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- 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 •