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Analysis of 3 D Motion Effects in Myocardial Elastography

In this paper, we investigate the elevational beamwidth and the extent of 3D decorrelation on 3D motion estimation of multiple short-axis slices under a previously established theoretical framework, which simulated two sequences, including passive filling and active contraction, in normal and ischemic canine finite-element (FE) left ventricular models. A 3D convolutional image formation model was developed to generate successive RF signals in 3D. Incremental lateral and axial motion components were estimated from lateral-axial plane using 1D cross-correlation and recorrelation techniques in a 2D search with a 1D matching kernel of 3 mm and 80% overlap. Similarly, the elevational displacement was estimated from elevational-axial plane using the same strategy. The cumulative 3D motion was obtained by accumulating the incremental motion from enddiastole to end-systole, while the cumulative lateral and axial strains were obtained using a least-squares strain estimator on the cumulative in-plane motion. Radial and circumferential strains, independent of the orientation of the transducer, were also computed from the in-plane finte strains. The elastographic estimation error was estimated by calculating the relative difference between the FEA and elastographic results. When the elevational beamwidth increased from 2 to 4 mm, the mean absolute differences for cumulative in-plane motion and systolic strains in both the normal and the ischemic cases, decreased. However, the larger the elevational beamwidth was, the larger elevational displacement estimation error was obtained. The findings showed that the elevational beamwidth of 2 mm was the optimal value for both in-plane deformation and out-of-plane motion estimation. In summary, even at the presence of physiologic elevational motion, the in-plane elastographic estimates remained at good agreement with the FE solutions. Error analyses indicated thus a substantial improvement in the performance of the recorrelation technique at various short-axis slices of the 3D left ventricle and at elevational beamwidths well within the clinical equipment range.