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- MRI Safety Update - RF Induced Heating

- 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.

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- MRI Safety Update - RF Induced Heating

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  1. - MRI Safety Update -RF Induced Heating presented to Society for Medical Innovation and Technology11-14 May 2006Pebble Beach, Monterey, CA, USA 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. 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

  5. Diffusion – Perfusion MRI MRI of Cancer Hematobiliary MRI Cardiovascular Imagingc Molecular Imaging Whole Body MRI Cellular Imaging Advanced Brain MRI Myocardial Functional Imaging Imaging of the Mother & Fetus Pediatric Brain MRI Degenerative Disease MRI Musculoskeletal Imaging Interventional MRI Quantitative Neuro MRI Multi-modal Functional MRI Cartilage Imaging MRI Angiography Spinal Cord Imaging Functional Breast Imaging Psychiatric MRS-I Functional Lung MRI Flow and Motion Quantitation MR Spectroscopy of the Brain MRI Contrast Agents Plus + 88 additional topics Evidence of Growth in MRI ISMRM 14th Scientific Meeting 6-12 May 2006

  6. Cardiac Rhythm Management Cochlear hearing implants Implantable (Automatic) Cardioversion-Defibrillation Gastric Simulation Pain Management Cardiac Resynchronization Therapy Bone Fusion Stimulation Bladder Control Neuromodulation Drug Infusion Pumps Cardiovascular Stenting Orthopedic Implants Simultaneous Growth in Use of Implanted Medical Devices Plus Many Others

  7. The Problem Implanted medical devices can create risks to their patients when exposed to MRI • Excessive heating of the device (multiple causes) capable of producing uncontrolled tissue heating and thermogenic damage. • 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.

  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. Representative MR Images Brain Tumor 3-D MR Angiography

  10. 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

  11. 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!

  12. 45°C Max 3-D Wire-in-Phantom Heating Heat Flux vectors showing conductive transport effect of the wire. Ambient = 25°C Ambient = 25°C 75°C Max 30°C Skin 30°C Skin Isothermal plot in phantom (Passive fixation lead) Close-up of isotherms (Active fixation lead) Substantial MRI-induced heating!

  13. Our Approach Tissue heating can be substantially reduced by increasing the high frequency (i.e. 64MHz) electrical impedance of the lead

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

  15. 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.

  16. Theory: Air Core Coils Simplified Impedance Equation Rd ≡ Distributed Resistance Cd ≡ Distributed Capacitance Resonance Condition Cs ≡ Parasitic Shunt Capacitance Rs ≡ Series Resistance Maximum coil impedance occurs at “self” resonance. Source: R.Ludwig, P. Bretchko, RF Circuit Design Theory and Applications, Prentice Hall, 1999

  17. 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

  18. Experimental Setup

  19. Results – Modified Wireform Leads designed with different inductance and capacitance. Changing the wire form design changes the capacitance-inductance characteristics of the lead and its impedance Two leads had less than 0.5°C temp. increase. Control

  20. Coil Impedance Values at 64 MHz In Air In-Situ 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 Control #2 (OEM #2) 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 Lead Impedance at 64 MHz

  21. Results - Discrete Component Solution Control #1 (Vendor A) Leads designed with different inductance and capacitance. Control #2 (Vendor B) Adding a discrete component, high frequency resonator to the lead changes the capacitance - inductance characteristics of the lead and its impedance 6 modified leads had < 1° C temp. increase.

  22. dB1 Induced Voltage ≈ AVL x dt MRI-induced Voltages 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.

  23. 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.

  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 Increasing Safety It is possible to produce devices that are inherently safe!

  25. Biophan Technology Overview The End

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