A freehand ultrasound elastography system with tracking for in vivo applications.

Ultrasound transducers are commonly tracked in modern ultrasound navigation/guidance systems. In this article, we demonstrate the advantages of incorporating tracking information into ultrasound elastography for clinical applications. First, we address a common limitation of freehand palpation: speckle decorrelation due to out-of-plane probe motion. We show that by automatically selecting pairs of radio frequency frames with minimal lateral and out-of-plane motions, combined with a fast and robust displacement estimation technique, greatly improves in vivo elastography results. We also use tracking information and image-quality measures to fuse multiple images with similar strains that are taken from roughly the same location so as to obtain a high-quality elastography image. Finally, we show that tracking information can be used to give the user partial control over the rate of compression. Our methods are tested on a tissue-mimicking phantom, and experiments have been conducted on intraoperative data acquired during animal and human experiments involving liver ablation. Our results suggest that in challenging clinical conditions, our proposed method produces reliable strain images and eliminates the need for a manual search through the ultrasound data in order to find radio frequency pairs suitable for elastography.