4-D echocardiography assessment of local myocardial strain using 3-D speckle tracking combined with shape tracking

In 4-D echocardiography (4DE), displacement estimates obtained solely from multi-dimensional speckle tracking can exhibit large variances and peak hopping, making it challenging to accurately calculate myocardial strains. 3-D phase-sensitive speckle tracking can produce sensitive estimates along the axial direction, but typically provides poorer estimates in orthogonal directions and at tissue boundaries. Shape tracking provides complimentary information, as it effectively tracks myocardial boundaries and does not depend on beam orientation. We propose a method combining 3-D speckle tracking with 3-D shape tracking using a quality-based radial basis function approach. Echocardiographic data (3D+t) were acquired in an open chest canine model at six weeks following surgical coronary occlusion using a commercial 2-D phased array, on which 3-D phase-sensitive speckle tracking and 3-D shape tracking were performed. An adaptive, multi-level radial basis function method was used to combine information from the two tracking methods, utilizing confidence metrics to weight the contribution of each estimate to generate a dense 3-D displacement field throughout the myocardium. A multi-level approach was used to capture smaller scales of motion in regions of fine deformation variation and high tracking confidence. The 3-D combined approach produced displacement estimates with greatly reduced variance and peak hopping compared to 3-D speckle tracking alone. Lower radial strains were observed in the myocardial infarct region, corresponding to reduced local contractility. Strong correlations were observed for both radial and circumferential strains between the combined method and estimates from magnetic resonance (MR) tagging studies.