Multi-band confidence processing for two-pass speckle tracking

In myocardial strain imaging using speckle tracking, large interframe strains can cause significant peak hopping. An iterative, or multi-pass, process for speckle tracking, using displacement estimates from the previous pass as an initial guess for a subsequent pass, can improve displacement accuracy if initial guesses are close to the true correlation coefficient peak. Different peak-hopping patterns are observed when speckle tracking is performed on different frequency components of radiofrequency (RF) data. To reduce these artifacts, we propose a two-pass approach using a multi-band algorithm to assess the quality of the displacement guess at each pixel by comparing correlation results from multiple sub-bands. In this study, RF data were acquired from the anterior wall of the left-ventricle in an open-chest dog using a commercial 2-D phased array. RF images were filtered into five frequency bands. In the first-pass, 2-D phase-sensitive correlation-based tracking (search region 33 x 9 pixels (axial x lateral)) was applied to adjacent frames of the broadband image and each sub-band image to produce six sets of 2-D displacement estimates. Following the first pass, a confidence index was found for each pixel based on the number of sub-bands with matching displacement estimates, the magnitude of the correlation coefficient, and estimated strain values. Cubic interpolation was performed between pixels in the broadband image for which the confidence weight exceeded a threshold. Second-pass tracking using the resultant displacements was performed with a small search region (5 x 3 pixels (axial x lateral)) to reduce peak hopping. The two-pass approach with multi-band selection criteria reduced peak hopping compared to the single-pass method without sacrificing spatial resolution. With the two-pass method, the variance of the interframe displacement was reduced by over 96% axially and 98% laterally. The variance of axial and lateral displacements accumulated from end dias- ole to peak systole was reduced by 99%.