SPECKLE REDUCTION IN PULSE ECHO ULTRASONIC-IMAGING USING A 2-DIMENSIONAL RECEIVING ARRAY

An experimental pulse-echo imager was developed for the purpose of reducing speckle in ultrasonic images. The system utilized a 64 element spherically focused segmented annuli array receiver with a common transmitter. Compounded images were formed using subapertures of varying size, shape and overlap, and the speckle and resolution characteristics of the final images observed. A point-like scatterer was imaged to determine the resolution, point spread function and sensitivity of the system along with a new measure called the resolution cell size. The response of the system was also simulated for comparisons. Images were made of a uniformly scattering material to determine the speckle signal-to-noise ratio, the speckle spot size and a measure of image quality, the normalized contrast-to-speckle ratio. Lateral resolution and resolution cell sizes only gradually increased with a decrease in subaperture size and system sensitivity was not greatly diminished. Incoherent summation of signals from small groups of elements, as compared to coherent summation, decreased the speckle noise by a factor of four while maintaining enough resolution to improve the image quality as measured by the CSR/d by a factor of almost two. The subaperture shape and orientation of overlap were also found to be significant.