ASA 124th Meeting New Orleans 1992 October

3pBV3. Limitations of diffraction correction techniques in the estimation of acoustic attenuation in biological media.

Fred K. Forster

Gregory V. Cullen

Tsu Hua Wang

Dept. of Mech. Eng., Univ. of Washington, Seattle, WA 98195

Numerous studies have addressed the accuracy of diffraction correction techniques for the measurement of the acoustic attenuation coefficient based on diffraction calculations or measurements in a lossless medium. While an often-sited limitation of the technique has been the effect of scatterer statistics, it is usually assumed that attenuation effects can be separated from diffraction in the Rayleigh--Sommerfeld diffraction integral. However, in this study, which utilized a high resolution pulse-echo system with a 30-MHz center frequency and an f/3 focused transducer, it was shown that the effects of attenuation on the diffraction cannot be separated. Using a graphite/silicon dioxide/agar tissue-mimicking phantom, measurements were made under lossless conditions that agree with calculations based on a current theoretical model [J. Cardoso and M. Fink, J. Acoust. Soc. Am. 90, 1074--1084 (1991)]. Using the same experimental model a simple test was developed that demonstrated attenuation cannot be separated from diffraction. It was also demonstrated that under these conditions using a lossless diffraction correction can lead to attenuation estimates that are less accurate than no correction at all when compared to a transducer translation method. [Work supported by NIH Grant No. AR 39818.]