Testing the limitations of 2-D companding for strain imaging using phantoms

Companding may be used as a technique for generating low-noise strain images. It involves warping radio-frequency echo fields in two dimensions and at several spatial scales to minimize decorrelation errors in correlation-based displacement estimates. For the appropriate experimental conditions, companding increases the sensitivity and dynamic range of strain images without degrading contrast or spatial resolution significantly. In this paper, we examine the conditions that limit the effectiveness of 2-D local companding through a series of experiments using phantoms with tissue-like acoustic and elasticity properties. We found that strain noise remained relatively unchanged as the applied compression increased to 5% of the phantom height, while target contrast increased in proportion to the compression. Controlling the image noise at high compressions improves target visibility over the broad range induced in elastically heterogeneous media, such as biological tissues. Compressions greater than 5% introduce large strains and complex motions that reduce the effectiveness of companding. Control of boundary conditions and ultrasonic data sampling rates is critical for a successful implementation of our algorithms.