Radio frequency (RF) data were acquired on all patients using a Siemens Acuson S2000 system and an 18L6 transducer (Siemens Medical Solutions, Malvern, PA, USA). Acquisitions over several cardiac cycles were made at the common carotid artery, the carotid bifurcation and internal carotid artery on both left and right carotid arteries for each patient. Plaque in each of the end diastolic frames were manually segmented by an experienced sonographer.
Displacement estimation was performed using a multi-level algorithm [16 (link), 17 (link)] with Bayesian regularization [18 (link)]. Displacement information was used to propagate the manual segmentations from end diastole to other frames over the cardiac cycle. Axial, lateral and shear strain were calculated from accumulated displacement vector estimates. A fast GPU version of the algorithm was used for the estimation reported in this paper [19 ].Figure 1 , shows an example of axial, lateral and shear strain maps obtained. The maximum accumulated strain index (MASI) was calculated from the estimated strain distribution as previously described [5 (link)]. MASI quantifies the strain distribution in a small region of interest with highest strain values over the entire plaque and over a cardiac cycle.
Displacement estimation was performed using a multi-level algorithm [16 (link), 17 (link)] with Bayesian regularization [18 (link)]. Displacement information was used to propagate the manual segmentations from end diastole to other frames over the cardiac cycle. Axial, lateral and shear strain were calculated from accumulated displacement vector estimates. A fast GPU version of the algorithm was used for the estimation reported in this paper [19 ].