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MRI Bioeffects and Safety

MRI Bioeffects and Safety. Dr. Peterson. 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.

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MRI Bioeffects and Safety

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  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 • 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/cm2 for this gradient. • Biological effects usually occur beyond 3 A/cm2

  24. Nerve/Muscle Stimulation • Example gradient induced a current of 1 A/cm2. • Need 15 to 100 A/cm2 for tetanic contractions of breathing muscles. • Need 0.2 to 1.0 A/cm2 to produce fibrillation.

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

  26. Magnetophosphenes • None at 17 A/cm2 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. FDA SAR Limits • 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 • Liquid Boils at: • ====== ================= • Air 86 k -187° C -305° F • Nitrogen 77 k -196° C -321° F • Helium 4.2 k -269° C -452° F

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