1. MRI Safety Jerry Allison, Ph.D. Department of Radiology
Medical College of Georgia
Augusta University

2. Projectile Accidents Foreign Materials in the Body Other Stuff
MRI Safety
Projectile Accidents
Foreign Materials in the Body
Other Stuff

3. Projectile Accidents
The MRI magnets are ALWAYS on (24 hours/day, 365 days/year)
There is a STRONG fringe magnetic field around the magnets
The fringe magnetic field is confined to the scan room 3

4. Projectile Accidents
The fringe magnetic field can cause ferrous metal objects to fly into the bore of the magnet with great force and speed
Ferrous metal objects contain iron (steel)
The closer you are to the bore of the magnet, the stronger the fringe field 4

5. As you approach the magnet, the fringe magnetic field gets STRONGER5

6. 6

7. Projectile Accidents The fringe magnetic field can be deadly
Valhalla, N.Y.
Westchester County Medical Center
July 27, 2001
Six year old Michael Colombini was fatally injured “when an oxygen canister was turned into a guided missile by the powerful MRI magnet” 7

8. Foreign Materials in the Body
Foreign material in the body is a concern in MRI
Foreign material is any material in the body other than the flesh and bones that we are born with
Wide variety of foreign material is intentionally and unintentionally present in patients, health care providers and others 8

9. Foreign Materials in the Body
The presence of certain foreign materials in the body can be fatal in an MRI magnet
Deaths have occurred due to the presence of aneurysm clips and cardiac pacemakers in patients undergoing MRI
Sight has been lost by movement of ferrous metal in the eye resulting in vitreous hemorrhage (2.0 mm x 3.5 mm iron 0.35 T)
Prostheses are usually securely anchored to bone and are usually non-ferrous. 9

10. Foreign Materials in the Body
Fringe magnetic fields stronger than 5 Gauss can affect cardiac pacemakers.
To protect patients and others having pacemakers there is a 5 Gauss Exclusion Zone around MRI magnets.
At AU Health, the Exclusion Zones are the MRI scan rooms 10

11. Fringe Diagram of Active Shield 1.5 T*
26’ 4”
5 Gauss
16’ 6”
*Siemens Symphony 11

12. Safety (continued)
-At least two deaths have resulted from MRI Scans on patients with pacemakers. 12

13. Foreign Materials in the Body
Heating of metals and implants has not been significant (even for large prostheses)
There are resources for assessing safety of foreign materials in MRI
F.G. Shellock. Reference manual for Magnetic Resonance Safety: 2017 Edition (© pages; ISBN ) 13

14. Aneurysm Clips 14

15. Signage
FDA Guidance for the Submission Of Premarket Notifications for Magnetic Resonance Diagnostic Devices states:
"The controlled access area should be labeled "Danger - High Magnetic Field" at all entries."
The term "warning" does not convey the importance of a situation that may not only be potentially hazardous, but has been responsible for serious injuries and deaths. 15

16. 16

17. Other Safety Issues Static Magnetic Fields
M R I Physics Course
Other Safety Issues
Static Magnetic Fields
Time Varying Magnetic Fields
RF Magnetic Fields
Auditory Safety
FDA Safety Guidelines
Contrast Agents 17

18. Other Safety Issues Psychological Distress Cryogen Safety
M R I Physics Course
Other Safety Issues
Psychological Distress
Cryogen Safety
Experiences at Higher Magnetic Fields
Pregnancy 18

19. Safety (continued) Biological effects of static magnetic fields
-Augmentation of the T wave during peak flow in the aortic arch
-No evidence of mutagenic effects (in vitro or in vivo)
-No evidence of an effect on organ development in vivo
-No pronounced threshold
-No trends suggestive of a dose response curve 19

20. Safety (continued) Biological effects of static magnetic fields
“In general, there is no conclusive evidence for irreversible or significant biological effects related to acute, short-term exposure of humans to static magnetic field strengths up to 2.0 T.”
MRI systems up to 8T are in use for human imaging. 20

21. Safety (continued) Biological effects of RF magnetic fields
-RF magnetic fields are oscillating magnetic fields
-These fields produce heating of tissue
-Energy deposited (degree of heating) is measured by the Specific Absorption Rate (SAR) in units of Watts/kilogram 21

22. Safety (continued) Biological effects of RF magnetic fields
- SAR depends upon flip angle, TR, tissue density, RF pulse duration and frequency, RF coil radius, tissue conductivity and static magnetic field.
-Guidelines restrict rise in core temperature to <1 oC 22

23. Safety (continued) Biological effects of RF magnetic fields
-Similar to ultrasound since as long as the energy deposited is below established limits, minimal heating of tissues occurs.
-Extra precautions are necessary for patients with compromised thermoregulation: fever, diabetes, obesity, cardiovascular disease, old age, medicated (  and Ca channel blockers), anesthesia 23

24. Safety (continued) Biological effects of RF magnetic fields
-Thermal burns
-Electrical conductors can absorb energy and become extremely hot
-Avoid having any cable wire or lead in contact with the patient
-Surface coil cables can be placed in sleeves to prevent burns 24

25. Safety (continued) -FDA Safety Guidance for MR Devices 25

26. Safety (continued)
Biological effects of time varying (gradient) magnetic fields
-Faraday’s Law: Induction of electric fields and currents in tissues (depends on tissue conductivity, gradient strength, risetime, waveshape, polarity, repetition time TR and duration of the scan)
-Eddy current formation 26

27. Safety (continued)
Biological effects of time varying (gradient) magnetic fields
-Nerve stimulation: electrical stimulation of peripheral and cutaneous nerves should precede any stimulation of ventricular fibrillation. Guidelines for time varying magnetic fields are designed to prevent any perceptible nerve stimulation ( with a safety factor of times three)
-Magnetophosphenes: perceptible light flashes caused by electrical stimulation of the retina (not experienced in imagers having field strengths less than 4.0 T) 27

28. Safety (continued)
Biological effects of time varying (gradient) magnetic fields
“…guidelines are believed to provide a wide margin of safety with respect to exposure to gradient magnetic fields during MRI…newer “fast scanning” techniques that may require more complex applications of gradient magnetic fields may easily exceed these recommended levels and must be thoroughly evaluated for potential health hazards.”
-Time varying magnetic fields are used to stimulate bone healing 28

29. Stimulation Threshold Levels
Mean Cardiac
Stimulation Threshold
Mean Respiratory
Mean Painful Nerve
Mean Peripheral Nerve
3600 T/s
T/s = Tesla per second
T/s = clinical mode
>20 T/s = First or Second
Controlled Modes*
* Limited by IRB or Human Ethical Committee
900 T/s
90 T/s
60 T/s
45 T/s Typical Operating Range
20 T/s 29
0 T/s
MR Safety Guide © 2003 by General Electric Company, Inc.

30. Due to the rapid rate of change of the magnetic fields (dB/dt) used during some EPI scans, a percentage of patients may experience a tingling or touch sensation. This means a percentage of patients may experience Peripheral Nerve Stimulation (PNS) at 45 T/s. If this sensation is bothersome or uncomfortable to the patient, stop the scan. Change to a non-EPI pulse sequence to continue scanning the patient.
MR Safety Guide © 2003 by General Electric Company, Inc. 30

31. Safety (continued) Auditory safety
Activation of gradient magnetic fields produces significant vibrations in the gradient coils.
MRI acoustical noise has been shown to produce reversible hearing impairment and could potentially produce permanent damage.
Hearing protection is recommended for all patients undergoing an MRI procedure on a high-field MRI system (1.5T and 3.0T).
Noise attenuating ear-plugs or head phones are routinely used in MRI 31

32. Safety (continued) -FDA Safety Guidance for MR Devices 32

33. Safety (continued)
-MRI manufacturers often use SAR level control (based on IEC guidance?):
Normal mode (or clinical mode): admissible for all patients
First level: increased SAR permissible under “medical supervision”
Second level: use of higher levels of SAR under IRB approval (research) 33

34. Safety (continued)
-GE SAR levels 34

35. Safety (continued)
-GE dB/dt levels 35

36. Safety (continued) -Contrast agents
-Gd-DTPA is significantly safer than iodine based contrast agents used in X-ray procedures (x 10)
-Severe reactions do occur
-Pregnancy: Gd-DTPA may have poor clearance from amniotic fluid
-Gadolinium may cause Nephrogenic Systemic Fibrosis (NSF) 36

37. Safety (continued) -Contrast agents
FDA Requests Boxed Warning for Contrast Agents Used to Improve MRI
… patients with severe kidney insufficiency who receive gadolinium-based agents are at risk for developing a debilitating, and a potentially fatal disease known as nephrogenic systemic fibrosis (NSF). In addition, it would state that patients just before or just after liver transplantation, or those with chronic liver disease, are also at risk for developing NSF if they are experiencing kidney insufficiency of any severity. 37

38. Safety (continued)
-Psychological Distress (before, during and after MRI)
Claustrophobia
Anxiety
Depression
Panic Disorders
-RF coils need to be near the body for high signal to noise
-Usually transient (2 cases of claustrophobia persisted requiring psychiatric treatment) 38

39. Safety (continued) -Psychological Distress -Countermeasures Briefing
Friend or relative in scan room
Physical or verbal contact with patient
Prone
Mirrors, prism glasses, video goggles
Blindfold
Relaxation strategies
Psychological desensitization techniques 39

40. Cryogen Safety
Superconducting MRI magnets use cryogens (1970 liters of liquid helium)
Magnets can QUENCH resulting in conversion of the liquid helium to gaseous helium
1970 liters of liquid helium becomes 53,000 cubic feet of gaseous helium in about 1 minute 40

41. Cryogen Safety
Magnets are designed to dump the gaseous helium directly outdoors through a quench pipe
If the quench pipe fails, gaseous helium could be dumped into the scan room, displacing the oxygen
Some AU Health MRI scan rooms have oxygen monitors that automatically activate emergency exhaust systems in the event the oxygen level drops below 18% (An Alarm Sounds) 41

42. Cryogen Safety
There has been one fatality by asphyxiation related to a liquid nitrogen leak in MRI 42

43. Safety (continued) -Experiences with higher magnetic fields (4 T)
Several 7 – 8 T systems
Several 4 T systems
Numerous 3T systems
Bioeffects:
magnetophosphenes
vertigo
nausea
headaches
metallic taste 43

44. Pregnancy MRI is not considered to be hazardous to the fetus
Limited studies have examined teratogenic effects
Cells that are reproducing rapidly, as in the first trimester, are susceptible to damage from a variety of physical agents 44

45. Pregnancy
FDA: “the safety of MR imaging when used to image the fetus and infant has not been established”
British National Radiological Protection Board: “It might be prudent to exclude pregnant women during the first three months of pregnancy”
“Particular caution should be exercised with the use of MRI during the first trimester because there is a high spontaneous abortion rate (30%) with associated potential medicolegal implications.”
May be preferred over studies requiring ionizing radiation
Should only be performed if the result is expected to be an important factor in the management of the patient 45

46. ACR Guidance http://onlinelibrary.wiley.com/doi/10.1002/jmri.24011/pdf46

47. AU Health MRI Safety Policy47

48. Vagal Nerve Stimulators
Cyberonics VNS Therapy System
Procedures
Several preparatory steps
Pulse generator to 0 mA
Transmit/receive head coil only
< 2 Tesla
SAR: <1.3 for a 70kg patient
Time-varying intensity: < 10 Tesla/sec 48

49. Medtronic Deep Brain Stimulators
Procedures
Several preparatory steps
Pulse generator to 0 mA
Transmit/receive head coil only
Only 1.5 Tesla systems
Use correct patient weight
SAR: <0.1
Gradient dB/dT: < 20 Tesla/sec 49

50. Projectile Accidents Foreign Materials in the Body Other Stuff
MRI Safety
Projectile Accidents
Foreign Materials in the Body
Other Stuff