- MRI Safety Update - RF Induced Heating. Society for Medical Innovation and Technology

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1 - MRI Safety Update - RF Induced Heating presented to Society for Medical Innovation and Technology May 2006 Pebble 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 2

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) 3

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 4

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

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

7 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

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. 8

9 Representative MR Images 3-D MR Angiography Brain Tumor 9

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 10

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! 11

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

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

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

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. 15

Theory: Air Core Coils Simplified Impedance Equation R d Distributed Resistance C d Distributed Capacitance R s Series Resistance C s Parasitic Shunt Capacitance

16 Theory: Air Core Coils Simplified Impedance Equation R d Distributed Resistance C d Distributed Capacitance R s Series Resistance C s Parasitic Shunt Capacitance Resonance Condition Maximum coil impedance occurs at self resonance. Source: R.Ludwig, P. Bretchko, RF Circuit Design Theory and Applications, Prentice Hall,

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 x ) as well as alternate (smaller) packaging designs 17

18 18 Experimental Setup

19 Results Modified Wireform 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 19

20 Lead Impedance at 64 MHz Sample Control #1 (SJM 1688T) OEM #1 4-2 OEM #1 4-1 OEM #1 1-2 OEM #1 3-2 OEM #1 3-3 OEM #1 1-1 OEM #2 1-6 Control #2 (OEM #2) OEM #2 3-6 Modified Wire Form Coil Impedance Values at 64 MHz In Air In-Situ Impedance (Ω) Z mag (Ω) Impedance (Ω) 57 93j j j j j j j j j j j j j j j j 57 48j j j j j j Z mag (Ω)

21 Results - Discrete Component Solution 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 21

22 MRI-induced Voltages Where; Induced Voltage A VL x db 1 dt A VL = Area of the virtual loop formed by the device, lead, and interconnecting tissue db 1 /dt = Rate of change of applied magnetic field Biophan has measured 1 induced voltages of ~ 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 =

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. 23

24 Typical Approach to Risk Management Increasing Safety 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 It is possible to produce devices that are inherently safe! 24

25 Biophan Technology Overview The End 25

26 26

27 Biophan Technology Overview Position of SFF Temperature Probe 27

28 Biophan Technology Overview - 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 28

29 Biophan Technology Overview Direct Measurement of Lead Impedance at 64 MHz Basic Parameters Testing (R-L and C-G) was conducted using an HP 4285A LCR Meter at 100 KHz. An HP 16095A Probe Fixture was used to connect the meter to the Pacing Lead Test Fixture. An HP 8752A Network Analyzer was used to conduct the s11 swept frequency response tests. Data was taken over the frequency range from 0.3 to 100 MHz. Each sweep contained 401 data points (return loss values) and was stored as a file in ASCII format. The impedance at 64 MHz of each pacing lead response was recorded using the Smith Chart display mode. Swept responses were made both in air and in fluid. The IPG was attached to the far end for all swept responses. 29

30 Biophan Technology Overview Tuned Resonance Inductance = L Resonating Stent Concept 1 ω 0 LC ω 0 = Resonance frequency (63.87 MHz at 1.5 Tesla) Capacitance = C Resonator 30

31 Biophan Technology Overview Biophan Resonator Technology - continued Wireless resonator technology B Resonator coil G MR-signal B G - Additional Benefits - No risk of heating or induced voltages! No connection to the MRI unit is required Does not change MRI product classification 31

32 Biophan Technology Overview Biophan Resonator Technology Prototypes - Applications - MR guided catheterization MR tiptracking MR endovascular imaging Thin-film versions of the wireless coil are also being developed 32

33 Biophan MRI Safety Overview Abstract The substantial benefits of magnetic resonance imaging are often denied to patients known to have implanted medical devices such as pacemakers and neurostimulators. Other patients are put at potential risk when they undergo a MRI procedure, even though specific informed consent is required regarding the possible MRI interactions with the implanted device. The medical community is currently divided over the actual extent of the MRI safety problem. In this report, insight is provided into the wide array of results achieved by many researchers, as well as several options for producing medical devices that are inherently safe under worst case MRI conditions. As the problem is very complex and the variety of implants is large, this paper focuses on the problems of MRI-induced lead heating. 33

Testing Methods and Standards for Magnetic Resonance (MR) Safety and Compatibility of Medical Devices - Update

Testing Methods and Standards for Magnetic Resonance (MR) Safety and Compatibility of Medical Devices - Update Testing Services for Magnetic Resonance Imaging Therapy Testing Methods Stards for Magnetic Resonance (MR) Safety Compatibility of Medical Devices - Update G. Schaefers MR:comp GmbH MR Testing Laboratory