Soft Tissue Modeling for Surgical Simulation

Motivation: A biomechanical model of soft tissue derived from experimental measurements is critical for developing 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. The findings of this research can be used to develop a realistic soft-tissue model for providing haptic feedback to the physician during surgical simulation training.

Project Highlights

  • Ex vivo experiments on pig liver tissues: In actual deformation of soft-tissue during surgical intervention, the tissue is subject to tension, compression, and shear. Therefore, characterization of soft-tissue in all these three deformation modes is necessary. We have designed and built an experimental setup to carry out soft-tissue tension, compression, and pure shear experiments while capturing in real-time the image of the tissue undergoing deformation. Because in these ex vivo tests, boundary conditions are simple and under relatively precise control, the results can be used directly for the constitutive modeling.

 stm_liver_tissue Figure 1: Liver Tissue Subjected to a) tension, b) compression and c) pure shear loading

  • Constitutive modeling of ex-vivo liver tissue: Based on the experimental result, a new combined logarithmic and Ogden strain energy was proposed and stress-strain relations were derived. This model is adequate to describe the full range of deformation for both compression and tension, and also good for low strain (<35%) deformation of pure shear.

stm_tension_curve

Figure 2: Nominal stress vs. stretch ratio curves for liver tissue under uniaxial tension and compression: the dashed lines are from experiments; the solid lines are the predicted by the models.

stm_shear_curveFigure 3: Nominal stress vs. stretch ratio curves for liver tissue under pure shear: the dashed lines are from experiments; the solid lines are the predicted by the models.

  • Model improvements based on in-vivo experimentation: When studying biological tissues variations in the properties of the tissue are often observed between the ex-vivo and in-vivo states. To provide a more accurate platform for surgical simulation, probing tests were carried out on in-vivo porcine liver while recording the force and displacement response of the liver. Using a finite element simulation approach, the above ex-vivo based constitutive models were modified to more accurately represent the loading of actual in-vivo experiments.

 stm_comparison

Figure 4: Comparison of experimental and simulated force response a) the ex-vivo constitutive model and b) the constitutive model modification process.

  • Development of cutting models through a fracture mechanics approach: The above models are sufficient to accurately describe the response of soft-tissue to general surgical loading; however the act of simulation the cutting process requires additional models. A cohesive zone approach was applied to the scalpel cutting process whereby the onset and propagation of the scalpel cutting process could be modeled. Cutting experiments were conducted on in-vivo porcine liver from which the cohesive parameters could be derived. Using this approach it is possible to simulate the cutting process using the finite element method.
  • 1st Generation real-time simulator: The first approach to the solution of the reality-based, haptics-enabled simulation of liver probing in a real-time sense makes use of preprocessed information. Many modern simulators make concessions in the accuracy of the applied constitutive models to simplify the complexity of the finite element approach; therefore, we proposed to use the accurate hyperelastic models previously derived to maintain the desired level of accuracy as a way to improve upon the overall system response. By using the reality-based models, the forces and deformations involved in the probing task will be much more realistic and lifelike, making for a superior training system. To use the accurate material models in real-time, a method of preprocessing the data has been developed for the implementation of an Ogden model in a real-time simulation of a probing task.

 

Personnel: Zhan Gao and Kevin Lister

Sponsor: NIH R01 Grant.

Archival publications:

  •  K. Lister, Z. Gao, J. P. Desai, “Development of in-vivo Constitutive Models for Liver: Application to Surgical Simulation”, In Annals Biomedical Engineering, Volume 39, Issue 3, Pages 1060-1073, 2011.
  •  Z. Gao, and J.P. Desai, “Estimating zero-strain states of very soft tissue under gravity loading using digital image correlation”, Medical Image Analysis, DOI: 10.1016/j.media.2009.11.002, 2010.
  •  Gao, K. Lister, and J. P. Desai, “Constitutive modeling of liver tissue: experiment and theory”, Annals Biomedical Engineering, DOI: 10.1007/s10439-009-9812-0, 2010.

Refereed Conference Publications:

  • K. Lister, Z. Gao, J.P. Desai, “Towards a Soft-Tissue Cutting Simulator using the Cohesive Zone Approach”, In Engineering in Medicine and Biology Society, EMBC, Boston, MA, USA, 2011.
  • K. Lister, Z. Gao, J. P. Desai, “A 3D In Vivo Constitutive Model for Porcine Liver: Matching Force Characteristics and Surface Deformations”, In Third IEEE/RAS-EMBS International Conference on Biomedical Robotics and Biomechatronics – BioRob, Tokyo, Japan 2010.
  • K. Lister, and J.P. Desai, “Real-time, haptics-enabled simulator for probing ex vivo liver tissue”, In Engineering in Medicine and Biology Society, 2009. EMBC 2009. Annual International Conference of the IEEE, pp 1196-1199, September, 2009.
  • Z. Gao, T. Kim, D.L. James, J.P. Desai, “Semi-automated soft-tissue acquisition and modeling for surgical simulation”, In 5th Annual IEEE Conference on Automation Science and Engineering, CASE 2009, pp. 268-273, August 22-25, 2009.
  • Z. Gao, K. Lister, J.P. Desai, “Constitutive Modeling of Liver Tissue: Experiment and Theory”, In Second biennial IEEE/RAS-EMBS International Conference on Biomedical Robotics and Biomechatronics – BioRob, October 2008