Sound speed estimation using ultrasound image registration

A method for estimating the average speed-of-sound (SOS) in tissue or other materials using automatic registration of electronically steered ultrasound images is presented. A mismatch between the SOS assumed for beam forming and image reconstruction and the actual SOS in the tissue to be imaged can lead to significant defocusing and geometric distortions of the image. The geometric distortions can be detected by registering multiple, overlapping images steered in different look directions. The average SOS in the field of view can be estimated from the geometric transformation required to register the images. The theoretical basis for SOS estimation using phased array and steered linear array transducers is derived. Simulations show a moderate sensitivity to noise, suggesting that SOS estimations can be accurate enough to improve image quality when used in the image formation process. Examples of SOS estimates in water and tissue-mimicking phantoms show promising results, with errors below 1%.