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The signal redundancy principle in the near field is analyzed quantitatively. It is found that common midpoint signals are not identical (or redundant) for echoes coming from arbitrary target distributions in the near field. A dynamic near-field correction is proposed to reduce the difference between common midpoint signals for echoes coming from the region of interest. When phase aberrations are present, it is shown that the dynamic correction can generally be done assuming no phase aberration, and the relative time-shift between common midpoint signals can be used to measure phase-aberration profiles. A phase-aberration correction algorithm based on that principle is proposed. In this algorithm, after common midpoint signals are collected they are dynamically corrected for near-field effects and cross-correlated with one another. In a related way, the phase errors are measured from peak positions of these cross-correlation functions. The phase-aberration profile across the array is derived from these measurements. The relationship between the errors in the derived phase aberration profile and the errors in the measured relative time-shift between common midpoint signals is derived. A method for treating the situation of different transmission and reception phase-aberration profiles is also proposed. This algorithm works for general target distributions, iteration is not required, and it can be used in other near-field, pulse-echo, imaging systems.
Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on (Volume:44 , Issue: 2 )
Date of Publication: March 1997