2-D COMPANDING FOR NOISE-REDUCTION IN STRAIN IMAGING

Companding is a signal preprocessing technique for improving the preci sion of correlation-based time delay measurements. In strain imaging, companding is applied to warp 2-D or 3-D ultrasonic echo fields to imp rove coherence between data acquired before and after compression. It minimizes decorrelation errors, which are the dominant source of strai n image noise. The word refers to a spatially variable signal scaling that compresses and expands waveforms acquired in an ultrasonic scan p lane or volume. Temporal stretching by the applied strain is a single- scale (global), 1-D companding process that has been used successfully to reduce strain noise. This paper describes a two-scale (global and local), 2-D companding technique that is based on a sum-absolute-diffe rence (SAD) algorithm for blood velocity estimation. Several experimen ts are presented that demonstrate improvements in target visibility fo r strain imaging. The results show that, if tissue motion can be confi ned to the scan plane of a linear array transducer, displacement varia nce can be reduced two orders of magnitude using 2-D local companding relative to temporal stretching.