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- MRI Safety Update - RF Induced Heating. presented to. Society for Medical Innovation and Technology 11-14 May 2006 Pebble Beach, Monterey, CA, USA. Jeffrey L. Helfer. Objective of this Presentation. - PowerPoint PPT Presentation
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- MRI Safety Update -
RF Induced Heating
Society for Medical Innovation and Technology11-14 May 2006
Pebble Beach, Monterey, CA, USA
presented to
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
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)
4 •
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
Background Information
5 •
ISMRM 14th Scientific Meeting6-12 May 2006
Imaging of the Mother & Fetus
Cardiovascular Imagingc
Spinal Cord Imaging
Degenerative Disease MRI
Flow and Motion Quantitation
Cellular Imaging
MRI of Cancer
Cartilage Imaging
Psychiatric MRS-I
MR Spectroscopy of the Brain
Interventional MRI
Hematobiliary MRI
Molecular Imaging
Functional Breast Imaging
Functional Lung MRI
Musculoskeletal Imaging
Diffusion – Perfusion MRI
Multi-modal Functional MRI
MRI Contrast Agents
Advanced Brain MRI
Pediatric Brain MRI
Quantitative Neuro MRI
MRI Angiography
Whole Body MRI
Myocardial Functional Imaging
Plus + 88 additional topics
Evidence of Growth in MRI
6 •
Simultaneous Growth in Use of Implanted Medical Devices
Plus Many Others
Cochlear hearing implants
Bladder Control
Implantable (Automatic) Cardioversion-Defibrillation
Neuromodulation
Pain Management
Drug Infusion Pumps
Cardiac Resynchronization Therapy
Cardiovascular Stenting
Cardiac Rhythm Management
Gastric Simulation
Bone Fusion Stimulation
Orthopedic Implants
7 •
Implanted medical devices can create risks to their patients when exposed to MRI
1. Excessive heating of the device (multiple causes) capable of producing uncontrolled tissue heating and thermogenic damage.
2. 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.
The Problem
8 •
A Dual Edged Sword!
The risk of using of 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.
9 •
Brain Tumor3-D MR Angiography
Representative MR Images
10 •
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
Managing MRI-induced Patient Risk is a Very Difficult Task!
Many of these parameters are currently either not recognized or inadequately addressed by existing testing methods
To Make Matters Worse
11 •
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!
To Make Matters Worse - continued
12 •
3-D Wire-in-Phantom Heating
Isothermal plot in phantom(Passive fixation lead)
45°CMax
30°CSkin
Close-up of isotherms(Active fixation lead)
75°CMax
30°CSkin
Ambient = 25°C
Substantial MRI-induced heating!
Ambient = 25°C Heat Flux vectors showing
conductive transport effect
of the wire.
13 •
Tissue heating can be substantially reduced
by increasing the high frequency
(i.e. 64MHz) electrical impedance of the lead
Our Approach
14 •
Circuit of pacing lead in MRI scanner is not simple…
IPG
Simple Model of Bipolar Lead Circuit Diagram
15 •
Theory: Shifting Self Resonance Of Lead
Maximum impedance at “self” resonance.
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.
64 MHz
16 •
Source: R.Ludwig, P. Bretchko, RF Circuit Design Theory and Applications, Prentice Hall, 1999
Theory: Air Core Coils
Simplified Impedance Equation
Resonance Condition
Maximum coil impedance occurs at “self” resonance.
Rd ≡ Distributed Resistance
Cd ≡ Distributed Capacitance
Rs ≡ Series Resistance
Cs ≡ Parasitic Shunt Capacitance
17 •
Attachment of components (side view).
Attachment of wires (side view
Discrete Component Solution
Smaller components are currently being evaluated (0.012” x 0.012” x 0.024”) as well as alternate (smaller) packaging designs
First Prototypes
18 •
Experimental Setup
19 •
Leads designed with
different inductance
and capacitance.
Two leads had less than 0.5°C temp. increase.
Control
Changing the wire form
design changes the
capacitance-inductance
characteristics of the
lead and its impedance
Results – Modified Wireform
20 •
287186 – 219j440280 – 340jModified Wire Form
484200 – 441j472204 – 426jOEM #2 3-6
136120 – 64j7557 – 48jControl #2 (OEM #2)
610215 – 571j606223 – 563jOEM #2 1-6
533124 – 518j534129 – 518jOEM #1 1-1
517203 – 476j527207 – 485jOEM #1 3-3
528208 – 485j542240 – 486jOEM #1 3-2
557162 – 533j568179 – 539jOEM #1 1-2
783220 – 751j784213 – 755jOEM #1 4-1
256178 – 184j232210 – 99jOEM #1 4-2
11796 – 67j10957 – 93jControl #1 (SJM 1688T)
Zmag ()Impedance ()Zmag ()Impedance ()Sample
In-SituIn Air
Coil Impedance Values at 64 MHz
Lead Impedance at 64 MHz
21 •
Results - Discrete Component Solution
Control #1(Vendor A)
Control #2(Vendor B)
6 modified leads had < 1° C temp. increase.
Leads designed with different inductance
and capacitance.
Adding a discrete component, high
frequency resonator to the lead changes the
capacitance - inductancecharacteristics of the
lead and its impedance
22 •
Induced Voltage ≈dB1
dtAVL
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
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.
Biophan has measured1 induced voltages of ~ 250 – 1000 mV in “anatomically reasonable” cardiac pacing lead configurations
Multiple solutions to this problem are available
MRI-induced Voltages
23 •
Conclusions
When implanted, these designs provide the potential to:
• Provide a greater margin of patient safety
• Allow a greater number of patients access to MRI
Minimally disruptive lead design options are available to reduce worst-case lead heating to acceptable levels
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.
Biophan has also developed easy to implement solutions for reducing or eliminating MRI-induced voltages in leads
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
In
crea
sin
g S
afet
y
It is possible to produce devices that are inherently safe!
25 •
Biophan Technology Overview
The End