Registration-based sound speed estimation in phantoms with acoustically vaporized droplets

A mismatch between the sound speed assumed for beam forming and scan conversion and the true sound speed in the field of view can lead to significant defocusing and some geometric distortions in ultrasound images. A method is presented for estimating the average sound speed based on detection of these distortions using automatic registration of overlapping, electronically steered images. An acrylamide gel phantom containing vaporized dodecafluoropentane (DDFP) droplets as point targets was constructed to evaluate the technique. Good agreement (errors < 1%) was found between the sound speeds measured in the phantom using a reference pulse-echo technique and the image-based sound speed estimates. A small improvement in accuracy was achieved by including the simulated sound field of the probe rather than assuming straight acoustic beams and refraction according to Snell's law.