METHODS FOR ESTIMATION OF SUBSAMPLE TIME DELAYS OF DIGITIZED ECHO SIGNALS

Time delay estimation (TDE) is commonly performed in practice by cross correlation of digitized echo signals. Since time delays are generally not integral multiples of the sampling period, the location of the la rgest sample of the crosscorrelation function (ccf) is an inexact esti mator of the location of the peak. Therefore, one must interpolate bet ween the samples of the ccf to improve the estimation precision. Using theory and simulations, we review and compare the performance of seve ral methods for interpolation of the ccf. The maximum likelihood appro ach to interpolation is the application of a reconstruction filter to the discrete ccf. However, this method can only be approximated in pra ctice and can be computationally intensive. For these reasons, a simpl e method is widely used that involves fitting a parabola (or other cur ve) to samples of the ccf in the neighborhood of its peak. We describe and compare two curve-fitting methods: parabolic and cosine interpola tion. Curve-fitting interpolation can yield biased time-delay estimate s, which may preclude the use of these methods in some applications. T he artifactual effect of these bias errors on elasticity imaging by el astography is discussed. We demonstrate that reconstructive interpolat ion is unbiased. An iterative implementation of the reconstruction pro cedure is proposed that can reduce the computation time significantly. (C) 1995 Academic Press, Inc.