Fixed Point Motion Estimation for frame rate up conversion in cardiac ultrasound

Frame rate up conversion in a low frame rate cardiac ultrasound scan makes the images run smoother and potentially increases the diagnostic value of the scan. Linear interpolation is one way of increasing frame rate; however, ghosting and blurring are the main drawbacks of this type of interpolation. In low frame rate cardiac ultrasound imaging, motion estimation and motion compensation techniques commonly used in optical imaging are not suitable, because of speckle noise as well as large movement of the cardiac valves compared to other anatomical features. We propose a novel method termed Fixed Point Motion Estimation(FPME) to handle these problems. FPME is a bidirectional tracker which extracts search regions from the previous and next frames in a pyramidal manner. The bottom of the pyramid uses a large search area around the moving object. At the higher level in the pyramid, the search region is narrowed down to capture the moving object. The motion vector at each level of the pyramid is used as an offset for the next level and the overall displacement will be the vector summation of motion vector at each level of the pyramid. This method works well if we do not have any out of plane movement. To have more reliable motion vectors, at each level a vector median filter is applied on the vector field. The result is used as an initialization for an optical flow regularization to capture subpixel movement. We demonstrate the performance of FPME on four standard cardiac ultrasound recordings. Our experiments show that, in ultrasound images, FPME works well compared to current motion estimation techniques in optical imaging; it removes ghosting and blurring artifacts in the movie. FPME frame rate up conversion reduces the average sum of squares error in the valve region by 5.6% relative to linear interpolation and increases the average peak signal-to-noise ratio by 2 db.