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Numerical Modeling of Treatment of Liver Tissue Cancer by Using High Intensity Focused Ultrasound (Hifu)

Mohammadpour Ghadikolaie, Maryam | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53784 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Firoozabadi, Bahar
  7. Abstract:
  8. In recent years, the incidence rate of liver cancer has increased sharply and nowadays liver cancer is known as the second leading cause of cancer death in the world. Surgical resection is usually used to treat malignant hepatic tumors. However, in some cases, surgical resection is not possible because of some reasons, such as tumor size and location. In these cases, other minimally invasive ablation methods such as radio-frequency ablation, microwave ablation, and high intensity focused ultrasound (HIFU) can be used. HIFU has some advantages such as noninvasiveness, high penetration depth, and low cost in comparison to the other ablation techniques such as radiofrequency ablation. In HIFU ablation procedure, high intensity acoustic waves are concentrated in cancerous region which results in tissue temperature elevation and death of cancerous cells. Although there are some preliminary successful clinical experiences with HIFU for cancer treatment, this ablation technique is still known as a developing technique. Therefore, numerical and experimental studies are needed to mature this technique for widespread clinical use.In the present work, heat transfer in a liver tumor and the cooling effect of liver vascular bed on it during high intensity focused ultrasound (HIFU) ablation is studied numerically. For this purpose, an acoustics-thermal-fluid coupling model based on the Westervelt, Pennes, porous and Navier-Stokes equations is used. For the first phase, a simple geometry is assumed for the liver vascular bed, and energy equations are solved in the tissue and the blood vessels. For the second phase of the project, a more realistic geometry comprising a tumor located close to the hepatic artery is assumed and the healthy tissue of the liver is considered as a porous medium at different values of permeability and porosity. Also, the effect of vascular porosity, acoustic streaming, and temperature variations of tissue properties on the HIFU treatment efficacy is investigated. It is found that the liver vascular bed and the porosity have a considerable cooling effect on HIFU ablation efficacy and this effect cannot be modeled correctly by using simplified Pennes bioheat equation. For example, based on the obtained results, it is shown that for a vascular bed comprising arterial branches, terminal arterial branches, venous branches and terminal veins at the porosity in the range of 0.05 to 0.3, the lesion size in the acoustic axis direction predicted by porous media theory is 3% (for venous branches at the porosity of 0.05) to 40% (for terminal arterial branches at the porosity of 0.3) lower than the one predicted by Pennes model. Also, it is shown that for all generations of the liver vasculature, maximum tissue and blood temperatures are 15% to 25% different and this reveals the requirement of using thermal models based on the local thermal non-equilibrium assumption between tissue and blood phases. Moreover, it is found that in the vascular bed comprising terminal arterial branches, the maximum velocity due to the acoustic streaming effect is 0.5% of the average blood velocity ,and hence, the effect of acoustic streaming on the velocity field and heat transfer can be neglected. Finally, by considering the dynamic temperature variations of tissue properties, it is concluded that the variation of absorption coefficient with temperature has a considerable effect on the lesion size and increases the lesion size in the acoustic axis direction and perpendicular direction by the amount of 40% and 15 %. However, it is shown that the effect of variation of specific heat capacity with tempertaure on treatment outcome is negligible
  9. Keywords:
  10. Liver ; Local Thermal Non-Equilibrium ; Porous Media ; Heat Transfer ; High Intensity Focused Ultrasound (HIFU)Method ; Vascular Bed ; Liver Cancer

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