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 dimensi ons and at several spatial scales to minimize decorrelation errors in correlation-based displacement estimates. For the appropriate experime ntal conditions, companding increases the sensitivity and dynamic rang e of strain images without degrading contrast or spatial resolution si gnificantly. In this paper, we examine the conditions that limit the e ffectiveness of 2-D local companding through a series of experiments u sing phantoms with tissue-like acoustic and elasticity properties. We found that strain noise remained relatively unchanged as the applied c ompression increased to 5% of the phantom height, while target contras t increased in proportion to the compression. Controlling the image no ise at high compressions improves target visibility over the broad ran ge induced in elastically heterogeneous media, such as biological tiss ues. Compressions greater than 5% introduce large strains and complex motions that reduce the effectiveness of companding. Control of bounda ry conditions and ultrasonic data sampling rates is critical for a suc cessful implementation of our algorithms.