Transcript RF Safety in Interventional MRI

RF Safety for Interventional MRI
Procedures
Ergin Atalar, Ph.D.
Bilkent University, Ankara, Turkey
Johns Hopkins University, Baltimore MD USA
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
Contraindication or Lower Power Threshold?
Devices include implants, surgical tools, internal imaging coils
Ergin Atalar, Ph.D.
Current FDA Guidelines
Regulatory Limits
Whole Body
Head
Local:
Torso
Extremities
• Core Temperature
• Daily Core Fluctuation
• Threshold for Skin Burn
oC
38
38
39
40
T(oC) SAR(W/kg)
1
4
1
8
over 1 g
2
8 Averaged
and 5 minutes
3
12
37
36-38
43
Current guidelines are appropriate for
external fields but not for internal
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
P(t )
SAR  E
Transmit
Pattern
SAR(r , t )
Bioheat
Transfer
T (r , t )
2


1 T (r , t )
2
  T (r , t )

t
Conduction


1
 v T (r , t ) 
SAR (r , t )
k
2
Perfusion
Power Source
Used extensively in hyperthermia field
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
Ergin Atalar, Ph.D.
Three MRI Situations
External transmitters
(e.g. diagnostic imaging)
Internal transmitters
(e.g. catheter tracking)
Passive devices
(e.g. guidewires, implants,
internal receivers)
P(t )
Transmit
Pattern
SAR  E
SAR(r , t )
Bioheat
Transfer
T (r , t )
2
Ergin Atalar, Ph.D.
1. External Transmitter
SAR (W/kg)
10
5
0
0
Finite Difference Solution: Boundary
condition of homogeneous B field on surface
50
100
radius(mm)
150
Ergin Atalar, Ph.D.
2. Internal Transmitting Antenna
Analytical Formulation for half wave
antenna in uniform homogeneous medium
2
101
100
10-1
coronal view
10-2
W/kg
SAR (W/kg)
10
102
1
10
0
10
-1
10
0
10
20
radius(mm)
Yeung CJ, Atalar E
JMRI 2000; 12:86-91
Ergin Atalar, Ph.D.
3. External Transmitter with Implant


E
Method of Moments
Ergin Atalar, Ph.D.
SAR Gain Prediction
RF (t )
Transmit
Pattern

SAR(r )

SAR (r )
SAR
Gain
7000
6000
6 cm
SAR gain
5000
12 cm
4000
18 cm
3000
24 cm
2000
30 cm
1000
0
-20
-10
0
length (cm)
10
20
Yeung CJ, Susil RC, Atalar E
MRM 2002; 47:187-193
Ergin Atalar, Ph.D.
P(t )
Transmit
Pattern
SAR(r , t )
Bioheat
Transfer
T (r , t )
t
1 T (r , t )
2
2
  T (r , t )  v T (r , t ) 
SAR(r , t )

t
k
Conduction
Perfusion
Power Source
Ergin Atalar, Ph.D.
Green’s Function Averaging
P(t )
Transmit
Pattern
SAR(r , t )
Bioheat
Transfer
T (r , t )

1 T (r , t )
  2 T (r , t ) v 2 T (r , t )  t SAR(r , t )

t
k
Conduction
Perfusion
Assumptions:
• homogeneous thermal parameters
• infinite boundary condition
LSI System :
Fully characterized by
impulse response
(Green’s Function)
1 vr
e
4kr
Power Source
Linear
Shift Invariant
Convolution
(weighted averaging)
Averaging Comparison 1. External Field
8
from Green’s Function
0.5
0.4
6
0.3
4
0.2
2
0.1
0
Yeung CJ, 0Atalar E
20
40
60
80
radius (mm)
Med Phys 2001; 28:826-832
100
SAR matched to
T scale based on
Green’s Function
Gain
T (deg C)
Raw SAR distribution
Estimated
Temperature
1 g averaged
SAR
SAR (W/kg)
10
0
120
Ergin Atalar, Ph.D.
Averaging Comparison
2. Transmit with Loopless RF Antenna
2
10
Raw SAR distribution
10
1
10
0
10
0
10
T (deg C)
SAR (W/kg)
1
1g averaged SAR
10g averaged SAR
Temperature Estimate
(resting muscle perfusion)
SAR matched to T scale based on
Green’s Function Gain
-1
10
-1
10
0
2
4
6
8 10 12 14 16 18 20
radius (mm)
Steady-State
Normalized to 100 mW input power
Yeung CJ, Atalar E.
Med Phys 2001; 28:826-832
Ergin Atalar, Ph.D.
New Guidelines ?
Regulatory Limits
Whole Body
Head
Local:
Torso
Extremities
oC
Regulatory Limits
Whole Body
Head
Local:
Torso
Extremities
oC
38
38
39
40
38
T(oC) SAR(W/kg)
1
4
1
8
2
8
3
12
T(oC) SAR(W/kg)
1
4
X
X*G(m)
Y
Y*G(m)
Z
Z*G(m)
Averaged over 1 g
and 5 minutes
Averaged with
Green’s Function
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
RF (t )
No wire
Wire
RF (t )
RF (t )
Transmit
Pattern
Transmit
Pattern
Transmit
Pattern

SAR(r )

SAR(r )

SAR(r )
SAR
Gain
Bioheat
Transfer

SAR (r )
Safety
Index

T (r )
Bioheat
Transfer

T (r )
peak SS T in vivo
Safety Index = F(device characteristics, thermal environment)
 F(transmit coil)
Ergin Atalar, Ph.D.
External Transmit with Wire Implant
75 m insulation
bare
9
oC/(W/kg)
8
Safety Index
7
6
Heat transfer
properties for
resting muscle
5
4
3
2
Wire-Free
Case
1
0
0
10
20
30
40
length (cm)
50
60
Yeung CJ, Susil RC, Atalar E
MRM 2002; 47:187-193
Ergin Atalar, Ph.D.
Safety Index: Effect of Perfusion
10
1.4
resonant bare wire
10cm insulated wire
without wire
8
10 cm insulated wire
without wire
1.2
Safety Index
Safety Index
1
6
4
0.8
0.6
0.4
2
0.2
0
bone
1.4
2.7
10
27
perfusion (ml/100g/min)
resting
muscle
54
exercising
muscle
100
0
1.4
2.7
10
27
perfusion (ml/100g/min)
54
100
brain
Yeung CJ, Susil RC, Atalar E
MRM 2002; 47:187-193
Ergin Atalar, Ph.D.
Device
Geometry
Perfusion
Thermal
Conductivity
Electrical
Conductivity
Electrical
Permittivity
Safety Index
New Paradigm: Any device can be safe
2 °C
Permitted Peak
Temperature °C
Permitted Peak
SS SAR
W/kg
Safety Index
°C/(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
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
Temperature (Degrees C)
22
21.5
21
20.5
20
19.5
19
18.5
0
100
200
300
400
500
600
Time (sec)
Ergin Atalar, Ph.D.
SAR Calculations
Slope Calculation (o C/sec)
Temperature ( Degrees C)
20
SAR  Slope   P
19.9
W  sec
 P  4180
kg C
19.8
19.7
19.6
19.5
19.4
Time (Sec)
Ergin Atalar, Ph.D.
Estimate In Vivo Temperature from
Phantom Temperature Measurements
Temperature (Degrees C)
22
21.5
21
20.5
20
Tvivo
19.5
19
18.5
0
100

200
300
Time (sec)
: perfusion time constant
400
500
600
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
•
•
•
•
Christopher Yeung
Rob Susil
Xiaoming Yang
Biophan, Inc.
•
•
•
•
Whitaker Foundation
NIH Training Grant
Surgi-Vision Inc.
NIH R01 HL61672
Ergin Atalar, Ph.D.