Electrical eddy currents in the human body: MRI scans and medical implants

1. Electrical eddy currents in the human body: MRI scans and medical implants Brent Hoffmeister Rhodes College Department of Physics

2. Research interests and collaborators • Ultrasonic bone assessment • Dr. Sue Kaste (St. Jude), Dr. Kendall Waters (NIST) • Students: Andy Whitten, Julie Javarone, Chad Jones, Garney Caldwell, Jeff France, John Janeski, David Johnson • Ultrasound therapy for osteoarthritis • Dr. Karen Hasty (U. Tennessee) • Ultrasonic imaging of cardiac electrical stimulation • Dr. Bob Malkin (Duke), Amy Curry (U. Memphis) • Students: Steve Smith, Will McKinney, Stu Johnston, Erin Sylvester, John Sexton, Chip Hartigan, Taylor Whaley • Magnetic Stimulation • Dr. Bob Malkin (Duke) • Student: Drew Shores

3. Magnetic stimulation • Problem • MRI scans expose patients to time varying magnetic fields • Faraday’s Law - time varying magnetic fields induce electrical “eddy” currents • Induced currents can cause muscle contraction, nerve stimulation, magnetophosphenes, cardiac arrhythmias • Effect of medical implants unknown • Goal • Model how implants affect magnetically induced currents in the body

4. MRI Y gradient coil Z gradient coil Transceiver X gradient coil Main coil Patient

5. MRI Magnetic Fields

6. Particularly interesting implant

7. Pacemakers and MRI • Concerns about sources of electromagnetic interference in patients with pacemakers. • Safe performance of magnetic resonance imaging on five patients with permanent cardiac pacemakers. • Loss prevention case of the month: not my responsibility! • Interference with cardiac pacemakers by magnetic resonance imaging: are there irreversible changes at 0.5 tesla? • MR imaging and cardiac pacemakers: in vitro evaluation and in vivo studies in 51 patients at 0.5 T. • Magnetic resonance imaging of the brain at 1.5 tesla in patients with cardiac pacemakers: can it be done? • MRI in patients with cardiac pacemakers: in vitro and in vivo evaluation at 0.5 tesla. • Magnetic resonance imaging and cardiac pacemaker safety at 1.5-Tesla • In vivo heating of pacemaker leads during magnetic resonance imaging.

8. Research question • Most of the research has focused on problems associated with RF fields. • Less research on switched gradient fields, and even less on interaction with implants. • Will metal and plastic objects in the chest increase the danger of switched gradient field stimulation? • The physics…

9. Faraday’s Law

10. Induced electric field Increasing uniform B Conducting cylinder (patient’s body)

11. Eddy currents Electric field Conductivity Current density

12. What’s going to happen? Metal/plastic object

13. Metal or plastic? • Both interesting • Plastics not studied • (plastics easier too)

14. Approach • Develop an analytical model based on Faraday’s law to predict current densities in the vicinity of a plastic implant. • Compare model to experiment and finite element analysis.

15. Experimental model Helmholtz coils (MRI Z-coils) 60 Hz AC Dish of physiologic saline (patient)

16. Apparatus

17. Saline dish Helmholtz coils Voltmeter Shielded cable to amplifier High impedance amplifier Field probe Probe tips Twisted insulated wires 1.00 cm Measurement system

18. “Implant” geometry? • Pick something easy to start with. • Ideas? Saline dish ?

19. Echg Effect of plastic “implant” - - - + + + Emag

20. y - - - + + + P x Analytic model - approach • Find charge density of accumulated charge • Use Coulomb’s law to find Echg • Enet = Emag - Echg

21. Saline conductivity Normal component At interface Free and bound charge Conservation of charge Surface charge density

22. Bound charge Electric susceptibility of water

23. Differential equation… …and steady state solution

24. 80 1.2 S/m 2p 60 Hz Simplifying assumption

25. y P x Coulomb’s Law z y 2W Plastic x 2L

26. Echg

27. Done! Compare to experiment…

28. Not done… Experiment

29. Saline-air interface plastic + + + + + + + + + Saline-dish bottom interface Side view + + + + + + + What’s wrong? • Math OK • Approximations OK • Input parameters OK

30. A fix • Use a deep beaker instead of a dish. Suspended from fishing line These interfaces far away from measurement region plastic

31. Theory and experiment Beaker of Saline

32. Another fix • Let W go to infinity in the model 2W Plastic 2L

33. Theory and experiment Dish of Saline

34. y L2 L1 P f x Generalizing the geometry where

35. Theory and experiment f = 90 deg f = 45 deg f = 15 deg

36. What we know so far • Plastics can significantly alter eddy current patterns • Basic effect: eddy currents are forced to flow around the plastic • Effect can be understood as result of charge accumulation at interfaces

37. Clinical significance • Plastics might redirect eddy currents toward sensitive tissues Heart Torso

38. Future work • Metal implants • More realistic geometries • More realistic B(t)