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Session: Ultrasound
Session starts: Thursday 26 January, 10:30
Presentation starts: 11:00
Room: Room 559

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Jan-Willem Muller (Eindhoven University of Technology)
Hans-Martin Schwab (Eindhoven University of Technology)
Marcel Rutten (Eindhoven University of Technology)
Marc van Sambeek (Catharina Hospital Eindhoven)
Richard Lopata (Eindhoven University of Technology)

Abstract:
Ultrasound (US) strain imaging and elastography rely on displacement tracking to characterize the mechanical properties of tissue. The measured displacement field may suffer from several inaccuracies, due to e.g. signal decorrelation, peak-hopping, and a lack of phase information in the lateral direction. Regularization techniques have been described in literature to increase robustness of strain determination. Typically, constraints are imposed using pixel-based methods, which need an equidistant grid and a relatively fine sampling of the estimated displacement field. In this study, a regularization method is proposed that is based on a finite element method (FEM), which allows for flexible regularization of sparse displacement data. The performance of the developed method was investigated in various in vitro phantoms and in vivo for both line-by-line scanning and ultrafast imaging. Block-matching was used to estimate the displacements for every node in the FEM mesh. The regularization method was applied on the inter frame displacement estimates. The L1-norm between the estimated and measured displacements is minimized, which increases the method’s robustness against outliers. The minimization is regularized by constraining the root mean square curl, divergence, and bending energy (smoothness) of the displacement field. The method was applied in two in vitro set-ups and in vivo. A pulsating polyvinyl alcohol vessel was scanned line-by-line using a 3.5 MHz curved probe (Esaote CA431) at a framerate of 28 Hz. Secondly, a pulsating porcine aorta, and a human volunteer were scanned with an ultrafast acquisition (>130 Hz), using a 3.7 MHz Verasonics C5-2v probe. The method improves the quality of the tracking, for both the line-by-line, and the ultrafast techniques. Specially, the regularization avoids the mesh from intersecting itself in all datasets, which normally results in non-physical displacement fields. Due to the accumulation of drift errors, the strains values were significantly overestimated without regularization. The regularization method could reduce the drift from 741 μm to 168 μm after one cycle in the porcine aorta experiment. Similar reductions in drift were observed for the other datasets. A thorough analysis of the impact on strain resolution and contrast will be determined in heterogenous phantoms and simulations in future work.