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Please use this identifier to cite or link to this item: http://hdl.handle.net/1860/882

Title: Reality-based soft tissue probing: experiments and computational model for application to minimally invasive surgery
Authors: Hu, Tie
Keywords: Mechanical engineering;Touch;Biomedical engineering
Issue Date: 28-Jul-2006
Abstract: Biomechanical model of soft tissue derived from experimental measurements is critical to develop a reality-based model for minimally invasive surgical training and simulation. In our research, we have focused on developing a biomechanical model of the liver with the ultimate goal of using this model for local tool-tissue interaction tasks and providing feedback to the surgeon through a haptic (sense of touch) display. Our method is to measure the tissue’s biomechanical properties both ex vivo and in vivo. The Local Effective Elastic Modulus (LEEM) values are derived by the inverse finite element method. LEEM describes the tissues’ stiffness and is the critical model parameter to develop a reality-based haptic display. Liver tissue is nonlinear, non-isotropic, non-homogeneous and inelastic material. It has an extremely nonlinear stress-strain relationship. Our initial research proposed a hybrid liver model derived from a series of indentation experiments, which is valid in both low strain and large strain regions. However, this model does not provide an accurate displacement field of the area surrounding the probe, though it can provide an accurate force-displacement behavior at the point of probing. To develop the model for soft-tissue response to probing, we developed a large probe model, whereby the probe is larger than the tissue sample being probed. An experimental apparatus was developed to perform large-probe compression tests for pig liver specimens with cylindrical geometry. The specimens were compressed to attain 30% nominal strain using a range of probing speeds (0.1016 mm/sec, 5.08 mm/sec, 12.70 mm/sec and 25.40 mm/sec). The LEEM was used to characterize the tissue material property for each infinitesimal region under large deformation. The sensitivity of LEEM on the tissue’s compressibility (Poisson’s ratio) and on the finite element mesh is presented. Additionally, the variation of LEEM of pig liver with the probing speed was revealed. Image tracking method is also used to study the internal deformation of the tissue under external load. The liver sample embedded with stainless steel beads was indented by a probe of diameter 7 mm. The displacements of the beads were captured by two CArms (OEC 7700 and OEC 9600). LEEM values were obtained from the force and displacement input by an inverse finite element method. A 3D finite element model was built by using the LEEM values. The beads were matched to the nodes with the corresponding coordinates in the finite element model. The actual displacement was compared with the numerical values. In vivo tissue has drastically different properties from the ex vivo tissue because of the changing factors in temperature, hydration, break-down of protein, and loss of blood. In surgery, the surgeons contact with in vivo tissues rather than ex vivo tissues. It is critical to obtain the mechanical properties of in vivo tissues for a surgical simulator. As a result, after completing large probe modeling work on ex vivo tissue, we proceeded to develop an in vivo model. We developed an experimental method for in vivo soft tissue test, and an axisymmetric finite element model to obtain the local effective elastic modulus (LEEM) of in vivo soft tissue. A microcontroller-based portable probe was developed to measure the force and displacement of in vivo soft tissue undergoing large deformation. The experiment was performed on a liver of a pig under general-anesthesia. The probe indented the liver up to 40% deformation at the speed of 1.5 mm/sec. Based on the experimental force and displacement data from five trials we obtained the LEEM by the inverse finite element method.
URI: http://hdl.handle.net/1860/882
Appears in Collections:Drexel Theses and Dissertations

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