Characterization of viscoelastic properties of intra-abdominal organs from in vitro indentation experiments and inverse finite element parameter optimization

Abstract

Soft tissue characterization based on experimental data has been investigated in order to provide a more realistic behavior in a virtual reality based surgical simulation. This thesis presents the characterization of the nonlinear properties of intra-abdominal organs from in vitro indentation experiments and the inverse finite element (FE) parameter optimization algorithm. To observe the soft tissue behavior, the in vitro experiments were conducted using three different inputs: a ramp and hold for relaxation, a sine wave for hysteresis, and a chirp wave for frequency responses. Soft tissues are modeled as a viscoelastic, nearly incompressible, and isotropic continuum. In the assumptions of a quasi-viscoelastic theory, the time dependent viscoelastic and time independent (strain energy dependent) hyperelastic material parameters were estimated separately. To characterize the parameters, the finite element (FE) model to simulate the forces at the indenters was developed. In addition, an optimization algorithm which updates new parameters and runs the simulation iteratively was also developed. As a result, we can develop the finite element model whose behavior is similar to that of porcine liver in the experiments. This implies that the tissue model could provide the information of the intra-abdominal organs and that the tissue model could be applied to the surgical simulation to provide realistic visual and haptic feedback.