For noninvasive and quantitative measurements of global two-dimensional (2D) heart wall motion, speckle tracking methods have been developed and applied. These methods overcome the limitation of tissue Doppler imaging (TDI), which is susceptible to aliasing, by directly tracking backscattered echoes by pattern matching techniques, i.e., the cross-correlation or the sum of absolute differences, in real time. In these conventional methods, the frame rate (FR) is limited to about 200 Hz, corresponding to the sampling period of 5 ms. However, myocardial function during the isovolumic contraction period obtained by these conventional speckle tracking methods remains unclear owing to low temporal and spatial resolutions of these methods. Moreover, the accuracy of the speckle tracking method depends on an important parameter, i.e., the size of the correlation kernel. To track backscattered echoes accurately, it is necessary to determine the optimal kernel size. However, the optimal kernel size has not been thoroughly investigated. In this study, correlation kernel size, which determines the tracking accurately, was optimized by evaluating root mean squared (RMS) errors in the lateral and axial displacements of a phantom estimated by speckle tracking methods at high spatial and temporal resolutions. For this purpose, the RF data from the longitudinal-axis cross-sectional view for the interventricular septum (IVS) were acquired on the basis of parallel beam forming (PBF) to improve temporal and spatial resolutions. A wide transmit beam scanned in 7 different directions sparsely and 16 receiving beams were generated for each transmission. The RF data of the phantom and the heart wall were obtained at high spatial (angle intervals of scan lines: 0.375 degrees) and temporal [frame rate (FR): 1020 Hz] resolutions. The determined optimal size of the correlation kernel was 7:9 Â 4:8 mm. Estimated displacements of the phantom were in good agreement with the actual displacement at an RMS error of 0.34 mm. Furthermore, the IVS motion during the isovolumic contraction (IC) was analyzed in detail. The speckle tracking method using the optimal kernel size 7:9 Â 4:8 mm was applied to multiple points in IVS to estimate 2D displacements during the IC period. In this period, a rapid displacement of IVS at a small amplitude of 1.5 mm, which suggests the expansion of the left ventricle and has not been measured by conventional tracking methods at a low temporal resolution, was detected by 2D tracking. Furthermore, the displacement on the apical side was found to be delayed by 10 ms compared with that on the basal side. These results indicate the potential of this method in the high-accuracy estimation of 2D displacements and detailed analyses of physiological function of the myocardium.
For noninvasive and quantitative measurements of global two-dimensional (2D) heart wall motion, speckle tracking methods have been developed and applied. In these conventional methods, the frame rate is limited to about 200 Hz, corresponding to the sampling period of 5 ms. However, myocardial function during short periods, as obtained by these conventional speckle tracking methods, remains unclear owing to low temporal and spatial resolutions of these methods. Moreover, an important parameter, the optimal kernel size, has not been thoroughly investigated. In our previous study, the optimal kernel size was determined in a phantom experiment under a high signal-to-noise ratio (SNR), and the determined optimal kernel size was applied to the in vivo measurement of 2D displacements of the heart wall by block matching using normalized crosscorrelation between RF echoes at a high frame rate of 860 Hz, corresponding to a temporal resolution of 1.1 ms. However, estimations under low SNRs and the effects of the difference in echo characteristics, i.e., specular reflection and speckle-like echoes, have not been considered, and the evaluation of accuracy in the estimation of the strain rate is still insufficient. In this study, the optimal kernel sizes were determined in a phantom experiment under several SNRs and, then, the myocardial strain rate was estimated such that the myocardial function can be measured at a high frame rate. In a basic experiment, the optimal kernel sizes at depths of 20, 40, 60, and 80 mm yielded similar results: in particular, SNR was more than 15 dB. Moreover, it was found that the kernel size at the boundary must be set larger than that at the inside. The optimal sizes of the correlation kernel were seven times and four times the size of the point spread function around the boundary and inside the silicone rubber, respectively. To compare the optimal kernel sizes, which was determined in a phantom experiment, with other sizes, the radial strain rates estimated using different kernel sizes were examined using the normalized mean-squared error of the estimated strain rate from the actual one obtained by the 1D phase-sensitive method. Compared with conventional kernel sizes, this result shows the possibility of the proposed correlation kernel to enable more accurate measurement of the strain rate. In in vivo measurement, the regional instantaneous velocities and strain rates in the radial direction of the heart wall were analyzed in detail at an extremely high temporal resolution (frame rate of 860 Hz). In this study, transition in contraction and relaxation was able to be detected by 2D tracking. These results indicate the potential of this method in the high-accuracy estimation of the strain rates and detailed analyses of the physiological function of the myocardium.
Methods for imaging of strain rate in the heart wall are useful for quantitative evaluation of regional heart function. We developed a method which can accurately measure the heart wall motion along an ultrasonic beam based on phase changes in rf echoes. However, there are some components in the wall motion which are not along each ultrasonic beam. Therefore, the measurement of motion in the direction perpendicular (lateral) to the beam has been required in addition to that in the axial direction, but some unsolved problems remain in estimation of lateral motion of the wall. In this study, two-dimensional displacement was estimated by 2D cross-correlation between rf echoes. Important parameters, the sizes of a region-of-interest and search region, which determine tracking accuracy, were adaptively optimized by referring to instantaneous wall velocities, in the respective cardiac phases. The correlation coefficient between the lateral displacement estimated by the 2D tracking with optimized parameters in longitudinal-axis view and axial displacement in apical view (corresponding to lateral displacement in longitudinal-axis view) separately and accurately estimated by the 1D phase-based method was 0.93. These results show possibility of this method for accurate measurement of two-dimensional heart motion to assess the regional myocardial strain rate.
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