Noncontact active sensing for viscoelastic parameters of tissue with coupling effect

Living soft tissues have two characteristics for an external force. One is the coupling effect (see Fig. 1) where the tissue deforms not only at the point of application of force but also at its surrounding area without any external force. The other is the direction-dependent response (see Fig. 2) where the response during the loading phase (when the force is applied with increasing displacement) is quicker than that during the unloading phase (when the force is shutdown). In order to represent these characteristics, this paper first proposes a single layered 3-D tissue model constructed by a network composed of two stiffness and two damping parameters, respectively. For such a single-layered model, we solve the inverse problem where four unknown viscoelastic parameters are obtained by assuming that both the applied force and surface deformation of the tissue are given with respect to time. Through both simulation and experimental results, we show that this model can describe good inherent characteristics of soft tissues, namely a direction-dependent response and a coupling effect.