Two-dimensional spatial compounding with warping.

Spatial compounding aims to improve image quality through signal averaging, but speed-of-sound (SoS) and refraction errors can misalign the component frames and blur the compound image. A 2-D compounding system is demonstrated that uses a nonrigid registration (warping) to realign the frames before compounding. Block-based estimates of local misalignments are interpolated smoothly to compute the warp vectors. Simulations and a specialized phantom, both with a 9% SoS distortion, were created, and compound images with and without warping were compared to the conventional image. Image sharpness was compared by measuring the diameter of point targets and directional edge sharpness. The average registration accuracy was 0.06 to 0.07 mm (approximately one pixel). The diameter of point targets increased only 2% with warping vs. 32% without warping and directional edge sharpness dropped 3.7% vs. 20.0%. Furthermore, most of the speckle reduction due to compounding is retained when warping is used. The tests on simulated and phantom data demonstrate that the method is capable of making a small, but significant, improvement to image quality. The examinations in vitro and in vivo show the correct operation of the method with real tissue features. Further clinical studies should be performed to compare spatial compounding with and without warping to see which applications would benefit from the small improvement.