Principles of Radiofrequency and Microwave Heating

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

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

3. 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, http://www.cosmanmedical.com/Pages/rfg_1a1.html

4. Basic Mechanisms:Interactions Induction Dielectric 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

5. Basic Mechanisms:Heating Mechanisms Induction Dielectric 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

6. 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 • +10oC heating Mehdizadeh 2009 , Shibata et al 2002

7. 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 Durneyet al 1985

8. 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

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

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

11. 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

12. 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-85-73 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 74 494-522 Jacob J and Boey F Y C 1995 Review: Thermal and non-thermal interaction of microwave radiation with materials Journal of Materials Science30 5321-27 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 Biology25 1149-59 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 Cancer74 817-25 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 Radiology223 331-37 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 TumorsArchives of Surgery141 181-90