1. - MRI Safety Update - RF Induced Heating
presented to
Society for Medical Innovation and Technology May 2006 Pebble Beach, Monterey, CA, USA
Jeffrey L. Helfer

2. Objective of this Presentation
Share with you a medical situation that is simultaneously very positive and potentially very dangerous
Briefly describe several options for helping to manage the risks
2 •

3. Acknowledgements Robert Gray (Biophan Scientist)
Andreas Melzer, M.D. (CTO - Biophan Germany)
Xingwu Wang, Ph.D. (Alfred University)
Susan Stalls (Biophan Program Manager)
Mark Bocko, Ph.D. (University of Rochester)
W. Timothy Bibens (Biophan Director of Operations)
Stuart G. MacDonald (Biophan VP of R&D)
Luxtron Corporation
University Medical Imaging (Rochester, New York)
3 •

4. Background Information
MRI is rapidly becoming a premiere non-invasive imaging modality due to the following capabilities:
1. Superb soft tissue contrast (greater detection sensitivity)
2. Functional analysis capabilities
3. No ionizing radiation to patients or healthcare providers
4. Very low toxicity of MRI contrast agents
Significantly less allergenic than iodinated contrast agents
Significantly less damage to kidneys (only for very high dosage)
5. Superior flow and temperature sensitivity
6. Multiplanar images and 3-D data sets without patient repositioning
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5. Evidence of Growth in MRI ISMRM 14th Scientific Meeting
6-12 May 2006
Musculoskeletal Imaging
Diffusion – Perfusion MRI
Multi-modal Functional MRI
MRI Contrast Agents
Advanced Brain MRI
Interventional MRI
Hematobiliary MRI
Molecular Imaging
Functional Breast Imaging
Functional Lung MRI
Cellular Imaging
MRI of Cancer
Cartilage Imaging
Psychiatric MRS-I
MR Spectroscopy of the Brain
Imaging of the Mother & Fetus
Cardiovascular Imagingc
Spinal Cord Imaging
Degenerative Disease MRI
Flow and Motion Quantitation
Pediatric Brain MRI
Quantitative Neuro MRI
MRI Angiography
Whole Body MRI
Myocardial Functional Imaging
Plus + 88 additional topics
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6. Simultaneous Growth in Use of Implanted Medical Devices
Cardiac Rhythm Management
Gastric Simulation
Bone Fusion Stimulation
Orthopedic Implants
Cochlear hearing implants
Bladder Control
Implantable (Automatic) Cardioversion-Defibrillation
Neuromodulation
Pain Management
Drug Infusion Pumps
Cardiac Resynchronization Therapy
Cardiovascular Stenting
Plus Many Others
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7. The Problem
Implanted medical devices can create risks to their patients when exposed to MRI
Excessive heating of the device (multiple causes) capable of producing uncontrolled tissue heating and thermogenic damage.
Induced voltages in the device that can interfere with organ function and device diagnostic and therapeutic capabilities.
3. MR image disruption and distortion that prevents visualization of
tissues “close” to the device.
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8. A Dual Edged Sword! The risk of using of MRI The risk of not using MRI
There are 2-3 million MRIs scanned per year in the U.S. and it is likely that hundreds of people receive scans despite the presence of a metallic implant.
The risk of not using MRI
Approximately 300,000 people per year are denied MRI and the associated health care and diagnostic benefits because of an implant.
Moreover, other diagnostic tools, e.g., invasive angiogram procedures, have undesirable risks such as toxic contrast media and exposure to ionizing radiation.
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9. Representative MR Images
Brain Tumor
3-D MR Angiography
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10. Managing MRI-induced Patient Risk is a Very Difficult Task!
To Make Matters Worse
Managing MRI-induced Patient Risk is a Very Difficult Task!
While it is relatively easy to demonstrate a heating or induced voltage problem, it is far more difficult to prove a solution to these problems, due to their complex and unpredictable nature, which includes factors such as:
• RF field strength • Patient position in the coil
• Type of imaging sequence • Patient characteristics
• Duration of imaging procedure • Body structure being imaged
• Lead design • Specific type of medical device
• Lead orientation within patient • The degree of perfusion near the device
• Temp. measurement procedure • Respiratory phase
Many of these parameters are currently either not recognized or inadequately addressed by existing testing methods
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11. We believe that patients deserve devices that are inherently safe!
To Make Matters Worse - continued
Proper understanding of the MRI safety situation is further exacerbated by the underreporting of adverse events, due to:
• Physician reluctance to report adverse events
• Litigation that shrouds the dissemination of circumstances
surrounding adverse events
MR systems using higher and faster gradient fields, and stronger RF fields will become increasingly common (e.g. move to 3T), maintaining the potential for insufficient safety awareness and risk to patients.
Guidelines alone do not guarantee patient safety.
We believe that patients deserve devices that are inherently safe!
11 •

12. 3-D Wire-in-Phantom Heating
Heat Flux vectors showing conductive transport effect of the wire.
Ambient = 25°C
Ambient = 25°C
45°C
Max
75°C
Max
30°C
Skin
30°C
Skin
Isothermal plot in phantom
(Passive fixation lead)
Close-up of isotherms
(Active fixation lead)
Substantial MRI-induced heating!
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13. (i.e. 64MHz) electrical impedance of the lead
Our Approach
Tissue heating can be substantially reduced by increasing the high frequency
(i.e. 64MHz) electrical impedance of the lead
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14. Simple Model of Bipolar Lead Circuit Diagram
IPG
Circuit of pacing lead in MRI scanner is not simple…
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15. Theory: Shifting Self Resonance Of Lead
64 MHz
MR scanner’s frequency
is fixed. So, we need to
shift lead’s self-resonance
frequency by changing
coil (i.e. lead) inductance and capacitance properties.
Maximum impedance at “self” resonance.
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16. Theory: Air Core Coils Simplified Impedance Equation
Rd ≡ Distributed
Resistance
Cd ≡ Distributed
Capacitance
Resonance Condition
Cs ≡ Parasitic
Shunt
Capacitance
Rs ≡ Series
Resistance
Maximum coil impedance occurs at “self” resonance.
Source: R.Ludwig, P. Bretchko, RF Circuit Design Theory and Applications, Prentice Hall, 1999
16 •

17. Discrete Component Solution
Attachment of components (side view).
First Prototypes
Attachment of wires (side view
Smaller components are currently being evaluated
(0.012” x 0.012” x 0.024”) as well as alternate (smaller) packaging designs
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18. Experimental Setup
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19. Results – Modified Wireform
Leads designed with different inductance and capacitance.
Changing the wire form design changes the capacitance-inductance
characteristics of the lead and its impedance
Two leads had less than 0.5°C temp. increase.
Control
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20. Coil Impedance Values at 64 MHz
Lead Impedance at 64 MHz
287
186 – 219j
440
280 – 340j
Modified Wire Form
484
200 – 441j
472
204 – 426j
OEM #2 3-6
136
120 – 64j 75
57 – 48j
Control #2 (OEM #2)
610
215 – 571j
606
223 – 563j
OEM #2 1-6
533
124 – 518j
534
129 – 518j
OEM #1 1-1
517
203 – 476j
527
207 – 485j
OEM #1 3-3
528
208 – 485j
542
240 – 486j
OEM #1 3-2
557
162 – 533j
568
179 – 539j
OEM #1 1-2
783
220 – 751j
784
213 – 755j
OEM #1 4-1
256
178 – 184j
232
210 – 99j
OEM #1 4-2
117
96 – 67j
109
57 – 93j
Control #1 (SJM 1688T)
Zmag ()
Impedance ()
Sample
In-Situ
In Air
Coil Impedance Values at 64 MHz
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21. Results - Discrete Component Solution
Control #1
(Vendor A)
Leads designed with different inductance and capacitance.
Control #2
(Vendor B)
Adding a discrete component, high frequency resonator to the lead changes the capacitance - inductance
characteristics of the lead and its impedance
6 modified leads had < 1° C temp. increase.
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22. MRI-induced Voltages Induced Voltage ≈ dB1 dt AVL x Where;
AVL = Area of the “virtual loop” formed by the device, lead, and interconnecting tissue
dB1/dt = Rate of change of applied magnetic field
Biophan has measured1 induced voltages of ~ 250 – 1000 mV in “anatomically reasonable” cardiac pacing lead configurations
Multiple solutions to this problem are available
Note 1: Test conditions consisted of RF switched off, scan sequence: Fast Spin Echo, TR = 300, TE = 4, Echo Train Length = 2, Freq = 256, Phase = 256, NEX = 2, Phase FOV = 1, FOV = 18, Spacing = 1.0.
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23. Conclusions
Minimally disruptive lead design options are available to reduce worst-case lead heating to acceptable levels
Biophan has also developed easy to implement solutions for reducing or eliminating MRI-induced voltages in leads
When implanted, these designs provide the potential to:
Provide a greater margin of patient safety
Allow a greater number of patients access to MRI
We believe that these design options can also be applied to other similar design conductive implants such as ICD and DBS leads as well as guidewires and catheters.
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24. Typical Approach to Risk Management
Training
Warnings and precautions in product labeling
Restrict product use (i.e. contraindications)
Protective measures (e.g. patient monitoring)
Product designs that reduce hazard likelihood
Product designs that eliminate the hazard
Increasing Safety
It is possible to produce devices that are inherently safe!
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25. Biophan Technology Overview
The End
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