Aberrator integration error in adaptive imaging

Tissue speed of sound inhomogeneities cause significant degradation of medical ultrasound images. In some cases these inhomogeneities may be modeled as a thin time delay screen located at the face of the transducer. The effects of such near-field aberrations can be reduced by adding compensating time delays to the normal system focusing delays. Unfortunately array elements are generally large in at least one dimension when compared to variations in the aberrator, thus correction of the mean time delay on an element leaves residual variations in the time delay profile across that element. This paper presents theoretical expressions and simulation results describing the magnitude of this aberrator integration error. Simulations results are also presented which show the distortion of received pulses and degradation of point spread functions which results from aberrator integration error. These results indicate that aberrator integration error may be the dominant source of error in the implementation of adaptive imaging techniques and in phase aberration measurements. Thus, correction of near-field aberrations may be significantly more difficult than previously suspected.