1. MRI Bioeffects and Safety Dr. Peterson

2. History and Background Bloch (Stanford) & Purcell (Harvard) - 1946 Nobel Prize - 1952 - Discovery of Spectroscopy Raymond Damadian – Rat Tumor – 1974 – Cover of Science Paul Lauterbur – Water image - 1973 Edinburgh Group/Peter Mansfield

3. New Diagnostic Modality Compared to x-rays (1895!) More Comparable to Ultrasound (1950’s) Need to understand physics for safety

4. Basic Magnetic Field Concepts Units of Field Strength Magnetic Domains Static Fields (Main Magnet) Time-Variant Fields (Gradient Magnets)

5. Units of Field Strength The Gauss (a.k.a. The Oersted) – Measures the Earth's Field (~0.5 G) The Tesla – Measures the MRI Field The Relationship: 1T = 10,000 G – Also used: 1 mT = 10 G – Also used: 1 kG = 0.1 T

6. Magnetic Domains Example: Hemoglobin – Iron atoms

7. The Static Field and the Environment Effects of the Environment on the Static Field Effects of the Static Field on the Environment

8. Effects of the Environment on the Static Field Why do we care? – Effects on the image Shimming to Compensate

9. Effects of the Environment on the Static Field

10. Effects of the Static Field on the Environment Why do we care? – Credit cards, computer disks, CRTs. Shielding to Protect

11. Effects of the Static Field on the Environment

12. Bioeffects of Static Magnetic Fields Cell Functions Growth and healing of bone Thrombolysis Nerve Function Cardiovascular Effects Temperature Magnetophosphenes

13. Cell Functions Rats had the permeability of the blood-brain barrier modified after 23 minutes at 0.15 T

14. Bone Growth and Healing Bone Growth and Healing There is much controversy on the effects of magnetic fields on the growth and healing of bone. No conclusions yet.

15. Thrombolysis Indeterminate experiments on rats and guinea pigs. Can’t tell.

16. Nerve Function Rats showed strong avoidance behavior to magnetic fields. Neuro function tests on humans before and after MR at fields up to 2T show that the bioelectric properties of neurons are not affected. Fields of 4T resulted in nausea, vertigo and metallic taste, indicating neural involvement.

17. Cardiovascular Effects Small increases in BP have been reported. Also a slight leukopenia.

18. Magnetohydrodynamic Effect Blood is a conductor flowing through the magnetic field. This usually results in an increase in T- wave amplitude.

19. Temperature Reports go both ways; the best studies say static fields don’t affect body temperature. Don’t confuse this with RF effects.

20. Magnetophosphenes Flashes of Light in the Visual Field – None at 2T – Reported at 4T

21. Static Field Bioeffects Summary Whole body: No effects at 2T Circulatory at 5T Extremities: No effects at 5T Discomfort; no adverse effects: 10T Conclusion: Don’t go above 2T for head & trunk; 5T for extremities

22. Bioeffects of Time-Variant Fields (Gradients) Mechanism Nerve/Muscle Stimulation Nerves and EPI Magnetophosphenes Auditory Considerations Other Biological Effects

23. Gradient Bioeffects Mechanism Changing magnetic field over time (dB/dt) induces voltage and current in conductors. Example: Peak gradient of 1 G/cm, rise time = 500  sec 30 cm from the isocenter. This is 2,000 G/sec  30 cm, or 60,000 G/sec, or 6 T/sec. The induced current is 1  A/cm 2 for this gradient. Biological effects usually occur beyond 3  A/cm 2

24. Nerve/Muscle Stimulation Example gradient induced a current of 1  A/cm 2. Need 15 to 100  A/cm 2 for tetanic contractions of breathing muscles. Need 0.2 to 1.0  A/cm 2 to produce fibrillation.

25. Nerves and EPI Twitching occurs beyond 60 T/sec

26. Magnetophosphenes None at 17  A/cm 2 in 1.95 T systems. Lots at 4 T and any gradient.

27. Auditory Considerations. Discussed later under Auditory Effects.

28. Other Biological Effects. Reduced effects of mannitol, fentanyl and morphine-induced analgesia. No effect on amphibian embryogenesis, murine cell cytotoxicity, and litter number or growth rates of pregnant mice.

29. Gradient Magnet Bioeffects Conclusions No effects at 6 T/sec. Nerve stimulation at 20 T/sec. Don’t go above 20 T/sec. Keep an eye on EPI in the future.

30. Quenches The Problem with the Field – Faraday’s Law The Oxford Pig Conclusions

31. The Problem with the Field Faraday’s Law: – When a conductor moves in a magnetic field, a voltage is induced in the conductor. It will also happen if the conductor is stationary and the magnetic field moves.

32. The Oxford Pig A pig was placed in the bore. 1.6T magnet quenched at 1.76 T Results: The pig was startled.

33. Quench Conclusions Quenches have happened several times with patients in the bore – Once it happened with the Chief Tech in the bore (in Queensland, Australia)! No deleterious effects have been reported, except for startle reflex.

34. Radiofrequency Bioeffects RF Defined Bioeffects Temperature-sensitive organs Quantifying Bioeffects Limits

35. RF Defined Non-ionizing electromagnetic radiation Frequency range: DC to 3,000 GHz Includes radar, TV, AM and FM radio

36. The Electromagnetic Spectrum

37. Bioeffects Include visual, auditory, endocrine, cardio- vascular, immune system, reproductive, and developmental functions. Simplified: It heats the tissues. Cardiac output & blood flow increase Sweat secretion & evaporation increase

38. Temperature-sensitive organs The testes The eyes Limits: 10 W/kg for more than 10 minutes

39. Quantify bioeffects using: SAR: Specific Absorption Rate – Rate at which RF energy is coupled into tissues. – Units: Watts per kilogram (W/kg) SAR - Time Product – Units: W-min/kg or W-hr/kg Temperature (°C)

40. FDA SAR Limits

41. Body average of 0.4 W/kg Peak for any gram of tissue of 8.0 W/kg Head average of 3.2 W/kg

42. FDA Temperature Limits A body core rise of less than 1  C No more than 38  C in the head No more than 39  C in the trunk No more than 40  C in the extremities Exceptions: Patients with compromised heat regulation

43. FDA Temperature Limits

44. Pregnancy

45. Bioeffects of the Static, Gradient and RF Fields Not enough is known for specific recommendations. See the following study of Health Care Workers and it’s Conclusions.

46. Health Care Workers A survey of 2,000 female MR operators reported 1421 pregnancies; 280 were MR techs or nurses; 894 worked at another job, 54 were students, 193 were homemakers. There were no statistically significant differences between MR operator pregnancies and the group before it worked in MR. Spontaneous abortion rate, pre-term delivery rate, low birth weight, infertility and offspring gender were analyzed. Adjustments for maternal age, smoking, and alcohol use didn’t change the results.

47. Pregnancy Conclusions “MR imaging may be used in pregnant women if other non- ionizing forms of diagnostic imaging are inadequate or if the examination provides important information that would otherwise require exposure to ionizing radiation (e.g., fluoroscopy, CT, etc.). It is recommended that pregnant patients be informed that, to date, there has been no indication that the use of clinical MR during pregnancy has produced deleterious effects. However, as noted by the FDA, the safety of MR during pregnancy has not been proved.” -SMRI Safety Committee

48. Bioeffects of Contrast Media Gadolinium is toxic but is caged by a chelate (DTPA). The chelate may be dissolved by the kidneys, releasing the gadolinium. Some IV contrast agents cross the placenta, are swallowed by the fetus, filtered and excreted by the kidneys as urine, swallowed; and the cycle then repeats many times. The end result of this cyclic process is not known.

49. Cryogenics Necessary for superconductivity – Niobium-titanium metal used for wires – Becomes superconducting at 10 kelvins Liquid helium boils at 4.2 kelvins Liquid nitrogen boils at 77 kelvins This classroom is at 295 kelvins

50. Cryogens LiquidBoils at: ======================= Air86 k -187° C-305° F Nitrogen77 k -196° C-321° F Helium4.2 k -269° C-452° F

51. Cryogens Helium is lighter than air Nitrogen is heavier than air They will drive the air out of the magnet room if released into it Both are incredibly cold and will burn flesh on contact

52. Auditory Effects Gradient magnet sounds – Protection or Distraction Ear plugs/Headsets Noise cancellation devices RF sounds – Clicks, buzzes, chirps or knocks – Seems to come from the back of the head – Usually masked by gradient noise Cryogen release sounds (think hurricane!)

53. Laser Positioning Systems Possibility of eye damage None has ever been reported Most manufacturers now use incandescent lamps instead of lasers

54. Bibliography and Links This information changes rapidly. See your handout for this class for the latest information.