Multiphysics Computational Algorithms

Despite excellent advancement in our understanding of the mechanics of soft tissues, the effects of cellular level mechanisms on the thermomechanical damage of soft tissue subjected to intense energy fields such as laser, radio frequency (RF) alternating current, and ultrasound have not been thoroughly understood. Such effects are critical in our understanding of emerging therapeutic technologies including thermal ablation of tumor, ultrasound activated soft tissue surgery and denervation techniques. To this end, our research is aimed at predicting the extent of damage of the soft tissue considering cellular level physics that is incorporated within the tissue level continuum model using a multiphysics modeling framework.

Modeling of Electrosurgical Procedures

Electrosurgery is a widely used procedure where the application of radiofrequency alternating current to tissue generates heat to conduct various surgical procedures including cutting, ablation, coagulation and desiccation. Radio-frequency heating of soft tissues during electrosurgical procedures is a fast process that involves phase change through evaporation and transport of intra- and extra-cellular water, and where variations in physical properties with temperature and water content play significant roles. Accurately predicting and capturing these effects would improve the prediction of temperature changes in the tissue allowing the development of improved instrument design and better prediction of tissue damage and necrosis. We are developing multiphysics models that account for phase change, mass transport, deformation, and damage associated with the process to study the energy dissipation in soft tissue.

Two-scale Model of Radio-frequency Electrosurgical Tissue Ablation

We propose a two-scale model to study the effects of microscale phase change and heat dissipation in response to radiofrequency heating in electrosurgical ablation procedures. At the microscale, the conservation of mass along with thermodynamic and mechanical equilibrium is applied to obtain an equation-of-state (EOS) relating vapor mass fraction to temperature and pressure. The evaporation losses are incorporated in the macro-level energy conservation and results are validated with mean experimental temperature distributions measured from electrosurgical ablation testing on ex vivo porcine liver at different power settings of the electrosurgical instrument. The two-scale multiphysics model is shown to capture the characteristics of radio-frequency activation of soft tissue (Figure 1).

Figure 1. Schamatic dipicts two-scale modeling paradigm. Middle panel compares mean experimental temperature to simulation results for the radial variation of temperature, whereas, right panel shows contours of fraction of remaining water content as predicted by the model for 50W power setting.

Mixture Theory-based Model of Radio-frequency Heating of Soft Tissues

We present a model of RF electrosurgical heating of hydrated soft tissue based on mixture theory. This allows the modeling of tissue that is multiphase in nature within a continuum framework, while simultaneously considering the transport, deformation and phase change losses due to evaporation that occur during electrosurgical heating. The model predictions are validated against data obtained from the in vivo ablation of porcine liver tissue at various power settings of the electrosurgical unit. The mixture model is shown to match the mean experimental temperature data with sharp gradients in the vicinity of the electrode during rapid low and high power ablation procedures (Figure 2). Additionally, the model is also able to capture fast vaporization losses (Figure 2) and the corresponding increase in pressure due to vapor buildup which have a significant effect on temperature prediction beyond 100 °C.

Figure 2. Variation in mean experimental temperature (solid lines) in the in vivo experiments compared to simulation results (dashed lines) with radial distance is shown in the left panel; whereas, temperature distribution in the electrode and tissue, and ratio of vapor content to initial water content for 50W power setting case is shown in the middle panel and right panel, respectively.

Relevant Publications

  1. Karaki, W., Rahul, Lopez, C., Borca-Tasciuc, D. A., and De, S. A two-scale model of radio-frequency electrosurgical tissue ablation. Submitted to Computational Mechanics.
  2. Karaki, W., Rahul, Lopez, C., Borca-Tasciuc, D. A., and De, S. A mixture theory-based model of radio-frequency heating of hydrated soft biological tissues. In preparation.

Modeling of Ultrasound Activated Scalpels

Ultrasonic surgical instruments have been gaining popularity among surgeons in the last decade. An increasing number of surgical procedures including but not limited to head, neck, gynecological, colorectal and gastrointestinal surgeries are performed using ultrasonic surgical instruments. These instruments utilize ultrasonic vibrations to cut, coagulate and dissect tissues, and seal vessels. They have been proven to be superior to conventional instruments and techniques such as electrosurgical and laser-based devices as they impose lesser thermal injury, desiccation and charring, lower mean blood loss during surgery, no risk of stray current, neuromuscular stimulation, lesser operation time and post-operative pain, and no smoke during the operation to occlude laparoscopic view.

Despite the increasing popularity of ultrasound-based surgical procedures, the affects of cellular level mechanisms on the thermomechanical response of ultrasonically activated soft tissues have not been understood completely. We have developed a multi-physics model to investigate the effects of cavitation, due to large transient pressure changes, on the thermomechanical response of soft tissue subjected to ultrasound vibrations. The cavitation model based equation-of-state provides the pressure arising from evaporation of intracellular and cellular water by absorbing heat due to viscoelastic heating in the tissue. The model is shown to capture characteristics of ultrasonically activated soft tissue deformation and temperature evolution. Further, with increasing operating frequency, the temperature rises faster leading to rapid evaporation of tissue cavity water, which may lead to accelerated protein denaturation and coagulation.

Relevant Publications

  1. Sankaranarayanan, G., Resapu, R. R., Jones, D. B., Schwaitzberg, S., and De, S. (2013). Common uses and cited complications of energy in surgery. Surgical endoscopy, 27(9), 3056-3072.
  2. Rahul, and De, S. (2017). A multi-physics model for ultrasonically activated soft tissue. Computer Methods in Applied Mechanics and Engineering, 314, 71-84.