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 de lays. These compensating delays are estimated from the arrival time di fferences between echoes received on different array elements. The acc uracy with which these arrival time differences can be estimated is li mited by the level of correlation between received speckle signals. Th is paper derives analytical expressions predicting the correlation bet ween speckle signals acquired by a pulse echo system with either point or larger receive elements in the presence of near-field phase aberra tions. Simulations are presented which are in good agreement with theo retical 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. Tra nsmit aperture amplitude apodization and a new translating aperture te chnique are presented as methods for improving speckle correlation. (C ) 1997 Acoustical Society of America.