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

Ergin Atalar, Ph.D. Introduction Interference with iMRI devices –Guidewires/Catheters –Needles –Surgical tools Excessive heating and burns

Ergin Atalar, Ph.D. 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

Ergin Atalar, Ph.D. RF Heating Sample heats during MRI due to absorption of energy from RF waves RF Transmitter (Body Coil)

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

Ergin Atalar, Ph.D. Current FDA Guidelines Core Temperature 37 Daily Core Fluctuation Threshold for Skin Burn 43 Current guidelines are appropriate for external fields but not for internal SAR(W/kg) Regulatory Limits Whole Body Head Torso Extremities o C Averaged over 1 g and 5 minutes  T( o C) Local:

Ergin Atalar, Ph.D. 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)

Ergin Atalar, Ph.D. 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)

Ergin Atalar, Ph.D. Framework: A RF Heating Model ConductionPower SourcePerfusion Used extensively in hyperthermia field Transmit Pattern Bioheat Transfer

Ergin Atalar, Ph.D. Outline 1.The coupled problem for 2 classes of internal devices (active and passive) 2.A metric for reporting the RF safety of a metallic device 3.A simple method for measuring the RF safety of a metallic device

Ergin Atalar, Ph.D. Outline 1.The coupled problem for 2 classes of internal devices (active and passive) 2.A metric for reporting the RF safety of a metallic device 3.A simple method for measuring the RF safety of a metallic device

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

Ergin Atalar, Ph.D. Transmit Pattern Bioheat Transfer

Ergin Atalar, Ph.D. 1. External Transmitter radius(mm) SAR (W/kg) Finite Difference Solution: Boundary condition of homogeneous B field on surface

Ergin Atalar, Ph.D. 2. Internal Transmitting Antenna Yeung CJ, Atalar E JMRI 2000; 12:86-91 W/kg coronal view Analytical Formulation for half wave antenna in uniform homogeneous medium radius(mm) SAR (W/kg)

Ergin Atalar, Ph.D. 3. External Transmitter with Implant Method of Moments

Ergin Atalar, Ph.D. SAR Gain Prediction length (cm) SAR gain Transmit Pattern SAR Gain 6 cm 12 cm 18 cm 24 cm 30 cm Yeung CJ, Susil RC, Atalar E MRM 2002; 47:

Ergin Atalar, Ph.D. Transmit Pattern Bioheat Transfer ConductionPower SourcePerfusion

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

Ergin Atalar, Ph.D. Averaging Comparison 1. External Field SAR (W/kg) radius (mm) Raw SAR distribution 1 g averaged SAR Estimated Temperature from Green’s Function T (deg C) Yeung CJ, Atalar E Med Phys 2001; 28: SAR matched to T scale based on Green’s Function Gain

Ergin Atalar, Ph.D. Averaging Comparison 2. Transmit with Loopless RF Antenna Steady-State Raw SAR distribution 1g averaged SAR 10g averaged SAR Temperature Estimate (resting muscle perfusion) SAR matched to T scale based on Green’s Function Gain Normalized to 100 mW input power SAR (W/kg) radius (mm) T (deg C) Yeung CJ, Atalar E. Med Phys 2001; 28:

Ergin Atalar, Ph.D. New Guidelines ? SAR(W/kg) Regulatory Limits Whole Body Head Torso Extremities o C Averaged over 1 g and 5 minutes  T( o C) Local: SAR(W/kg) 4 X*G(m) Y*G(m) Z*G(m) Regulatory Limits Whole Body Head Torso Extremities o C 38 Averaged with Green’s Function  T( o C) 1 X Y Z Local:

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

Ergin Atalar, Ph.D. Outline 1.The coupled problem for 2 classes of internal devices (active and passive) 2.A metric for reporting the RF safety of a metallic device 3.A simple method for measuring the RF safety of a metallic device

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

Ergin Atalar, Ph.D Safety Index length (cm) External Transmit with Wire Implant o C/(W/kg) bare 75  m insulation Heat transfer properties for resting muscle Wire-Free Case Yeung CJ, Susil RC, Atalar E MRM 2002; 47:

Ergin Atalar, Ph.D Safety Index perfusion (ml/100g/min) resonant bare wire 10cm insulated wire without wire Safety Index: Effect of Perfusion bone resting muscle exercising muscle brain Yeung CJ, Susil RC, Atalar E MRM 2002; 47:

Ergin Atalar, Ph.D. New Paradigm: Any device can be safe Device Geometry Perfusion Thermal Conductivity Electrical Conductivity Electrical Permittivity Safety Index °C/(W/kg) Permitted Peak Temperature °C Permitted Peak SS SAR W/kg (as currently determined) Safety Index Resonant bare wire in resting muscle8 °C/(W/kg)0.25 W/kg 9 cm wire (75  m insul.) in resting muscle 0.6 °C/(W/kg)3.3 W/kg 2 °C 9 cm wire (75  m insul.) in exercising muscle 0.2 °C/(W/kg)10 W/kg Resonant bare wire in exercising muscle5.5 °C/(W/kg)0.36 W/kg

Ergin Atalar, Ph.D. 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.

Ergin Atalar, Ph.D. Outline 1.The coupled problem for 2 classes of internal devices (active and passive) 2.A metric for reporting the RF safety of a metallic device 3.A simple method for measuring the RF safety of a metallic device

Ergin Atalar, Ph.D. Temperature to SAR

Ergin Atalar, Ph.D. SAR Calculations Slope Calculation ( o C/sec) Time (Sec) Temperature ( Degrees C)

Estimate In Vivo Temperature from Phantom Temperature Measurements Time (sec) Temperature (Degrees C)  T vivo   : perfusion time constant

Ergin Atalar, Ph.D. Summary - 3 It is possible to estimate the in vivo temperature from phantom temperature measurements In vivo temperature value depends on the perfusion level

Ergin Atalar, Ph.D. Conclusion New local RF heating guidelines Safety thresholds for internal transmitter and passive wires Safety Index – easy to use metric Simple measurement method

Ergin Atalar, Ph.D. Acknowledgements Whitaker Foundation NIH Training Grant Surgi-Vision Inc. NIH R01 HL61672 Christopher Yeung Rob Susil Xiaoming Yang Biophan, Inc.