Speckle coherence and implications for adaptive imaging.

Tissue speed of sound inhomogeneities cause significant degradation of medical ultrasound images. In certain cases these inhomogeneities can be modeled as a thin, spatially varying time delay screen located at the face of the transducer. Correction of such aberrators requires the addition of compensating time delays to the normal system focusing delays. These compensating delays are estimated from the arrival time differences between echoes received on different array elements. The accuracy with which these arrival time differences can be estimated is limited by the level of correlation between received speckle signals. This paper derives analytical expressions predicting the correlation between speckle signals acquired by a pulse echo system with either point or larger receive elements in the presence of near-field phase aberrations. Simulations are presented which are in good agreement with theoretical predictions. Similarities between the derived expressions and the Van Cittert-Zernike Theorem are discussed. These results indicate that near-field phase aberration correction may be far more difficult than previous analyses suggest because of the low correlation between echoes received by adjacent elements in elevation in 1.5-D arrays. Transmit aperture amplitude apodization and a new translating aperture technique are presented as methods for improving speckle correlation.