Optimizing Ultrasonic Intensity for High Intensity Focused Ultrasound Therapy
Carter, Matt; Sullivan, Art; Byers, Kenny; Jessel, Michael
Every year as many as one million individuals worldwide are diagnosed with hepatocellular carcinoma (HCC), the most common form of liver cancer. Caused by cirrhosis, HCC is typically treated with surgery or chemotherapy. High intensity focused ultrasound (HIFU) therapy, an emerging treatment option, is a noninvasive alternative to these methods. HIFU targets a cancerous tumor and induces necrosis while reducing damage to surrounding tissue. Acoustic pressure waves propagate from a curved transducer head into the tissue medium. The curved nature of the transducer surface focuses the pressure waves into a selected region and the energy of the beam is converted into heat. HIFU allows for precise targeting of tumor regions and reduced necrosis of healthy tissue. It is easier to control the depth and position of interstitial ultrasound than it is for other interstitial heating methods, such as percutaneous ethanol injection and radiofrequency. This project models the treatment of liver cancer using HIFU therapy. We model the thermal necrosis of a liver tumor caused by an ultrasonic ransducer, and we optimize the process to maximize tumor ablation and minimize tissue damage. The process is modeled in COMSOL Multiphysics using 2-D axisymmetric oordinates which simplifies the tumor geometry as symmetric and includes the HIFU probe and surrounding tissue. Transducer size and parameters are that of the JC-model HIFU transducer from Haifutech, Inc. Relevant tumor and tissue parameters are taken from the literature. Pressure waves are modeled using the Helmholtz equation and heat transfer utilizes the Bioheat Equation. Tumor and tissue ablation are evaluated with a thermal dose equation. Our results show pressure wave propagation focused at the center of the liver tumor. Maximum heating occurs at the tumor center where pressures were the highest and lower temperatures are seen in healthy tissue regions, indicating a proper coupling of the ultrasound and heat transfer physics. A transducer frequency of 1 MHz with a power of 200W and a sonication time of 3.2 seconds maximizes tumor ablation while minimizing healthy tissue damage in a 0.8 cm diameter tumor. This model demonstrates the effective heating of HCC tumors by HIFU, and can be used as a reference for optimizing a heating dose for tumors of known sizes.