Phase rotation in correlation coefficient filtering and multi-pass methods for 3-D speckle tracking in 4-D echocardiography

In 4-D echocardiographic (4DE) strain estimation using speckle tracking, large interframe strains resulting from moderate volumetric frame rates can cause significant peak hopping artifacts. Conventional correlation filters applied spatially at constant lags are effective for reducing peak hopping in low strain cases, but can exacerbate false peak artifacts when large strains are present. Alternatively, a tilted method of filtering 3-D correlation coefficients can correct for strain-induced incoherence across the correlation function by applying phase rotations in accordance with the local strain distribution prior to filtering. In addition, 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. 3-D speckle tracking with tilt filtering and two-pass processing was implemented on 4DE data acquired in the left ventricle (LV) of an open chest canine at six weeks following surgical coronary occlusion. Tilt filtering reduced peak hopping compared to filtering without phase rotation. The most notable improvements were seen in the transitions zones where axial displacements had a high degree of variation along the orthogonal directions. In the transition zones, larger correlation filters without tilt processing exacerbated peak hopping, whereas larger tilt filters were able to overcome peak hopping artifacts. The lowest peak hopping was seen when both tilt filtering and multi-pass methods were applied. Tilt filtering and two-pass processing can improve the quantification of myocardial deformation using 3-D speckle tracking and may be valuable in the assessment of cardiac function using echocardiography.