In Situ Mechanical Characterization of Multilayer Soft Tissue Using Ultrasound Imaging

In this paper, we report the development of a technique to characterize layer-specific nonlinear material properties of soft tissue in situ with the potential for in vivo testing. A soft tissue elastography robotic arm system comprising of a robotically manipulated 30 MHz high-resolution ultrasound probe, a custom designed compression head, and load cells has been developed to perform compression ultrasound imaging on the target tissue and measure reaction forces. A multilayer finite element model is iteratively optimized to identify the material coefficients of each layer. Validation has been performed using tissue mimicking agar-based phantoms with a low relative error of similar to 7% for two-layer phantoms and similar to 10% error for three layer phantoms when compared to known ground-truth values obtained using a commercial material testing system. The technique has then been used to successfully determine the in situ layer-specific mechanical properties of intact porcine stomach. The mean C-10 and C-20 for a second-order reduced polynomial material model were determined for the muscularis (6.41 +/- 0.60, 4.29 +/- 1.87 kPa), submucosal (5.21 +/- 0.57, 3.68 +/- 3.01 kPa), and mucosal layers (0.06 +/- 0.02, 0.09 +/- 0.24 kPa). Such a system can be utilized to perform in vivo mechanical characterization, which is left as future work.