Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.