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 36-38

• Threshold for Skin Burn 43

Current guidelines are appropriate for external fields but not for internal

SAR(W/kg)

4

8

8

12

Regulatory Limits

Whole Body

Head

Torso

Extremities

oC

38

38

39

40

Averaged over 1 g and 5 minutes

T(oC)

1

1

2

3

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

2ESAR

Conduction Power SourcePerfusion

Used extensively in hyperthermia field

),(1

),(),(),(1 22 trSAR

ktrTvtrT

t

trT

Transmit Pattern

Bioheat Transfer

( , )SAR r t

( , )T r t

( )P t

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

2ESAR

( , )SAR r t

( , )T r t

( )P t

Ergin Atalar, Ph.D.

1. External Transmitter

0 50 100 1500

5

10

radius(mm)

SA

R (

W/k

g)

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/kg100

102

10-1

101

10-2

coronal view

Analytical Formulation for half wave antenna in uniform homogeneous medium

0 10 2010

-1

100

101

102

radius(mm)S

AR

(W

/kg)

Ergin Atalar, Ph.D.

3. External Transmitter with Implant

E

Method of Moments

Ergin Atalar, Ph.D.

SAR Gain Prediction

-20 -10 0 10 200

1000

2000

3000

4000

5000

6000

7000

length (cm)

SA

R g

ain

Transmit Pattern

( )RF t )(rSAR

)(rRSASAR

Gain

6 cm

12 cm

18 cm

24 cm

30 cm

Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193

Ergin Atalar, Ph.D.

Transmit Pattern

Bioheat Transfer

Conduction Power SourcePerfusion

2 21 ( , )( , ) ( , ) ( , )tT r t

T r t v T r t SAR r tt k

( , )SAR r t

( , )T r t

( )P t

Green’s Function Averaging

2 21 ( , )( , ) ( , ) ( , )tT r t

T r t v T r t SAR r tt k

Convolution(weighted averaging)

LSI System :Fully characterized by

impulse response (Green’s Function)

vrekr

4

1

Conduction Perfusion Power Source

Assumptions:• homogeneous thermal parameters Linear• infinite boundary condition Shift Invariant

Transmit Pattern

Bioheat Transfer

( , )SAR r t

( , )T r t

( )P t

Ergin Atalar, Ph.D.

Averaging Comparison 1. External Field

0 20 40 60 80 100 1200

2

4

6

8

10

SA

R (

W/k

g)

radius (mm)

Raw SAR distribution 1 g averaged SAR

Estimated Temperature from Green’s Function

0 20 40 60 80 100 1200

0.1

0.2

0.3

0.4

0.5

T (

deg

C)

Yeung CJ, Atalar EMed Phys 2001; 28:826-832

SAR matched to T scale based on Green’s Function

Gain

Ergin Atalar, Ph.D.

Averaging Comparison2. Transmit with Loopless RF Antenna

Steady-State

Raw SAR distribution

1g averaged SAR

10g averaged SARTemperature Estimate (resting muscle perfusion)

SAR matched to T scale based on Green’s Function Gain

Normalized to 100 mW input power

0 2 4 6 8 10 12 14 16 18 2010

-1

100

101

102

SA

R (

W/k

g)

radius (mm)0 2 4 6 8 10 12 14 16 18 20

10-1

100

101

T (

deg

C)

Yeung CJ, Atalar E.Med Phys 2001; 28:826-832

Ergin Atalar, Ph.D.

New Guidelines ?SAR(W/kg)

4

8

8

12

Regulatory Limits

Whole Body

Head

Torso

Extremities

oC

38

38

39

40

Averaged over 1 g and 5 minutes

T(oC)

1

1

2

3

Local:

SAR(W/kg)

4

X*G(m)

Y*G(m)

Z*G(m)

Regulatory Limits

Whole Body

Head

Torso

Extremities

oC

38

Averaged with Green’s Function

T(oC)

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

)(rSAR

Safety Index

vivoinTSSpeak ( )RF t

No wire

WireTransmit Pattern

)(rRSASAR

GainBioheat Transfer

)(rT

( )RF t )(rSAR

Transmit Pattern

Bioheat Transfer

)(rT

( )RF t )(rSAR

Ergin Atalar, Ph.D.

0 10 20 30 40 50 600

1

2

3

4

5

6

7

8

9S

afet

y In

dex

length (cm)

External Transmit with Wire Implant

oC/(W/kg)

bare 75 m insulation

Heat transfer properties for resting muscle

Wire-FreeCase

Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193

Ergin Atalar, Ph.D.

1.4 2.7 10 27 54 1000

2

4

6

8

10

Saf

ety

Inde

x

perfusion (ml/100g/min)

resonant bare wire 10cm insulated wirewithout wire

0

2

4

6

8

10

Safety Index: Effect of Perfusion

1.4 2.7 10 27 54 1000

0.2

0.4

0.6

0.8

1

1.2

1.4

Saf

ety

Inde

x

perfusion (ml/100g/min)

10 cm insulated wirewithout wire

0

0.2

0.4

0.6

0.8

1

1.2

1.4

boneresting muscle

exercising muscle brain

Yeung CJ, Susil RC, Atalar E MRM 2002; 47:187-193

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)

Saf

ety

Inde

x

Resonant bare wire in resting muscle 8 °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 muscle 5.5 °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

Distal Tip Temperature Verses Time at 8cm Insertion Depth During 7.5W/kg Applied Whole Body SAR

18.5

19

19.5

20

20.5

21

21.5

22

0 100 200 300 400 500 600

Time (sec)

Tem

pe

ratu

re (

De

gre

es

C)

Distal Tip Temperature Verses Time at 8cm Insertion Depth During 7.5W/kg Applied Whole Body SAR

18.5

19

19.5

20

20.5

21

21.5

22

0 100 200 300 400 500 600

Time (sec)

Tem

pe

ratu

re (

De

gre

es

C)

Ergin Atalar, Ph.D.

SAR Calculations

Slope Calculation (oC/sec)

19.4

19.5

19.6

19.7

19.8

19.9

20

Time (Sec)

Tem

per

atu

re (

Deg

rees

C)

sec4180

P

P

SAR Slope

W

kg C

sec4180

P

P

SAR Slope

W

kg C

Estimate In Vivo Temperature from Phantom Temperature Measurements

18.5

19

19.5

20

20.5

21

21.5

22

0 100 200 300 400 500 600

Time (sec)

Tem

per

atu

re (

Deg

rees

C)

Tvivo

: 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.