4B-6 A Novel Method for Direct Localized Sound Speed Measurement Using the Virtual Source Paradigm

Accurate sound speed estimates are desirable in a number of fields, particularly adaptive imaging, and tissue and phantom characterization. In an effort to increase the spatial resolution of sound speed estimates, a new method is proposed for direct measurement of sound speed between arbitrary spatial locations. The method utilizes the sound speed estimator developed by Anderson and Trahey. Their least-squares fit of the received waveform´s curvature provides the wave´s point of origin. The point of origin and the delay profile calculated from the fit are used to arrive at a spatially registered virtual detector. Between a pair of registered virtual detectors a spherical wave is propagated. By beamforming the received data the time of flight between the two virtual sources can be calculated. From this information the local sound speed can be estimated. Validation of the estimator used both phantom and simulation results. The phantom consisted of two wire targets located near the transducer´s axis at depths of 17 and 28 mm. Using this phantom the sound speed between the wires was measured for a homogeneous (water) medium and for two inhomogeneous (DB-grade castor oil and water) mediums. The inhomogeneous mediums were arranged as an oil layer, one 6 mm thick and the other 11 mm thick, on top of a water layer. To complement the phantom studies, sources of error for spatial registration of virtual detectors were simulated. The sources of error presented here are multiple sound speed layers, and signal-to-noise-ratio. Results are shown for 3 different media. The local sound speed estimates had mean relative errors and standard deviations of 0.0991% plusmn0.655% for a homogeneous medium, and -0.0673%plusmn0.279% and -0.0343%plusmn0.119% for inhomogeneous media with an oil layer of 6 mm and 11 mm respectively. Simulations are shown as well.