofibers and the smallest for regions of approximately parallel insonification, with the minimum to maximum values of apparent backscatter differing by approximately 10 dB. The measured anisotropy of backscatter was similar for end-systole and end-diastole and was in good agreement with the predicted anisotropy obtained from the computer simulations.

Estimating myocardial attenuation from M-mode ultrasonic backscatter

The objective of this investigation was to determine whether measurements of myocardial attenuation can be obtained from analyses of M-mode images. We exploited the inherent anisotropy of myocardial properties as a means of systematically varying the attenuation to evaluate this M-mode image-based method for myocardial tissue characterization. A commercially available ultrasonic imaging system was used to acquire M-mode images of 24 excised cylindrical specimens from six formalin-fixed sheep hearts that were analyzed using video signal analysis. Data were compensated for the presence of bright intramural myocardial echoes, a potentially significant contributor to uncertainty in measurements of attenuation from backscattered ultrasound. The estimated attenuation coefficient in dB/cm at an effective center frequency of 2.75 MHz as a function of angle of insonification for measurements obtained from analyses of M-mode images is presented. Given a linear frequency-dependence of attenuation in myocardial tissue over frequencies ranging from 1.5 MHz to 8 MHz, as has been previously reported, M-mode image-based analyses were used to estimate the slope of attenuation. Results showed slopes of attenuation (over a -10 dB transmit bandwidth of 1.875 MHz to 3.75 MHz) ranging from 1.00 +/- 0.07 to 1.81 +/- 0.08 dB/(cm.MHz) for perpendicular and parallel insonification, respectively. These values were in good agreement with contemporaneously measured values (0.99 +/- 0.02 to 1.77 +/- 0.04 dB/(cm.MHz)) obtained over a frequency bandwidth of 4 MHz to 7 MHz using a through-transmission radio-frequency-based approach. These data suggest that robust measurements of myocardial attenuation can be obtained from analyses of M-mode images and that this method may be diagnostically feasible in the clinical setting.

Effects of region-of-interest length on estimates of myocardial ultrasonic attenuation and backscatter

Measurements of tissue properties using an image-based technique that makes use of an external reference may have the potential for practical clinical implementation in echocardiography. The objective of this study was to quantify the ability of this technique to distinguish myocardial attenuation and backscatter properties for specific lengths of the region-of-interest (ROI). We chose to exploit the anisotropic properties of the myocardium as a model for distinguishing tissue with different acoustic properties. Excised lateral wall segments from seven healthy adult sheep hearts were imaged using a commercially available (Philips/ATL) clinical scanner operating in the fundamental imaging mode with a linear array (L 7-4). Statistical and receiver operating characteristic (ROC) analyses were used to evaluate the ability of the video signal analysis method to differentiate midmyocardial from subendocardial regions based on measurements of the acoustic properties for specific lengths of the ROI. Results demonstrate that the ability to distinguish tissue properties increases with ROI length for both slope of attenuation and backscatter coefficient measurements. Statistically significant differences were observed for measurements utilizing the ROI lengths as short as 0.4 cm with corresponding progressively increasing areas under the ROC curves for increasing ROI lengths. [NIH R37 HL40302]

Relation of ultrasonic tissue characterization with integrated backscatter to contractile reserve in patients with chronic coronary artery disease

BACKGROUND

Previous studies have shown that viable but stunned myocardium displays contractile reserve and exhibits cardiac cycle-dependent variations of integrated backscatter (CVIB), whereas infarcted myocardium does not.

HYPOTHESIS

This study was designed to clarify whether assessment of the acoustic properties of the myocardium can predict contractile reserve in patients with chronic coronary artery disease (CAD).

METHODS

In all, 21 patients with chronic CAD and 19 normal control subjects were studied. The magnitude of CVIB of the myocardium was measured in the basal and mid segment of the anterior septum and posterior wall of the left ventricle, using a real-time, two-dimensional integrated backscatter imaging system. The results were compared with the percent systolic wall thickening and the wall motion before and after revascularization. The wall motion was graded as normal, hypokinetic, or akinetic, and contractile reserve was considered present when an akinetic or hypokinetic segment improved after revascularization.

RESULTS

The average magnitude of CVIB was lower among dysfunctional segments of CAD than among normal segments of controls (3.73 +/- 1.71 vs. 6.35 +/- 0.69, p < 0.001). Of the 77 segments examined, 38 showed reversible dysfunction. Before revascularization, percent systolic wall thickening was similar among segments showing contractile reserve compared with those with persistent dysfunction myocardium (17.97 +/- 8.41 vs. 16.83 +/- 6.37%, p = 0.19), and the mean CVIB was significantly greater in segments with than in those without contractile reserve (4.73 +/- 1.47 vs. 2.75 +/- 1.31, p < 0.001). The CVIB above 3 dB before percutaneous transluminal coronary angioplasty predicted segments with contractile reserve with a sensitivity and specificity of 84.2 and 79.5%, respectively.

CONCLUSIONS

Cardiac cycle-dependent variations of integrated backscatter reflected myocardial contractility and functional capacity of the myocardium. They predicted segmental contractile reserve in patients with CAD.

Ultrasound tissue characterization detects preclinical myocardial structural changes in children affected by Duchenne muscular dystrophy

Our goal was to identify early changes in myocardial physical properties in children with Duchenne muscular dystrophy (DMDch). Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin, which triggers complex molecular and biological events in skeletal and cardiac muscle tissues. Although about 30% of patients display overt signs of cardiomyopathy in the late stage of the disease, it is unknown whether changes in myocardial physical properties can be detected in the early (preclinical) stages of the disease. We performed an ultrasonic tissue characterization (UTC) analysis of myocardium in DMDch with normal systolic myocardial function and no signs of cardiomyopathy. Both the cyclic variation of integrated backscatter (cvIBS) and the calibrated integrated backscatter (cIBS) were assessed in 8 myocardial regions of 20 DMDch, age 7 +/- 2 years (range 4 to 10 years), and in 20 age-matched healthy controls. We found large differences in the UTC data between DMDch and controls; the mean value of cvIBS was 4.4 +/- 1.5 dB versus 8.8 +/- 0.8 dB, whereas the mean value of cIBS was 36.4 +/- 7.1 dB versus 26.9 +/- 2.0 dB (p < 10(-6) for both). In DMDch, all eight sampled segments showed cIBS mean values to be significantly higher and cvIBS mean values to be significantly lower than those in the controls. Finally, interindividual differences were greater in DMDch than in controls for both parameters.The myocardium in DMDch displays UTC features different from those in healthy controls. These results show that lack of dystrophin is commonly associated with changes in myocardial features well before the onset of changes of systolic function and overt cardiomyopathy.

Radio frequency dual-spectra analysis of regional myocardial perfusion: Comparison with harmonic densitometric method

Our objective was to compare harmonic-to-fundamental frequency ratio peak (HFRp) analysis with conventional harmonic gray-scale densitometric analysis on the basis of ability to eliminate heterogeneity in ultrasound signals. Broadband radio frequency and harmonic data were obtained by using intermittent short-axis scans in 10 open-chest pigs before and during infusion of contrast microbubbles. HFRp and gray-scale intensity values were measured in 6 segments of left ventricular myocardium. In baseline images, the influence of anisotropy on HFRp values was significantly less than that in gray-scale intensities. In perfusion assessment with subtraction, contrast heterogeneity in HFRp values was significantly smaller than that in gray-scale intensities. The increase in HFRp values after subtraction was significantly greater than that in gray-scale intensities in lateral (P <.001), posterior (P <.0001), and inferior (P <.01) myocardium. HFRp analysis can compensate for baseline myocardial anisotropy and regional contrast heterogeneity. With background subtraction, HFRp analysis allows better quantification of myocardial perfusion.

Myocardial elastography--a feasibility study in vivo

Early detection of cardiovascular diseases has been a very active research area in the medical imaging field. Assessment of the local and global mechanical functions is one of the major goals of accurate diagnosis. In this study, we investigated the feasibility of elastography for estimation and imaging of the local cardiac muscle displacement and strain in a human heart in vivo. In its noninvasive applications, elastography has been typically used to determine local tissue strain through the use of externally applied compression. For our study, we utilized the cardiac muscle motion during a cardiac cycle as the mechanical stimulus, and acquired successive radiofrequency (RF) data frames of the septal and posterior walls over a few cardiac cycles in parasternal and apical views, respectively. High-quality ciné-loop elastograms were obtained due to high frame rates and the resulting low decorrelation noise. Furthermore, the strain contrast was higher in the parasternal case, when only the posterior wall was imaged, and strain estimation was more robust in the apical view. High repeatability of the results was observed through elastographic measurements over several cardiac cycles. Finally, an M-mode version of elastography was used to follow part of the interventricular septum or the posterior wall over the course of two cardiac cycles. Not only do these preliminary results show that elastography is feasible in cardiac applications in vivo, but also that it can provide new information regarding cardiac motion and mechanical function. Future prospects include assessment of the role of elastography in detection of ischemia and infarction.

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.

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.

Ultrasonic tissue characterization with integrated backscatter during inotropic stimulation

Ultrasonic tissue characterization with integrated backscatter is an objective method to quantitatively define the physical state of the myocardium. To determine if backscatter imaging during inotropic stimulation could be used objectively to determine the myocardial viability and ischemia in patients with ischemic heart disease, the backscatter changes were examined in 23 patients with myocardial infarction during dobutamine stress two-dimensional (2-D) echocardiography. Coronary angiography was performed within 1 to 2 days after the stress test. The results of this study demonstrated that changes in backscatter variability correlated significantly with the wall motion changes in stress echocardiography during dobutamine infusion (p < 0.0001). In addition, it was shown that the backscatter changes were significantly different in various types of myocardial tissue. In 23 healthy control segments, the ultrasonic backscatter variability was preserved and unchanged during inotropic stimulation (p = NS). In 15 viable infarct zones, restoration or an increase in backscatter variability during low-dose dobutamine infusion was noted, this being lost when ischemia developing during high-dose dobutamine infusion (p < 0.01). In 9 nonviable infarct zones, the phase-weighted variation was usually < or = 0 and did not change significantly during inotropic stimulation, regardless of the patency of the infarct-related arteries. In 15 remote ischemic myocardial zones, the backscatter variability was preserved at the baseline level, did not change during low-dose dobutamine infusion, but decreased significantly during high-dose dobutamine stress (p < 0.01). In conclusion, dobutamine stress tissue characterization could offer an objective approach for the detection of myocardial viability and ischemia, and might be a useful adjunct to the conventional stress echocardiography.

Detection of residual tissue viability within the infarct zone in patients with acute myocardial infarction: ultrasonic integrated backscatter analysis versus dobutamine stress echocardiography

OBJECTIVES

The goals of this study were to analyze temporal changes in cardiac cyclic variation of integrated backscatter (CVIB) in acute myocardial infarction (AMI) and to investigate the predictive value of CVIB normalization compared with that of dobutamine stress echocardiography (DSE) in the assessment of functional recovery after revascularization.

BACKGROUND

The normal CVIB is blunted by ischemia and recovers early after reperfusion, faster than wall motion improvement. Analysis of CVIB has been widely investigated for its potential to detect viable myocardium in the early stage of infarction. No studies have compared CVIB analysis with other techniques for viability assessment in patients with acute ischemic.

METHODS AND RESULTS

Integrated backscatter images were obtained in 12 patients with AMI on days 1, 3, and 7 after admission and 1 month after revascularization. On day 7, DSE was performed in all patients. On admission, 22 of 144 segments were dyssynergic. On day 1, CVIB was abnormal in all 22 infarcted segments, on day 3, in 16, and on day 7, in only 10 infarcted segments. Eight of 10 segments nonviable by CVIB (CVIB-nonviable) were also nonrespondent by DSE; whereas 12 of 14 segments viable by DSE (DSE-viable) were also CVIB-viable. At follow-up, 10 CVIB-viable segments and 1 CVIB-nonviable segment showed functional recovery; whereas 10 of 14 DSE-viable segments showed functional recovery. Thus the positive predictive value of CVIB and DSE was 83% and 72%, respectively, with a diagnostic agreement between techniques in 77% of segments.

CONCLUSIONS

Our data suggest that the normalization in CVIB in the first week after AMI accurately predicts residual tissue viability within the infarct zone. We also observed that the initial pattern of cyclic variation may be predictive of functional recovery. Finally, we found a good correlation between the recovery of a normal CVIB in segments that were still dysfunctional and a more validated method to assess tissue viability, such as the dobutamine test.

The extracellular matrix is an important source of ultrasound backscatter from myocardium

Ultrasound tissue characterization with measurement of backscatter has been employed in numerous experimental and clinical studies of cardiac pathology, yet the cellular components responsible for scattering from cardiac tissues have not been unequivocally identified. This laboratory has proposed a mathematical model for myocardial backscatter that postulates the fibrous extracellular matrix (ECM) as a significant determinant of backscatter. To demonstrate the importance of ECM, this group sought to determine whether measurements of backscatter from the isolated ECM could reproduce the known directional dependence, or anisotropy of backscatter, from intact cardiac tissues in vitro. Segments of left ventricular free wall from ten formalin fixed porcine hearts were insonified at 50 MHz, traversing the heart wall from endo- to epicardium to measure the anisotropy of myocardial backscatter, defined as the difference between peak (perpendicular to fibers) and trough (parallel to fibers) backscatter amplitude. The tissue segments were then treated with 10% NaOH to dissolve all of the cellular components, leaving only the intact ECM. Scanning electron micrographs (SEM) were obtained of tissue sections to reveal complete digestion of the cellular elements. The dimensions of the residual voids resulting from cell digestion were approximately the diameter of the intact myocytes (10-30 microm). These samples were reinsonified after seven days of treatment to compare the anisotropy of integrated backscatter. The magnitude of anisotropy of backscatter changed from 15.4 +/- 0.8 to 12.6 +/- 1.1dB for intact as compared with digested specimens. Because digestion of the myocardium leaves only extracellular sources of ultrasonic scattering, and because the isolated ECM exhibits similar ultrasonic anisotropy as does the intact myocardium, it is concluded that there is a direct association between the ECM and the anisotropy of backscatter within intact tissue. Thus, it is suggested that ultrasonic tissue characterization represents a potentially clinically applicable method for delineating the structure and function of the ECM.

Alterations in ultrasonic backscatter during intra-aortic balloon counterpulsation support in patients with acute myocardial infarction

Alterations of ultrasonic backscatter parameters have been evident in humans with myocardial infarction or ischemia. The backscatter variability could be restored in ischemic or stunned myocardium after reperfusion. The aims of this study were to determinate changes in regional myocardial ultrasonic backscatter during intra-aortic balloon counterpulsation (IABP) support in patients with acute myocardial infarction (AMI), and to evaluate whether backscatter imaging could be a functional guide of IABP support. A total of 9 patients with AMI were investigated during IABP support with a two-dimensional (2-D) ultrasonic backscatter imaging approach for parasternal short-axis view. Coronary angiography was performed in 6 of the 9 patients. A total of 21 vessel territories were studied in different modes of IABP support: 1:1, 1:2 and standby. Restoration of cyclic variation of backscatter after IABP support was demonstrated in 10 vessel territories. Failure of restoration of cyclic variation of backscatter after IABP support was noted in 6 vessel territories with severe coronary lesions (total or nearly total occlusion) or scar tissue. No changes of the ultrasonic backscatter were found in nonischemic vessel territories with patent coronary arteries or TIMI III coronary flow. In addition, the wall motion score did not change significantly with different IABP support. These results suggest that IABP could restore the cyclic variation of backscatter in ischemic myocardium. Myocardial anisotropy may play an influential role in the alterations of ultrasonic backscatter. We propose that ultrasonic backscatter could be a noninvasively functional guide of IABP use in patients with AMI.

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.

Ultrasonic tissue characterization in predicting residual ischemia and myocardial viability for patients with acute myocardial infarction

The identification of viable myocardium and residual ischemia in patients with acute myocardial infarction has important prognostic implications. The ultrasonic tissue characterization with integrated backscatter and dobutamine-atropine stress echocardiography were performed 8.3+/-3 days after AMI in 30 patients. After coronary angioplasty for the residual stenosis of infarct-related artery, both modalities were repeated. The parameter obtained from ultrasonic tissue characterization, phase-weighted variation, could differentiate the myocardium with residual coronary stenosis or nonviable myocardium from the viable myocardium without residual coronary stenosis (p < 0.001). Using the cutoff value of 5.8 dB, the sensitivity, specificity and accuracy for detecting viable myocardium without residual coronary stenosis were 75%, 100% and 90.2%, respectively. The phase-weighted variation of the viable infarction zone restored after the coronary stenosis was relieved. In contrast, the nonviable myocardium had a small phase-weighted variation that was irrelevant to the patency of the infarct-related artery. The ultrasonic tissue characterization may be used in identifying patients with acute myocardial infarction whose infarction zones are viable without residual ischemia.

Effects of myocardial fiber orientation in echocardiography: quantitative measurements and computer simulation of the regional dependence of backscattered ultrasound in the parasternal short-axis view

We measured the regional disparity in backscattered ultrasound by means of obtaining integrated backscatter images of 10 healthy subjects and placing a region of interest in 18 distinct positions. A computer model simulating the short-axis view was implemented on the basis of previously measured values for the anisotropic ultrasonic properties of myocardium. Measurements showed that the integrated backscatter value was greatest for the anterior septum and decreased by 15.9 +/- 3.5 dB for the lateral wall and 17.7 +/- 3.5 dB for the inferior septum. The value in the posterior wall was 8.1 +/- 3.8 dB below the value for the anterior septum. The regional variation of backscatter predicted with the simulation correlated well with the clinical measurements. These results suggested that analyses based on measurements of backscatter may require compensation for the inherent anisotropic properties of myocardium.

Effects of tissue anisotropy on the spectral characteristics of ultrasonic backscatter measured with a clinical imaging system

In this paper, we report the effects of inherent tissue anisotropy on the spectral properties of backscattered ultrasound when measured with a commercially-available imaging system. We insonified five specimens of bovine tendon immersed in a water tank and rotated in 10 degrees increments while being imaged with a Hewlett-Packard Sonos 1500 system. The backscattered RF signals corresponding to each angle of insonification were digitized and the spectral characteristics of the backscattered ultrasound were determined. The mean anisotropy, defined as the average difference between values at perpendicular and parallel insonification, for band-limited estimates of backscattered power, centroid frequency, upper-band to lower-band power ratio, and upper-band to total-band power ratio were found to be 24.6 +/- 1.1 dB, 142 +/- 27 kHz, 32 +/- 13%, and 22 +/- 5%, respectively (mean +/- SE). The magnitude of each of these backscatter spectral parameters was larger at perpendicular insonification compared with the corresponding values at parallel insonification, consistent with previous measurements of the inherent anisotropy of ultrasonic attenuation and backscatter in tissue.

Ultrasonic tissue characterization for coronary care unit patients with acute myocardial infarction

The ultrasonic integrated backscatter of myocardium changes in infarction and ischemia. On the third day after acute myocardial infarction, 30 patients underwent ultrasonic tissue characterization from the parasternal short-axis view. With a composite parameter, the phase-weighted variation, sensitivity, specificity, and accuracy for diagnosing multivessel coronary artery disease were 84.6%, 52.9% and 66.6%, respectively. Using 67 degrees as the cutoff value for the phase deviation of the backscatter power curve, the recanalization of the infarct-related artery could be detected with a positive predictive value of 77.7% and a negative predictive value of 66.6%. Ultrasonic tissue characterization is a feasible technique for detecting the multivessel coronary artery diseases and the recanalization of infarct-related artery for patients with acute myocardial infarction. The diminished cardiac cycle-dependent variation in integrated backscatter and increased phase deviation can differentiate patent coronary arteries from those coronary arteries with anatomically significant stenoses.

Relation of ultrasonic tissue characterization with integrated backscatter to contractile reserve in chronic left ventricular ischemic dysfunction

Previous studies have shown that viable but stunned myocardium displays contractile reserve and exhibits cardiac cycle-dependent variations of integrated backscatter, whereas infarcted myocardium does not. The present study was designed to evaluate whether integrated backscatter imaging could be useful in identifying segments with recruitable inotropic reserve in patients with chronic left ventricular (LV) ischemic dysfunction. We studied 15 patients (mean age 59 +/- 10 years) with chronic coronary artery disease, anterior or inferior wall dysfunction, and depressed LV ejection fraction (35 +/- 12%), and 6 noncardiac control subjects (mean age 49 +/- 18 years). Cardiac cycle-dependent variations of integrated backscatter were measured in anterior and inferior segments during transesophageal echocardiography and compared with the contractile response (% wall thickening) of these segments to low doses of dobutamine (5 to 10 microg/kg/min). The average magnitude of cardiac cycle-dependent variations of integrated backscatter was greater among normally contracting segments of both patients and controls (5.67 +/- 0.88 and 5.64 +/- 2.26 dB, respectively, p = NS) than among dysfunctional segments (2.77 +/- 3.05 dB, p <0.01 vs control and remote segments). Dysfunctional segments were further categorized into those with and without dobutamine-induced contractile reserve. At baseline, systolic wall thickening was similar among segments responding to dobutamine than among those that did not (3.6 +/- 2.3% vs 2.9 +/- 1.6%, p = NS). During dobutamine, systolic wall thickening increased only in segments showing improvement in wall motion score (to 24.5 +/- 4.7%), whereas it remained unchanged in segments not responding to dobutamine (to 2.0 +/- 3.7%, p <0.01). The magnitude of resting cardiac cycle-dependent variations of integrated backscatter was larger in segments responding to dobutamine than in those with persistent dysfunction (5.31 +/- 2.06 vs 0.23 +/- 0.94 dB, p <0.01) and correlated significantly (r = 0.74, p <0.01) with systolic wall thickening during dobutamine. Our data demonstrate that resting cardiac cycle-dependent variations of integrated backscatter closely parallel contractile reserve in patients with chronic LV ischemic dysfunction. This suggests that tissue characterization with integrated backscatter could be a useful adjunct to the delineation of myocardial viability in these patients.

Comparison of integrated backscatter values obtained with acoustic densitometry with values derived from spectral analysis of digitized signals from a clinical imaging system

Time-domain-based integrated backscatter values obtained with the use of acoustic densitometry (AD) were compared with values determined from a spectral-based analysis of the radio-frequency (RF) signals with a modified Hewlett-Packard Sonos 1500 imaging system. Integrated backscatter images of five specimens of bovine tendon were acquired in the AD acquisition mode, and the corresponding signals related to the backscattered RF were digitized for each angle of insonification as the specimens were rotated in 10-degree increments. The integrated backscatter images were analyzed with the AD analysis package, and the corresponding values determined from the RF power spectra were obtained from the digitized ultrasonic signals. Good agreement was found between the two methods over the entire range of measured values. The mean anisotropy in the measured integrated backscatter (mean +/- standard error) was found to be 27 +/- 2 dB for time-domain-based analysis and 25 +/- 2 dB for RF spectral-based analysis.

Influences of ultrasonic machine settings, transducer frequency and placement of region of interest on the measurement of integrated backscatter and cyclic variation

Integrated backscatter and its cyclic variation are potentially important parameters to discriminate normal from diseased myocardium. Cyclic variation of integrated backscatter is expected to be independent of machine settings. Backscatter images of swine hearts were taken using a two-dimensional backscatter system while acoustic power was varied at different time gain control (TGC) settings. Cyclic variation was measured in vivo with various acoustic power and TGC settings using different transducer frequencies. Three different regions were analyzed. For any given TGC setting, the relationship between acoustic power and integrated backscatter in vitro was linear only over a narrow range. In vivo, cyclic variation was present at all regions studied in both long- and short-axis views. However, lower acoustic power (< 15 dB) and TGC (< 20 dB), or excessive settings of acoustic power (> 35 dB) and TGC (> 50 dB), produced minimal cyclic variation. Appropriate acoustic power (20-35 dB) and TGC (30-50 dB) produced larger and more consistent cyclic, variation at the posterior region of the left ventricle. These data indicate that each region has specific, appropriate machine settings to maximize the magnitude of cyclic variation.

Influence of data processing on cyclic variation of integrated backscatter and wall thickness in stunned porcine myocardium

This study was performed to investigate the relationship between the cyclic variation of integrated backscatter and myocardial wall thickening in stunned myocardium. Different definitions of cyclic variation were evaluated to be able to compare with other studies. Ultrasound data were acquired from 10 open-chested Yorkshire pigs (25-33 kg) at baseline, during regional ischemia and during 30 min of stunning, using a broadband ultrasound transducer (3-7 MHz) sutured directly upon the left ventricular myocardial wall. Cyclic variation of integrated backscatter and myocardial wall thickening were calculated using three definitions obtained from the literature. Independent of the definition, cyclic variation of wall thickness and integrated backscatter were blunted during acute ischemia and returned transiently to or above baseline during the first minute of reperfusion, followed by a gradual decrease to a level under baseline during stunning. An early return of the cyclic variation of the integrated backscatter was not observed in pigs, independent of the data processing used. The relationship between integrated backscatter and wall thickness was maintained.

The relationship between myocardial integrated backscatter, perfusion pressure and wall thickness during isovolumic contraction: an isolated pig heart study

To investigate the independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter, experiments were designed in which integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. In nine blood-perfused isolated pig hearts, perfusion pressure was uncoupled from left ventricular pressure generation (Langendorff method) and isovolumic contraction and relaxation were realized by inserting a noncompressible water-filled balloon into the left ventricle. In a first experiment, at constant perfusion pressure (85 mmHg), the integrated backscatter (3-7 MHz), the myocardial wall thickness and the left ventricular pressure were determined simultaneously at various balloon volumes (5-25 mL). A quasistatic increase of balloon volume by 50% resulted in an average decrease of wall thickness of 6.5% (p < 0.01) and a mean increase in the integrated backscatter level of 1.1 dB (p < 0.01). Integrated backscatter levels increased statistically significant by 0.14 +/- 0.014 dB per percent decrease of wall thickness. Measurements of percentage end-systolic myocardial wall thickening ranged from -10% to +10%, mean 0.15 +/- 4.5% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -3.9 to +3.9 dB, mean 0.19 +/- 1.5 dB (NS from zero). In a second experiment, at a constant midrange balloon volume, the same parameters were determined simultaneously at various perfusion pressures (20-120 mmHg). An increase in perfusion pressure by 50% resulted in a small but statistically significant increase of 1.5% in myocardial wall thickness, which could be explained by an increase of intravascular volume. The integrated backscatter levels did not change statistically significantly. Measurements of percentage end-systolic myocardial wall thickening ranged from -8.9 to +7.8%, mean 0.13 +/- 4.0% (NS from zero); whereas cyclic variation of integrated backscatter ranged from -1.8 to +4.2 dB, mean 0.37 +/- 1.3 dB (NS from zero). The magnitude of cyclic variation of integrated backscatter of myocardial tissue in a contractile state is reduced if myocardial muscle is prevented from normal thickening. In addition, changes in intravascular volume during the cardiac cycle have a negligible influence on the absolute backscatter level or its cyclic variation. We conclude, if only wall thickness and perfusion pressure are involved, that integrated backscatter is mainly determined by myocardial wall thickness.