Radiofrequency: Principles & Devices

Transcription

1 Radiofrequency: Principles & Devices J W HAND Radiological Sciences Unit, Hammersmith Hospitals NHS Trust, London and Imaging Sciences Dept, Faculty of Medicine, Imperial College London Mayneord-Phillips Summer School Ultrasound and other Minimally Invasive Therapies July 2005

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3 Rev. John Wesley MA ( ) Pioneer Electrotherapist In Primitive Physick, or An Easy and Natural Way of Curing Most Diseases' (1747) electricity is recommended as a cure for over twenty illnesses It was one of his favourite remedies and he describes it as "far superior to all the medicines I know". In the preface of the 1760 edition he spoke enthusiastically of electricity, 'certainly it comes the nearest an universal medicine of any yet known in the world' (Wesley 1760). See and

4 A Short History of RF Ablation Bovie and Cushing (mid 1920s): electrocoagulation by high-frequency currents Sweet and Mark (1953) reported that RF currents created more reproducible and controllable neural lesions than direct currents. Doss and McCabe (1976): interstitial LFC hyperthermia RF ablation for cardiac arrhythmias occurred first in the early 1980s (Huang et al 1985) RF ablation for hepatic tumours ~ 1990 (McGahan et al ) Currently RF ablation systems have embedded advanced features such as temperature monitoring, temperature-based power control, impedance monitoring and simultaneous RF energy delivery to multiple electrodes under temperature control.

5 Electromagnetic Spectrum Increasing wavelength Decreasing frequency Wavelength (in air): m Frequency: 5x x 10 7 Hz

6 Electromagnetic Fields & Tissues Drift of free conduction charges in response to an applied E-field Conduction current is σe (σ is the conductivity (S/m)) Dielectric polarisation displacement of positive and negative charges from their equilibrium position due to the applied E field. Gives rise to a polarisation charge alignment of polar molecules with applied E-field Tissues are essentially nonmagnetic

7 Electromagnetic Fields & Tissues Interaction can be described through the complex permittivity ε of the tissue (units F/m) ( ε ε ) ε = ε j o Permittivity of free space = x F/m f ω = 2π Real part: Relative permittivity (dielectric constant) Imaginary part: ε = σ ωε o

8 Permittivity & Conductivity of Tissues Relative Permittivity λ tissue (27 MHz): 0.78 m, 0.59 m, 3 m eps (liver) eps (prostate) eps (breast) sig (liver) sig (prostate) sig (breast) Conductivity (S/m) Frequency (MHz) 0 λ tissue (500 khz): 9 m, 5.7 m, 27.6 m

9 Energy Absorption ε is a measure of the energy transferred to the tissue from the applied field - the larger ε, the more lossy is the tissue. Loss tangent: tan δ = ε / ε Time-averaged power absorbed per unit volume is: P = 2 σ E = ωε ε o E 2 W/m 3 rms E-field

10 Specific Absorption Rate (SAR) SAR is defined as: The time derivative of the incremental energy (dw) absorbed by (dissipated in) an incremental mass (dm) in a volume element (dv) of given density (ρ) SAR = d dt dw dm = d dt dw ρdv = σ E ρ 2 = ωε o ε ρ E 2 Units are W/kg

11 Electric & Magnetic Fields & Time Maxwell s equations: B E = t D H = J + t D = ρ B = 0 but for the frequency range of interest here, wavelength >> body dimensions and so, to a good approximation: time variation can be ignored E and H fields can be considered to be independent spatial distribution of fields is that of static fields

12 Basic RF technique - bipolar An RF voltage is applied between needle-like electrodes inserted into the tissue and the current flowing between the electrodes produces Ohmic heating in the tissue spacing = 7.5 x diameter

13 Basic RF technique -bipolar spacing = 7.5 x diameter

14 Basic RF technique - bipolar Choice of frequency Upper limit: need conduction current >> displacement current de JC = σ E J ε D = dt σ >> ωε ε o satisfied in high water content tissues < few 10s MHz satisfied in low water content tissues < few MHz Lower limit: avoid direct electrical stimulation of nerve and muscle generally satisfied above a few 10s of khz

15 Basic RF technique - bipolar Typical diameter of electrodes is mm Typical spacing is 10 x diameter Typical frequency used is 400 f 1000 khz

16 What if needle separation is increased? spacing = 15 x diameter

17 Basic RF technique - monopolar Single interstitial electrode and large are return electrode from Panescu D et al. IEEE Trans Biomed Eng

18 Problems & Solutions in RF Lesioning The dimensions of the volume of ablated tissue are determined largely by the magnitude of the RF current, the length and diameter the electrode tip, and the time for which RF energy is applied. Tumours smaller than 20 mm in diameter can be ablated with a conventional single needle electrode Ablation of larger tumors is possible with recent technical improvements to create overlapping ablation fields (use of multiple-prong (clustered) electrodes or an expandable electrode with multiple retractable J hooks) to create overlapping ablation fields

19 Problems & Solutions in RF Lesioning When tissue temperature at the electrode becomes excessive, carbonization occurs around the electrode which results in a sharp rise in tissue impedance and interruption of the RF current. This effectively limits the volume of tissue that can be ablated. A cooled-tip electrode avoids charring of tissue immediately around the electrode by cooling the internal chamber of the needle via cold saline infusion, and allows the use of a higher power than the conventional needle.

20 Problems & Solutions in RF Lesioning Controlling the temperature at the catheter tip prevents tissue boiling formation of blood coagulum at the electrode surface subsequent impedance changes at the catheter/tissue interface and is important in achieving reproducibility

21 Problems & Solutions in RF Lesioning Other ways of increasing the ablated volume Pulsed RF current Golderg et al. J Vasc Interv Radiol 1999; 10:

22 Problems & Solutions in RF Lesioning Other ways of increasing the ablated volume Use of saline prior to or during ablation Burdio et al Radiology 2003; 229:

23 Clinical Applications of RF Ablation Cancer Tumours in breast bung bone kidney spleen pancreas Non Cancer applications in cardiology endometrial ablation treatment of gastroesophogeal junction anal canal spinal pain volumetric reduction of the palate/tongue base chronic nasal obstruction Treatment of Parkinson s disease

24 Clinical Applications of RF Ablation The three sensing electrodes and the ablation electrode can be used for unipolar or bipolar electrogram recording and for cardiac pacing (from Panescu D et al. IEEE Trans Biomed Eng ). Lesions produced by a typical RF catheter with, say, a 4 mm diameter tip, are small. (from Lardo AC et al. Circulation. 2000; 102: 698 Attempts at producing larger lesions have included the use of larger electrodes and saline irrigated and cooled electrode tips.

25 Commercial RF devices Medtronic Inc. Cardioblate BP Surgical Ablation System Cardioblate Surgical Ablation Pen Bipolar device Irrigated electrode Medtronic, Inc. 2005

26 Commercial RF devices RITA Medical Systems Inc, CA Treatment of unresectable liver lesions & pain relief for patients with metastatic bone lesions 14g/15g trocar Starburst electrodes with integrated temperature sensors (4-9 tines) Real-time temperature control Copyright 2004, RITA Medical Systems Designed for lesions up to 5 cm

27 Commercial RF devices Radiotherapeutics Inc (now Boston Scientific) Soft tissue lesions, including nonresectable liver lesions The umbrella-shaped array design of the LeVeen Needle Electrode, available in 3.0 and 4.0 cm array diameters 15g trocar Inter-tine spacing of 1 cm. Treatment controlled through monitoring of impedance to prevent charring around electrodes Copyright 2005 Boston Scientific Corporation or its affiliates

28 Commercial RF devices Radiotherapeutics Inc (now Boston Scientific) Concerto bipolar needle electrode Dual arrays 4.2cm spread between arrays 3cm width of array larger ablations, up to 4cm wide by 5.5cm deep designed to reduce the need for overlapping ablations. uses less power (~110W ) for thermal coagulation. designed to achieve clinical endpoint in less time (12-15 minutes?) safety and convenience of not requiring grounding pads. Copyright 2005 Boston Scientific Corporation or its affiliates

29 Commercial RF devices Radiotherapeutics Inc (now Boston Scientific) Thermal lesion begins at tine tips Lesion forms around tines Lesion spreads to gaps between tines and along needle shaft Lesion continues to spread Lesion complete Copyright 2005 Boston Scientific Corporation or its affiliates

30 Commercial RF devices Radionics Inc (now Valleylab) Non-resectable liver lesions Cooled electrode - prevents charring, maintains low impedance Impedance and temperature monitored Treatment controlled through impedance feedback Pulsed current

31 Commercial RF devices EMcision Ltd Hand held- bipolar device designed for rapid tissue ablation and coagulation Designed to minimise blood loss during surgical resection mm diameter needles spaced at ~7mm

32 Potential Hazards Explosions of combustible mixtures (e.g. anaesthetic gases, bowel gas), Interference with instruments and pacemakers, Stimulation of excitable tissues which may lead to ventricular fibrillation Accidental RF burns

33 Summary Monopolar and bipolar techniques Need to monitor/control electrode temperature and/or tissue impedance Various methods to enlarge ablated volume (electrode dimensions, multi-electrodes, multi-tined electrodes, cooled electrodes, saline irrigated electrodes) Essentially low cost technology Large range of clinical applications