Download MRI Overview_SMIT - Biophan Technologies

- 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
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 •
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 •
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
4 •
Evidence of Growth in MRI
ISMRM 14th Scientific Meeting
6-12 May 2006
Diffusion – Perfusion MRI
MRI of Cancer
Molecular Imaging
Whole Body MRI
Cellular Imaging
Advanced Brain MRI
Imaging of the
Mother & Fetus
Pediatric Brain MRI
Degenerative Disease MRI
Interventional MRI
Cardiovascular
Imagingc
Myocardial Functional
Imaging
Musculoskeletal
Imaging
Quantitative Neuro MRI
Multi-modal
Functional MRI
MRI Angiography
Psychiatric MRS-I
Functional Breast
Imaging
Flow and Motion
Quantitation
MR Spectroscopy
of the Brain
Plus + 88 additional topics
5 •
Hematobiliary MRI
Cartilage Imaging
Spinal Cord
Imaging
Functional Lung MRI
MRI Contrast Agents
Simultaneous Growth in Use of
Implanted Medical Devices
Cardiac Rhythm Management
Cochlear hearing implants
Implantable (Automatic)
Cardioversion-Defibrillation
Gastric Simulation
Cardiac Resynchronization
Therapy
Pain Management
Bone Fusion Stimulation
Bladder Control
Neuromodulation
Cardiovascular Stenting
Drug Infusion Pumps
Orthopedic Implants
Plus Many Others
6 •
The Problem
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.
7 •
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.
8 •
Representative MR Images
3-D MR Angiography
9 •
Brain Tumor
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
10 •
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 •
3-D Wire-in-Phantom Heating
Ambient = 25°C
Ambient = 25°C
45°C
Max
30°C
Skin
75°C
Max
Heat Flux vectors showing
conductive transport effect
of the wire.
30°C
Skin
Isothermal plot in phantom
(Passive fixation lead)
Close-up of isotherms
(Active fixation lead)
Substantial MRI-induced heating!
12 •
Our Approach
Tissue heating can be substantially reduced
by increasing the high frequency
(i.e. 64MHz) electrical impedance of the lead
13 •
Simple Model of Bipolar Lead
Circuit Diagram
IPG
Circuit of pacing lead in MRI scanner is not simple…
14 •
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.
15 •
Theory: Air Core Coils
Simplified Impedance Equation
Rd ≡ Distributed
Resistance
Cd ≡ Distributed
Capacitance
Resonance Condition
Rs ≡ Series
Resistance
Cs ≡ Parasitic
Shunt
Capacitance
Maximum coil impedance occurs at “self” resonance.
Source: R.Ludwig, P. Bretchko, RF Circuit Design Theory and Applications, Prentice Hall, 1999
16 •
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
17 •
Experimental Setup
18 •
Results – Modified Wireform
Leads designed with
different inductance
and capacitance.
Changing the wire form
design changes the
Control
Two leads had less than
0.5°C temp. increase.
capacitance-inductance
characteristics of the
lead and its impedance
19 •
Lead Impedance at 64 MHz
Coil Impedance Values at 64 MHz
In Air
Sample
Impedance ()
Zmag ()
Impedance ()
Zmag ()
Control #1 (SJM 1688T)
57 – 93j
109
96 – 67j
117
OEM #1 4-2
210 – 99j
232
178 – 184j
256
OEM #1 4-1
213 – 755j
784
220 – 751j
783
OEM #1 1-2
179 – 539j
568
162 – 533j
557
OEM #1 3-2
240 – 486j
542
208 – 485j
528
OEM #1 3-3
207 – 485j
527
203 – 476j
517
OEM #1 1-1
129 – 518j
534
124 – 518j
533
OEM #2 1-6
223 – 563j
606
215 – 571j
610
57 – 48j
75
120 – 64j
136
OEM #2 3-6
204 – 426j
472
200 – 441j
484
Modified Wire Form
280 – 340j
440
186 – 219j
287
Control #2 (OEM #2)
20 •
In-Situ
Results - Discrete Component Solution
Control #1
(Vendor A)
Control #2
(Vendor B)
6 modified leads had <
1° C temp. increase.
21 •
Leads designed with
different inductance
and capacitance.
Adding a discrete
component, high
frequency resonator to
the lead changes the
capacitance - inductance
characteristics of the
lead and its impedance
MRI-induced Voltages
Induced Voltage ≈
AVL
x
dB1
dt
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.
22 •
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.
23 •
Typical Approach to Risk Management
Training
Increasing Safety
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
It is possible to produce devices
that are inherently safe!
24 •
Biophan Technology Overview
The End
25 •