| We have previously developed regularized 2D and 3D elastography methods using Dynamic Programing (DP) \cite{RivazTMI:08,RivazMICCAI:08}. A cost function which incorporates similarity of echo amplitudes and displacement continuity was minimized using DP to obtain the displacement map. In this work, we present a novel hybrid method for calculating the displacement map between two ultrasound images. The method uses DP in the first step to find an initial estimate of the motion field. In the second step, we assume a linear interpolation for the reference image and obtain a closed-form solution for a subpixel accuracy motion field. The closed-form solution enables fast displacement estimation. We present four \textit{in-vivo} patient studies of monitoring liver ablation with freehand 2D DP elastography. The thermal lesion was not discernable in the B-mode image but it was clearly visible in the strain image as well as in validation CT. We also present 3D strain images from thermal lesions in \textit{ex-vivo} ablation. We introduce a novel volumetric rendering model for visualization of the volumetric B-mode images. We exploit strain values in the opacity of the volumetric B-mode data to better classify soft tissue. From a single 3D rendering picture, it is a possible to observe the surface of the hard lesions, its size, and its appearance and texture. |