Evaluation Of The Local Speed-Of-Sound Estimation For The Correction Of Ultrasound Compound Imaging By Speckle Analysis

In several applications like ultrasound based registration in navigated surgery, geometrically correct reconstructed ultrasound images are crucial for diagnostic and therapeutic success. The spatial information in ultrasound images is derived from the round trip propagation delay and an assumed speed-of-sound, which is 1540m/s for most ultrasound systems. Since different tissue types have different speeds-of-sound, geometrical information can be incorrect and spatial deformation in the resulting B-mode image might account for up to 5 mm.To solve that problem, an algorithm has been developed to estimate local speed-of-sound variations in compound images by speckle analysis employing three different criteria: signal-to-noise ratio (SNR), entropy and contrast. For the evaluation of this method, B-mode images of a speckle phantom with a cylindrical inclusion with a differing speed-of-sound are acquired from different directions and segmented. In order to estimate the optimal speed-of-sound combination, the speed-of-sound for each image segment is varied between 1435 m/s and 1615 m/s. That creates image deformation due to stretching and refraction. Deformed images are then superpositioned to generate a compound image. Speckle regions in the compound image are subsequently evaluated by the criteria. The speed-of-sound combination with minimal SNR, maximal entropy and maximal contrast are interpreted as the optimal parameters.The best estimation result for the phantom body is accomplished by the SNR criterion with an error of 0.04%. Best estimation for both phantom segments generated the contrast criterion with a mean error of 0.46%. Combining all criteria, the mean error is reduced by the factor of 6 for the phantom body and the mean factor of 1.5 for the total phantom, compared to the assumed speed-of-sound 1540 m/s.This geometrical correction improves compound imaging and therefore registration accuracy in navigated surgery and has a positive impact on therapeutic success.