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Myocardial Strain by Doppler Echocardiography Validation of a New Method to Quantify Regional Myocardial Function

BackgroundMyocardial strain is a measure of regional deformation, and by definition, negative strain means shortening and positive strain, elongation. This study investigates whether myocardial strain can be measured by Doppler echocardiography as the time integral of regional velocity gradients, using sonomicrometry as reference method. Methods and ResultsIn 13 anesthetized dogs, myocardial longitudinal strain was measured on apical images as the time integral of regional Doppler velocity gradients. Ultrasonic segment-length crystals were placed near the left ventricular (LV) apex and near the base. Apical ischemia was induced by occluding the left anterior descending coronary artery (LAD), and preload was increased by saline. Percentage systolic strain by Doppler correlated well with strain by sonomicrometry (y =0.82 x −1.79, r =0.92, P <0.01). During LAD occlusion, apical myocardium became dyskinetic, as indicated by positive strain values and negative Doppler velocities. At the LV base, myocardial strain by Doppler, strain by sonomicrometry, and velocity of shortening by sonomicrometry (dL/dt) were unchanged during apical ischemia. However, myocardial Doppler velocities at the base decreased from 4.2±0.7 (±SEM) to 2.7±0.4 cm/s (P <0.05), probably reflecting loss of motion caused by tethering to apical segments. Volume loading increased myocardial Doppler velocities from 2.2±0.3 to 4.1±0.8 cm/s (P <0.05) and Doppler-derived strain from −12±1% to −22±2% (P <0.05), whereas peak LV elastance remained unchanged. ConclusionsMyocardial strain by Doppler echocardiography may represent a new, powerful method for quantifying regional myocardial function and is less influenced by tethering effects than Doppler tissue imaging. Like myocardial Doppler velocities, strain is markedly load-dependent.

Real-Time Strain Rate Imaging of the Left Ventricle by Ultrasound

The regional function of the left ventricle can be visualized in real-time using the new strain rate imaging method. Deformation or strain of a tissue segment occurs over time during the cardiac cycle. The rate of this deformation, the strain rate, is equivalent to the velocity gradient, and can be estimated using the tissue Doppler technique. We present the strain rate as color-coded 2-dimensional cine-loops and color M-modes showing the strain rate component along the ultrasound beam axis. We tested the method in 6 healthy subjects and 6 patients with myocardial infarction. In the healthy hearts, a spatially homogeneous distribution of the strain rate was found. In the infarcted hearts, all the infarcted areas in this study showed up as hypokinetic or akinetic, demonstrating that this method may be used for imaging of regional dysfunction. Shortcomings of the method are discussed, as are some possible future applications of the method.

Segmental and global longitudinal strain and strain rate based on echocardiography of 1266 healthy individuals: the HUNT study in Norway

AIMS To study the distribution of longitudinal systolic strain and strain rate (SR) as indicators of myocardial deformation according to age and sex in a healthy population. METHODS AND RESULTS Longitudinal strain and SR were determined in 1266 healthy individuals from three standard apical views, using a combination of speckle tracking (ST) and tissue Doppler imaging (TDI) to track regions of interest (ROIs). To test applicability of the reference values, we used a subset of the population to compare four methods of assessing myocardial deformation: (1) a combination of TDI and ST; (2) TDI with fixed ROIs; (3) TDI with tracking of ROIs; and (4) ST. Mean (SD) overall global longitudinal strain and SR were -17.4% (2.3) and -1.05 s(-1) (0.13) in women, and -15.9% (2.3) and -1.01 s(-1) (0.13) in men. Deformation indices decreased with increasing age. The combined and ST methods showed identical SR, but values were significantly lower than those obtained by TDI. Strain was overestimated by the ST method (18.4%) compared with the combined method (17.4%). CONCLUSION The reference values for global and segmental longitudinal strain and SR obtained from this population study are applicable for use in a wide clinical setting.

Myocardial strain imaging: how useful is it in clinical decision making?

Abstract Myocardial strain is a principle for quantification of left ventricular (LV) function which is now feasible with speckle-tracking echocardiography. The best evaluated strain parameter is global longitudinal strain (GLS) which is more sensitive than left ventricular ejection fraction (LVEF) as a measure of systolic function, and may be used to identify sub-clinical LV dysfunction in cardiomyopathies. Furthermore, GLS is recommended as routine measurement in patients undergoing chemotherapy to detect reduction in LV function prior to fall in LVEF. Intersegmental variability in timing of peak myocardial strain has been proposed as predictor of risk of ventricular arrhythmias. Strain imaging may be applied to guide placement of the LV pacing lead in patients receiving cardiac resynchronization therapy. Strain may also be used to diagnose myocardial ischaemia, but the technology is not sufficiently standardized to be recommended as a general tool for this purpose. Peak systolic left atrial strain is a promising supplementary index of LV filling pressure. The strain imaging methodology is still undergoing development, and further clinical trials are needed to determine if clinical decisions based on strain imaging result in better outcome. With this important limitation in mind, strain may be applied clinically as a supplementary diagnostic method.

Strain Rate Imaging by Ultrasound in the Diagnosis of Regional Dysfunction of the Left Ventricle

Background: Regional strain rate in the left ventricle can be assessed by tissue Doppler velocity gradient and color mapped in real time. Regional contractility thus can be visualized and graded. To validate the method, we made a comparison with standard echocardiography. Methods and Results: Fifteen patients with recent myocardial infarction were examined with the use of strain rate imaging (SRI). Velocity gradients were mapped by color. Gray‐scale imaging was performed using the second harmonic mode. Cine loops of two‐dimensional echocardiography (2‐D echo) and SRI images from three standard apical planes were analyzed offline. A four‐grade scale in 16 segments was used to score wall motion by 2‐D echo and by SRI. Of a total of 236 segments, 235 segments were analyzable by 2‐D echo and 218 segments were analyzable by SRI. Correlation of wall motion score index with ejection fraction was – 0.84 by 2‐D echo and – 0.92 by SRI. One hundred fourteen segments had an equal score by the two methods: 51 segments differed by 1 degree and 14 segments differed by 2 degrees (K = 0.45). Conclusions: SRI agrees well with echocardiography in grading regional wall function, and the method can be seen as validated in a clinical setting for assessment of regional systolic wall function and is demonstrated to be applicable for semiquantitative wall motion assessment. SRI has theoretical advantages and may be a valuable addition to standard echocardiography, especially in the field of stress echocardiography.

Clutter filters adapted to tissue motion in ultrasound color flow imaging

The quality of ultrasound color flow images is highly dependent on sufficient attenuation of the clutter signals originating from stationary and slowly moving tissue. Without sufficient clutter rejection, the detection of low velocity blood flow will be poor, and the velocity estimates will have a large bias. In some situations, e.g., when imaging the coronary arteries or when the operator moves the probe in search for small vessels, there is considerable movement of tissue. It has been suggested that clutter rejection can be improved by mixing down the signal with an estimate of the mean frequency prior to high pass filtering. In this paper, we compare this algorithm with several other adaptive clutter filtering algorithms using both experimental data and simulations. We found that realistic accelerations of the tissue have a large effect on the clutter rejection. The best results were obtained by mixing down the signal with non-constant phase increments estimated from the signal. This adapted the filter to a possibly accelerated tissue motion and produced a significant improvement in clutter rejection.

Regional myocardial long-axis strain and strain rate measured by different tissue Doppler and speckle tracking echocardiography methods: a comparison with tagged magnetic resonance imaging

AIMS Compare four different echocardiographic methods, based on tissue Doppler imaging (TDI) and speckle tracking (ST) separately or combined, for long-axis strain and strain rate (SR) measurements, using magnetic resonance imaging (MRI) tagging as a reference. METHODS AND RESULTS In 21 subjects (10 with myocardial infarction) peak systolic strain and systolic and early diastolic SR were measured by four different echo methods: (i) two-dimensional (2D) strain (B-mode); (ii) ST (custom software) of segment end-points (B-mode); (iii) similar to (ii), but combining ST with tissue Doppler tracking; (iv) strain from tissue Doppler velocity gradients (VG). Agreement with MRI tagging was better for strain than for SR. Ninety-five per cent limits of agreement were wider for the TDI-VG method, and 2D strain showed negative bias compared with MRI tagging and the other echo methods. Reproducibility was better for 2D strain than for MRI tagging and the other echo methods. CONCLUSION ST alone or combined with TDI seems to be suitable for automated measurements of regional myocardial deformation. The study gives important information on the strengths and weaknesses of the different methods, which is important for further development to increase accuracy and applicability.

Interpolation methods for time-delay estimation using cross-correlation method for blood velocity measurement

The cross-correlation method (CCM) for blood flow velocity measurement using Doppler ultrasound is based on time delay estimation of echoes from pulse-to-pulse. The sampling frequency of the received signal is usually kept as low as possible in order to reduce computational complexity, and the peak in the correlation function is found by interpolating the correlation function. The parabolic-fit interpolation method introduces a bias at low sampling rate to the ultrasound center frequency ratio. In this study, four different methods are suggested to improve the estimation accuracy: (1) Parabolic interpolation with bias-compensation, derived from a theoretical signal model. (2) Parabolic interpolation combined with linear filter interpolation of the correlation function. (3) Parabolic interpolation to the complex correlation function envelope. (4) Matched filter interpolation applied to the correlation function. The new interpolation methods are analyzed both by computer simulated signals and RF-signals recorded from a patient with time delay larger than 1/f/sub 0/, where f/sub 0/ is the center frequency. The simulation results show that these methods are more accurate than the parabolic-fit method. From the simulation, the worst estimation accuracy is about 1.25% of 1/f/sub 0/ for the parabolic-fit interpolation, and it is improved by the above methods to less than 0.5% of 1/f/sub 0/ when the sampling rate is 10 MHz, the center frequency is 2.5 MHz and the bandwidth is 1 MHz. This improvement also can be observed in the experimental data. Furthermore, the matched filter interpolation gives the best performance when signal-to-noise ratio (SNR) is low. This is verified both by simulation and experimentation.

Ultrasound-based vessel wall tracking: an auto-correlation technique with RF center frequency estimation

Vessel diameter is related to the distending blood pressure, and is used in estimations of vessel stiffness parameters. The vessel walls can be tracked by integrating wall velocities estimated by ultrasound (US) Doppler techniques. The purpose of this work was to evaluate the performance of the modified autocorrelation estimator when applied on vessel wall motion. As opposed to the conventional autocorrelation method that only estimates the mean Doppler frequency, the modified autocorrelation method estimates both the mean Doppler frequency and the radiofrequency (RF) center frequency. To make a systematic evaluation of the estimator, we performed computer simulations of vessel wall motion, where pulse bandwidth, signal-to-noise ratio (SNR), signal-to-reverberation ratio, packet size and sample volume were varied. As reference, we also analyzed the conventional autocorrelation method and the cross-correlation method with parabolic interpolation. Under the simulation conditions considered here, the modified autocorrelation method had the lowest bias and variance of the estimators. When integrating velocity estimates over several cardiac cycles, the resulting tissue displacement curves might drift. This drift is directly related to the magnitude of the estimator bias and variance. Hence, the modified autocorrelation method should be the preferred choice of method.

3-D Speckle Tracking for Assessment of Regional Left Ventricular Function

Speckle tracking in 2-D ultrasound images has become an established tool for assessment of left ventricular function. The recent development of ultrasound systems with capability to acquire real-time full volume data of the left ventricle makes it possible to perform speckle tracking in three dimensions, and thereby track the real motion of the myocardium. This paper presents a method for assessing local strain and rotation from 3-D speckle tracking in apical full-volume datasets. The method has been tested on simulated ultrasound data based on a computer model of the left ventricle, and on patients with myocardial infarction. When applied on simulated ultrasound data, the method showed good agreement with strain and rotation traces calculated from the reference motion, and the method was able to capture segmental differences in the deformation pattern, although the magnitudes of strains were systematically lower than the reference strains. When applied on patients, the method demonstrated reduced strain in the infarcted areas. Bulls-eye plots of regional strains showed good correspondence with wall motion scoring based on 2-D apical images, although the dyskinetic and hypokinetic regions were not apparent in all strain components.