RF Safety for Interventional MRI Procedures

1. RF Safety for Interventional MRI Procedures Ergin Atalar, Ph.D. Bilkent University, Ankara, Turkey Johns Hopkins University, Baltimore MD USA

2. Introduction • Interference with iMRI devices • Guidewires/Catheters • Needles • Surgical tools • Excessive heating and burns

3. RF Heating of Guidewires • Problem is extensively studied • Heating is real • Sources of problem are well-known • Conflicting measurement methods are proposed • Guidelines are not well-established

4. RF Heating • Sample heats during MRI due to absorption of energy from RF waves RF Transmitter (Body Coil)

5. RF Heating with Metallic Devices Contraindication or Lower Power Threshold? Devices include implants, surgical tools, internal imaging coils

6. T(oC) 1 1 2 3 • Regulatory Limits • Whole Body • Head • Torso • Extremities oC 38 38 39 40 SAR(W/kg) 4 8 8 12 Local: Averaged over 1 g and 5 minutes Current FDA Guidelines • Core Temperature 37 • Daily Core Fluctuation 36-38 • Threshold for Skin Burn 43 Current guidelines are appropriate for external fields but not for internal

7. Reported Observations • Guidewire tip heating in a phantom • +11°C in 12 s, est. SAR 1 W/kg (Nitz et al. 2001) • +20°C(Wildermuth et al. 1998, Ladd et al. 1998, Liu et al. 2000) • +50°C in 30 s, est. SAR 4 W/kg (Konings et al. 2000) • Broken spinal fusion stimulator lead • +14°C in 4 min, est. SAR 1 W/kg (Chou et al. 1997)

8. Problems With Previous Work: Temperature vs. SAR • Fluid Bath (Ladd 98, Achenbach 97, Sommer 00, Tronnier 99) • Introduces convection – not physiological • Causes underestimation (up to 80 %) • Gel (Smith 00, Nyenhuis 99, Shellock 01, Luechinger 01) • Thermal conductivity not necessarily physiological – under/over estimation (50/100%) • Perfusionless – overestimation (500% or more)

9. Transmit Pattern Bioheat Transfer Framework: A RF Heating Model Conduction Perfusion Power Source Used extensively in hyperthermia field

10. Outline • The coupled problem for 2 classes of internal devices (active and passive) • A metric for reporting the RF safety of a metallic device • A simple method for measuring the RF safety of a metallic device

11. Outline • The coupled problem for 2 classes of internal devices (active and passive) • A metric for reporting the RF safety of a metallic device • A simple method for measuring the RF safety of a metallic device

12. Three MRI Situations External transmitters (e.g. diagnostic imaging) Internal transmitters (e.g. catheter tracking) Passive devices (e.g. guidewires, implants, internal receivers)

13. Transmit Pattern Bioheat Transfer

14. 10 SAR (W/kg) 5 0 0 50 100 150 radius(mm) 1. External Transmitter Finite Difference Solution: Boundary condition of homogeneous B field on surface

15. 2 10 1 10 SAR (W/kg) 0 10 102 -1 101 10 0 10 20 100 W/kg radius(mm) 10-1 coronal view 10-2 2. Internal Transmitting Antenna Analytical Formulation for half wave antenna in uniform homogeneous medium Yeung CJ, Atalar E JMRI 2000; 12:86-91

16. Method of Moments 3. External Transmitter with Implant

17. 6 cm 12 cm 18 cm 24 cm 30 cm SAR Gain Prediction Transmit Pattern SAR Gain 7000 6000 5000 4000 SAR gain 3000 2000 1000 Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193 0 -20 -10 0 10 20 length (cm)

18. Transmit Pattern Bioheat Transfer Conduction Perfusion Power Source

19. Assumptions: • homogeneous thermal parameters Linear • infinite boundary condition Shift Invariant LSI System : Fully characterized by impulse response (Green’s Function) Convolution (weighted averaging) Green’s Function Averaging Transmit Pattern Bioheat Transfer Conduction Perfusion Power Source

20. Averaging Comparison1. External Field 10 Raw SAR distribution 0.5 1 g averaged SAR Estimated Temperature from Green’s Function 8 0.4 SAR matched to T scale based on Green’s Function Gain 6 0.3 SAR (W/kg) T (deg C) 4 0.2 2 0.1 0 0 0 0 20 20 40 40 60 60 80 80 100 100 120 120 radius (mm) Yeung CJ, Atalar E Med Phys 2001; 28:826-832

21. 1 10 Raw SAR distribution 1g averaged SAR 10g averaged SAR T (deg C) Temperature Estimate (resting muscle perfusion) 0 10 -1 10 Averaging Comparison2. Transmit with Loopless RF Antenna 2 10 1 10 SAR (W/kg) 0 10 SAR matched to T scale based on Green’s Function Gain -1 10 0 0 2 2 4 4 6 6 8 8 10 10 12 12 14 14 16 16 18 18 20 20 radius (mm) Yeung CJ, Atalar E. Med Phys 2001; 28:826-832 Steady-State Normalized to 100 mW input power

22. New Guidelines ? T(oC) 1 1 2 3 • Regulatory Limits • Whole Body • Head • Torso • Extremities oC 38 38 39 40 SAR(W/kg) 4 8 8 12 Local: Averaged over 1 g and 5 minutes • Regulatory Limits • Whole Body • Head • Torso • Extremities oC 38 T(oC) 1 X Y Z SAR(W/kg) 4 X*G(m) Y*G(m) Z*G(m) Local: Averaged with Green’s Function

23. Summary - 1 • Using the Green’s function solution to the bioheat equation, established a rationale for updated guidelines for local RF heating

24. Outline • The coupled problem for 2 classes of internal devices (active and passive) • A metric for reporting the RF safety of a metallic device • A simple method for measuring the RF safety of a metallic device

25. Transmit Pattern Bioheat Transfer No wire Transmit Pattern SAR Gain Bioheat Transfer Wire Safety Index Transmit Pattern A Useful Metric for RF Heating Safety Index = F(device characteristics, thermal environment)  F(transmit coil)

26. 75 m insulation External Transmit with Wire Implant bare 9 oC/(W/kg) 8 7 6 Heat transfer properties for resting muscle 5 Safety Index 4 3 2 1 Wire-Free Case 0 0 10 20 30 40 50 60 length (cm) Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193

27. 1.4 1.4 10 cm insulated wire without wire 1.2 1.2 1 1 0.8 0.8 Safety Index 0.6 0.6 0.4 0.4 0.2 0.2 0 0 1.4 2.7 10 27 54 100 perfusion (ml/100g/min) exercising muscle resting muscle bone brain Safety Index: Effect of Perfusion 10 10 resonant bare wire 10cm insulated wire without wire 8 8 6 6 Safety Index 4 4 2 2 0 0 1.4 2.7 10 27 54 100 perfusion (ml/100g/min) Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193

28. Permitted Peak Temperature °C Permitted Peak SS SAR W/kg (as currently determined) Resonant bare wire in resting muscle 8 °C/(W/kg) 0.25 W/kg Resonant bare wire in exercising muscle 5.5 °C/(W/kg) 0.36 W/kg 9 cm wire (75 m insul.) in resting muscle 0.6 °C/(W/kg) 3.3 W/kg 9 cm wire (75 m insul.) in exercising muscle 0.2 °C/(W/kg) 10 W/kg New Paradigm: Any device can be safe Device Geometry 2 °C Perfusion Thermal Conductivity Safety Index Electrical Conductivity Safety Index °C/(W/kg) Electrical Permittivity

29. Summary - 2 • Question of “Is this implant safe?” is wrong. • Correct question is “what is the power threshold?” • Safety Index is a measure of a passive device’s RF safety • Independent of RF transmitter E distribution • Easy to use at the scanner • Depends upon thermal environment (perfusion) • A power threshold can be established based on safety index.

30. Outline • The coupled problem for 2 classes of internal devices (active and passive) • A metric for reporting the RF safety of a metallic device • A simple method for measuring the RF safety of a metallic device

31. Temperature to SAR

32. SAR Calculations o Slope Calculation ( C/sec) 20 19.9 19.8 Temperature ( Degrees C) 19.7 19.6 19.5 19.4 Time (Sec)

33. Estimate In Vivo Temperature from Phantom Temperature Measurements 22 21.5 21 20.5 Temperature (Degrees C) 20 Tvivo 19.5  19 18.5 0 100 200 300 400 500 600 Time (sec) : perfusion time constant

34. Summary - 3 • It is possible to estimate the in vivo temperature from phantom temperature measurements • In vivo temperature value depends on the perfusion level

35. Conclusion • New local RF heating guidelines • Safety thresholds for internal transmitter and passive wires • Safety Index – easy to use metric • Simple measurement method

36. Acknowledgements • Whitaker Foundation • NIH Training Grant • Surgi-Vision Inc. • NIH R01 HL61672 • Christopher Yeung • Rob Susil • Xiaoming Yang • Biophan, Inc.