Transcript Effect of Position in MRI Tube on Heating

Biophan Technologies, Inc.
Jeffrey L. Helfer, Vice President of Engineering
ISMRM Workshop on MRI Safety
November 6, 2005
- Acknowledgements Robert Gray
Xingwu Wang, Ph.D.
W. Timothy Bibens
Mark Bocko, Ph.D.
Stuart G. MacDonald
Jeffrey L. Helfer
University Medical Imaging, Rochester, New York
- Introduction Pacemakers and other 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 the visualization of
tissues “close” to the device.
Introduction - continued
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 either not recognized or poorly addressed
by existing testing methods (i.e. ASTM 2182)
Introduction - 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!
- Purpose of this Investigation • To examine the effects of lead design on
MRI-induced heating
• To utilize these insights to develop inherently safe
lead designs
- Key Assumptions MRI energy is coupled into conductive leads in two
major ways:
- Antennae effect
- Electrical potential induced within the body
(Implant acts as an electrical “short circuit”)
High electrical current densities at the lead-tissue
interface induces resistive heating in tissue
Coiled Lead Wires
- Hypothesis Tissue heating can be substantially reduced by
increasing the high frequency (i.e. 64MHz)
electrical impedance of the lead
at
- Materials and Methods • Proprietary design bipolar pacing lead prototypes,
52 cm in length. Connected to IPG.
• Standard active fixation bipolar pacing lead, 52 cm in
length. Connected to IPG. (Control)
• Luxtron® fluoroptic thermometry system
• Head/torso phantom
• Gelled-saline solution: 5.8 g PAA, 0.8g NaCl per liter
of de-ionized water
• GE 1.5-T MR system (GE), FSE-XL, Whole body avg.
SAR: 1.79 W/kg
Experimental
Setup
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
Theory: Shifting Self Resonance Of Lead
MR scanner’s frequency
is fixed. So, need to
shift lead’s self-resonance
frequency by changing
coil inductance and
capacitance properties.
Maximum impedance at “self” resonance.
Simple Model Of Bipolar Lead Circuit Diagram
Circuit of pacing lead in MRI scanner is not simple…
IPG
- Results MRI Heating of Pacing Leads
Leads designed with
different inductance
and capacitance.
Control
Two leads had less than
0.5°C temp. increase.
Changing the wire form
design changes the
capacitance-inductance
characteristics of the
lead and its impedance
- Results MRI Heating of Pacing Leads
Leads designed with
different inductance
and capacitance.
Control #1
(Vendor A)
Control #2
(Vendor B)
6 modified leads had <
1° C temp. increase.
Adding a discrete
component, high
frequency resonator to
the lead changes the
capacitance - inductance
characteristics of the lead
and its impedance
- Conclusions Lead design geometry has a strong influence on worst case
MRI-induced heating at 1.5T
Worst-case lead heating can be reduced to acceptable
levels in several ways:
● Proper choice of wire form design geometry
● Use of discrete component resonator
- Commentary 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,
guidewires, catheters, etc.