RF Safety for Interventional MRI Procedures - PowerPoint PPT Presentation

Loading...

PPT – RF Safety for Interventional MRI Procedures PowerPoint presentation | free to download - id: 692242-ZmZiM



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

RF Safety for Interventional MRI Procedures

Description:

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

Number of Views:18
Avg rating:3.0/5.0
Slides: 37
Provided by: Christophe783
Learn more at: http://www.mri.jhu.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: 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
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
  • 11C in 12 s, est. SAR 1 W/kg (Nitz et al. 2001)
  • 20C (Wildermuth et al. 1998, Ladd et al. 1998,
    Liu et al. 2000)
  • 50C in 30 s, est. SAR 4 W/kg (Konings et al.
    2000)
  • Broken spinal fusion stimulator lead
  • 14C 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
Framework A RF Heating Model
Conduction
Power Source
Perfusion
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
1. External Transmitter
Finite Difference Solution Boundary condition of
homogeneous B field on surface
15
2. Internal Transmitting Antenna
Analytical Formulation for half wave antenna in
uniform homogeneous medium
Yeung CJ, Atalar E JMRI 2000 1286-91
16
3. External Transmitter with Implant
17
SAR Gain Prediction
Transmit Pattern
SAR Gain
7000
6000
5000
4000
SAR gain
3000
2000
1000
Yeung CJ, Susil RC, Atalar E MRM 2002 47187-193
0
-20
-10
0
10
20
length (cm)
18
Transmit Pattern
Bioheat Transfer
Conduction
Power Source
Perfusion
19
Greens Function Averaging
Transmit Pattern
Bioheat Transfer
Conduction
Perfusion
Power Source
20
Averaging Comparison 1. External Field
10
Raw SAR distribution
0.5
1 g averaged SAR
Estimated Temperature from Greens Function
8
0.4
SAR matched to T scale based on Greens Function
Gain
6
0.3
SAR (W/kg)
T (deg C)
4
0.2
2
0.1
0
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
radius (mm)
Yeung CJ, Atalar E Med Phys 2001 28826-832
21
Averaging Comparison2. Transmit with Loopless RF
Antenna
2
10
1
10
SAR (W/kg)
0
10
SAR matched to T scale based on Greens Function
Gain
-1
10
0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
12
14
16
18
20
radius (mm)
Yeung CJ, Atalar E. Med Phys 2001 28826-832
Steady-State
Normalized to 100 mW input power
22
New Guidelines ?
Regulatory Limits Whole Body Head Torso Extremitie
s
oC 38 38 39 40
?T(oC) 1 1 2 3
SAR(W/kg) 4 8 8 12
Local
Averaged over 1 g and 5 minutes
Regulatory Limits Whole Body Head Torso Extremitie
s
oC 38
?T(oC) 1 X Y Z
SAR(W/kg) 4 XG(m) YG(m) ZG(m)
Local
Averaged with Greens Function
23
Summary - 1
  • Using the Greens 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
A Useful Metric for RF Heating
Safety Index F(device characteristics, thermal
environment) ? F(transmit coil)
26
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 47187-193
27
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 47187-193
28
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 devices
    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.
About PowerShow.com