A correction scheme for refraction and time-of-flight artifacts in limited-angle spatial compound imaging with high-frequency ultrasound

In high-frequency ultrasound (HFUS) imaging systems usually mechanically moved single-element fixed-focus transducers are used, and a water path between transducer and the object (tissue area) is required. The speed of sound (SoS) differences of water and tissue causes beam refraction at the boundary. When applying spatial compounding by superposition of B-mode images obtained from different angles of insonation, time-of-flight (ToF) and refraction artifacts appear and reduce image resolution and contrast. Both are analyzed based on an analytical model, which assumes different but spatially constant SoS for the water and the tissue regions, respectively. For correction, a ray tracing algorithm is applied to the individual images used for compounding. The SoS of water is obtained experimentally by wire phantom measurements, and the SoS of the tissue, which is not exactly known, is iteratively changed to optimize the compound image in terms of resolution. Due to refraction and ToF errors, imaged structures appear at different positions for different angles of insonation. The position shift of a point-like scatterer is taken as a measure of the image correction quality. By minimizing this shift compared to uncorrected images, the mean SoS of the tissue can be determined. The superposition of image frames after the final correction step delivers the compound image with optimized resolution and image contrast. For evaluation, the scheme has been applied to wire phantom measurements and to measurements on a rat pup cadaver. The corrected compound images show improved image contrast and resolution.