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 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 either not recognized or poorly addressed by existing testing methods (i.e. ASTM 2182)

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! Introduction - continued

To examine the effects of lead design on MRI-induced heating To utilize these insights to develop inherently safe lead designs - Purpose of this Investigation -

- 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

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 - Materials and Methods - at

Experimental Setup

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. R d ≡ Distributed Resistance C d ≡ Distributed Capacitance R s ≡ Series Resistance C s ≡ Parasitic Shunt Capacitance

Theory: Shifting Self Resonance Of Lead Maximum impedance at “self” resonance. MR scanner’s frequency is fixed. So, need to shift lead’s self-resonance frequency by changing coil inductance and capacitance properties.

Simple Model Of Bipolar Lead Circuit Diagram Circuit of pacing lead in MRI scanner is not simple… IPG

MRI Heating of Pacing Leads Leads designed with different inductance and capacitance. - Results - 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

MRI Heating of Pacing Leads - Results - 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 - 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 - 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, guidewires, catheters, etc. Biophan has also developed easy to implement solutions for reducing or eliminating MRI-induced voltages in leads