The measurement accuracy of sound velocity by reflection mode methods is estimated theoretically for several versions of the technique suitable for in vivo application. Focal fields used in versions of the focus adjustment method (FAM) are analyzed by computer simulation as well as the incoherent crossed-beam method (ICM) and the cross-transmission-and-reception-beam method (CTRM) for the same acoustic conditions. The results show that FAM with a large reception aperture, and both crossed-beam methods, produce identical velocity measurement accuracies. A novel version of FAM with a modified synthetic aperture is presented that produces twice the accuracy of the other methods discussed. In vivo as well as in vitro sound velocity measurements have been made using high-resolution scanner with pixel-by-pixel real-time dynamic focusing based on the FAM concept with a large reception aperture. Significant sound velocity differences have been detected in normal, cirrhotic, and fatty livers. Since the potential measurement accuracy of FAM is at least twice that of the tested version, even better instrumentation for measurement of in vivo sound velocity is possible.
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