Rapid tracking of small displacements using ultrasound

Time-delay estimators, such as normalized cross correlation and phase-shift estimation, form the com- putational basis for elastography, blood flow measure- ments, and acoustic radiation force impulse (ARFI) imag- ing. This paper examines the performance of these algo- rithms for small displacements (less than half the ultra- sound pulse wavelength). The effects of noise, bandwidth, stationary echoes, kernel size, downsampling, interpolation, and quadrature demodulation on the accuracy of the time delay estimates are measured in terms of bias and jitter. Particular attention is given to the accuracy and resolu- tion of the displacement measurements and to the compu- tational efficiency of the algorithms. In most cases, Loupas' two-dimensional (2-D) autocorrelator performs as well as the gold standard, normalized cross correlation. However, Loupas' algorithm's calculation time is significantly faster, and it is particularly suited to operate on the signal data format most commonly used in ultrasound scanners. These results are used to implement a real-time ARFI imaging sys- tem using a commercial ultrasound scanner and a computer cluster. Images processed with the algorithms are examined in an ex vivo liver ablation study.

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