Chris Roy September 28 th 2010 MBP 1028 Optical, Thermal and Radiation Biophysics Principles of Radiofrequency and Microwave Heating

Outline Background Radiofrequency radiation (RF) Microwave radiation Basic mechanisms Interactions Heating Calculation of deposited power Absorption in tissue Application

Background: Electromagnetic Radiation Radiofrequency 30 kHz to 300GHz Radiofrequency Ablation 460 kHz Microwave 300 MHz to 300 GHz Percutaneous Microwave Coagulation 2.45 GHz Non-Ionizing Shibata et al 2002, Image : Cosman Medical,

Basic Mechanisms: Interactions InductionDielectric Polarization Electric field acts on charged particles in a material Particles move freely Result : Induced current Dielectric Polarization Particles bound in the material Electrons surrounding nuclei Nuclei Charges at interfaces Permanent Dipoles Reorientation in phase with the field ICNIRP 1998, Jacob and Boey 1995

Basic Mechanisms: Heating Mechanisms InductionDielectric Polarization Joule’s loss effect Moving charges Finite conductivity Excess energy gained from the field transforms into heat Result : Induction Heating Dielectric loss effect Dipoles try to align with the reversing field Polar molecules can’t keep up and lag behind Excess energy gained from the field transforms into heat Result : Dielectric Heating Mehdizadeh 2009

Power deposition Power Density Watts per cubic metre Joule loss Conductivity Magnitude of electric field Dielectric loss Permittivity Frequency Magnitude of electric field Therapy 30W to 90W in 10W/min intervals +10 o C heating Mehdizadeh 2009, Shibata et al 2002

Absorption in Tissue: Energy absorbed by the human body Penetrate, reflected, absorbed Absorbed energy over four ranges : 100 kHz to 20 MHz : Decreases rapidly with frequency Examples : Neck and legs 20 MHz to 300 MHz : High absorption for whole body, highest values at extremities Examples : Fingers, Toes, Head 300 MHz to 10 GHz : Local non uniform absorption >10 GHz : Body surface Durney et al 1985

Absorption in Tissue: Specific Absorption Rate (SAR) SAR : Watts per unit mass Dissipated energy Density of the tissue Electric field Reflection in tissue Conductivity NCRP 1981

Absorption in Tissue: Specific Absorption Rate (SAR) Schepps and Foster 1980

Radiation Generation RFA and PMC Equipment Generator Cable Electrode Needle Grounding Pads Result Tissue damage Cell death (tumour) Sutherland et al 2006, Seki et al 1994

Summary Heating via high frequency radiation Induction Dielectric polarization Absorption in tissue Function tissue-specific parameters Dielectric properties Therapy Radiofrequency Ablation Microwave Coagulation Therapy Shibata et al 2002

References Durney C H, Massoudi H, Iskander 1985 M F Radiofrequency radiation dosimetry handbook, Brooks Air Force Base, TX U.S. Air Force School of Aerospace, Medical Division; Reg. No. SAM-TR International Commission on Non-Ionizing Radiation Protection (ICNIRP) 1998 Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (Up to 300 GHz) Health Physics Society Jacob J and Boey F Y C 1995 Review: Thermal and non-thermal interaction of microwave radiation with materials Journal of Materials Science Mehdizadeh M 2009 Microwave/RF applicators and probes for material heating, sensing, and plasma generation Elsevier 5-7 National Council on Radiation Protection and Measurements (NCRP) 1981 Radiofrequency Electromagnetic Fields – Properties, Quantities and Units, Biophysical Interaction and Measurements Report No. 67 Schepps J and Foster K R 1980 The UHF and microwave dielectric properties of normal and tumour tissues: variation in dielectric properties with tissue water content Physics in Medicine and Biology Seki T, Wakabayshi M, Nakagawa T, Itho T, Shiro T, Kunieda K, Sato M, Uchiyama S, Inoue K 1994 Ultrasonically Guided Percutaneous Microwave Coagulation Therapy for Small Hepatocellular Carcinoma Cancer Shibata T, Limuro Y, Yamamoto Y, Maetani Y, Ametani F, Itoh K, Konishi J 2002 Small Hepatocellular Carcinoma: Comparison of Radio-frequency Ablation and Percutaneous Microwave Coagulation Therapy Radiology Sutherland L M, Williams J A R, Padbury R T A, Gotley D C, Stokes B, Maddern G J 2006 Review : Radiofrequency Ablation of Liver Tumors Archives of Surgery