- 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

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sin

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afet

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It is possible to produce devices that are inherently safe!

25 •

Biophan Technology Overview

The End