radio frequency induced heating near medical devices during mri justin peterson university of...
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Radio Frequency Induced Heating Near Medical Devices During MRI
Justin PetersonUniversity of Western Ontario
Medical BiophysicsMarch. 23, 2011
MR System ComponentsMain Magnet (3 T)
Gradient Coils
RF Coils (64 or 127 MHz)
y
x
B0
z
F
Applied Field
B0
Bz
z
∆ z
Radio-frequencysignals
MR Induced Heating
• Factors affecting rate of heating:• Strength of magnet• Geometry • Composition• Position in bore
Device• Stainless steel pins connected by a carbon
fiber/fiberglass hybrid bar
• Used to brace damaged joints while healing
• Simple configuration shown:
Objectives
• Determine the relationship between radial position and rate of temperature change in the device
• Determine the ideal “worst case scenario” for future testing
Methods• Phantom resembles human head and
torso
• Fill with gel made of NaCl, Polyacrylic acid and distilled water
• Gel has conductivity similar to human tissue (4.80 mS/cm)
• Brace device and attach optical temperature sensors
Methods
Methods
-240 mm -210 mm -160 mm
0 mm +230 mm
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 1061131201271341411481551621691761831901970
5
10
15
20
25
30
35
40
Temperature Change +230 mm from Centre of Bore
Sensor 1Sensor 2Sensor 3Sensor 4Sensor 5Sensor 6Sensor 7Sensor 8
Time (s)
∆ T
em
pera
ture
(˚
C)
Results
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200
2
4
6
8
10
12
14
16
Varying Radial Position of Device on Tem-perature
- 240 mm - 210 mm - 160 mm 0 mm + 230 mm
Time (s)
∆ T
em
pera
ture
(˚
C)
Results
Results
-300 -200 -100 0 100 200 3000.300
0.350
0.400
0.450
0.500
0.550
0.600
0.650
0.700
0.750
0.800
Rate of Temperature Change Dependency on Radial Position in MR Bore
Radial Position in Bore (mm)
dT/d
t (˚
C/s
)
Discussion
• Strong trend showed an increased rate of temperature change as radial distance from the centre increases • electric field associated with gradient coils
• Ideal “worst case scenario” for radial position:• Along the edge of the bore
• Device pin closest to the ‘head’ of the phantom has the highest rate of temperature change
Conclusion/Future Studies
• Highest rate of temperature change along edge of bore
• Higher sample size for each group
• Different heating of two pins
• Tests down the length of the bore
Acknowledgements
• Blaine Chronik, PhD
• Ian MacDonald, PhD
• Baraa Al-Khazraji
• Nicole Niovielli