Alwyn D'Sa

Integrated backscatter for quantification of left atrial spontaneous echo contrast

OBJECTIVES This study was designed to develop a quantitative method of spontaneous echo contrast (SEC) assessment using integrated backscatter and to compare integrated backscatter SEC measurement with independent qualitative grades of SEC and clinical and echocardiographic predictors of thromboembolism. BACKGROUND Left atrial SEC refers to dynamic swirling smokelike echoes that are associated with low flow states and embolic events and have been graded qualitatively as mild or severe. METHODS We performed transesophageal echocardiography in 43 patients and acquired digital integrated backscatter image sequences of the interatrial septum to internally calibrate the left ventricular cavity and left atrial cavity under different gain settings. Patients were independently assessed as having no, mild or severe SEC. We compared intensity of integrated backscatter in the left atrial cavity relative to that in the left ventricular as well as to the independently assessed qualitative grades of SEC. Fourier analysis characterized the temporal variability of SEC. The integrated backscatter was compared with clinical and echocardiographic predictors of thromboembolism. RESULTS The left atrial cavity integrated backscatter intensity of the mild SEC subgroup was 4.7 dB higher than that from the left ventricular cavity, and the left atrial intensity of the severe SEC subgroup was 12.5 dB higher than that from the left ventricular cavity. The left atrial cavity integrated backscatter intensity correlated well with the qualitative grade. Fourier transforms of SEC integrated backscatter sequences revealed a characteristic dominant low frequency/high amplitude spectrum, distinctive from no SEC. There was a close relationship between integrated backscatter values and atrial fibrillation, left atrial size, left atrial appendage flow velocities and thrombus. CONCLUSIONS Integrated backscatter provides an objective quantitative measure of SEC that correlates well with qualitative grade and is closely associated with clinical and echocardiographic predictors of thromboembolism. The relationship between integrated backscatter measures and cardioembolic risk will be defined in future multicenter studies.

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.

Myocardial contrast echocardiography: relation between on-line and off-line assessment of myocardial perfusion.

Background: Quantitative assessment of myocardial perfusion by myocafdial contrast echocardiography has been made possible by the use of custom‐made off‐line video‐intensity programs. A standardized program that could be used by all investigators would improve the reproducibility of results and enhance its clinical utility. Methods and Results: The purpose of this study was to determine if the assessment of myocardial perfusion by contrast echocardiography using a new commercially available, quantitative on‐line software program correlates with an off‐line custom‐made video‐intensity program previously validated by our laboratory and with radiolabeled microspheres, under various experimental myocardial perfusion conditions. Two of the measured myocardial contrast echocardiographic parameters (peak intensity, area under the time‐intensity curve [area]) correlated well among on‐line and off‐line methods and radiolabeled microspheres, especially when the data were “normalized” by comparing percent change from baseline or a ratio of ischemic to nonischemic myocardium. The third myocardial contrast echocardiographic parameter examined, half‐time of the peak intensity on the washout limb of the curve (t1/2), correlated only when the percent change from baseline was compared between the two methods or when the off‐line method was compared with radiolabeled microspheres. Conclusion: The results of this investigation add further support to the potential use of myocardial contrast echocardiography to evaluate serial changes in myocardial perfusion. Whether using the off‐line or on‐line analysis system, it is important that: (1) multiple regions of interest are examined during a single injection: (2) no log compression or postprocessing is performed on the returning ultrasound signal to avoid distortion of the acquired data; and (3) myocardial translation and rotation are corrected with an internal reference system.

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° 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.

946-107 Altered Global and Regional Myocardial Backscatter Characteristics in Cardiac Fatigue Following Prolonged Exercise

Prolonged exercise causes a transient reduction in LV systolic function, including regional wall motion abnormalities in the absence of ischemic ECG or enzyme abnormalities (cardiac fatigue). To assess the possible contribution of local tissue changes such as ischemia, ultrasonic integrated backscatter images (lBS) were obtained in 16 endurance athletes before and immediately after the Ironman triathlon (2.4 mi swim, 112 mi bike, 26 mi run; average duration 12 hrs 03 min). Echoes were recorded using identical acquisition parameters with transmit gain adjusted individually to preclude saturation. Mean IBS (absolute value, dB) at 2 transmit settings was measured using acoustic densitometry for pericardium (P), and septal (S) and posterior walls (PW) and converted to linear mV scale for normalization to pericardial reflectivity (%PR). Results were: P(dB) PW(dB) S(dB) P(mV) PW(mV) S(mV) PW-%PR S-%PR Pre 35.0 12.3 17.7 437 53 90 12.4 21.6 † Finish 34.0 10.1 * 13.4 *† 402 42 * 57 * 10.9 * 15.5 †* %Δ -2 -13 -21 -3 -13 -27 -5 -27t * p l 0.01 vs pre, † p l 0.05 PW vs S by ANOVA The lack of significant change in pericardial reflectivity at finish indicates that the acoustic properties of the chest were unchanged. Therefore, reductions in Sand PW IBS and reflectivity represent global alterations in myocardial characteristics, possibly due to edema andlor ischemia. These changes are more marked in the septum and correlate with previously described wall motion abnormalities in this area. In conclusion careful acquisition of ultrasonic backscatter images with normalization to pericardium allows serial noninvasive tissue characterization in man. IBS changes following prolonged exercise suggest that both global and regional metabolic or perfusion abnormalities may occur following physiologic stimuli and contribute to cardiac fatigue.