MRI Safety. Jerry Allison, Ph.D.

Transcription

1 MRI Safety Jerry Allison, Ph.D.

2 Caveat This lecture emphasizes MRI safety for high field (1.5T and 3.0T) superconducting magnets such as those used at MCGHI.

3 MRI Safety Projectile Accidents Foreign Materials in the Body Other Stuff

4 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 4

5 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 5

6 As you approach the magnet, the fringe magnetic field gets STRONGER 6

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11 My Favorite Horror Story Here's one I heard from an Oxford Magnets engineer which was later independently verified by a technologist who had been hired to work at this site. A brand new magnet had just finished being installed into a brand new building. All of the acceptance testing had been completed and the magnet was to be turned over to the customer the very next day. There was only one minor problem to be dealt with first. One of the sprinklers in the scan room had a tiny leak. A welder was brought in to fix the leak, but somebody forgot to tell him that the magnet was at field. So... in walks this welder with his acetylene torch system. His tank flies into the magnet, the valve breaks off, sparks and catches fire. Since he was in there to fix a leak in the sprinkler system, it had been turned off first. The brand new building burned to the ground! 11

12 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 12

13 Fatal Accident Details Michael Colombini was in the bore of a GE Signa An anesthesiologist was in the scan room with the patient The built-in oxygen delivery system failed A nurse heard the anesthesiologist calling for oxygen 13

14 Fatal Accident Details The nurse delivered a STEEL oxygen cylinder to the anesthesiologist thinking it was safe (ALUMINUM) Within 3 to 6 feet of the magnet, the STEEL cylinder flew into the bore, fatally injuring the patient 14

15 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 15

16 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. 16

17 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 MCGHI, the Exclusion Zones are the MRI scan rooms 17

18 Fringe Diagram of Active Shield 1.5 T* 5 Gauss *Siemens Symphony 18

19 Safety (continued) -At least two deaths have resulted from MRI Scans on patients with pacemakers. 19

20 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: 2003 Edition Salt Lake City: Amirsys Inc

21 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. 21

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23 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 23

24 M R I Physics Course Other Safety Issues Psychological Distress Cryogen Safety Experiences at Higher Magnetic Fields Pregnancy 24

25 Safety Why worry about biological and safety issues? -The patient is exposed to: Static magnetic field B0: B < 3.0 T Time varying magnetic fields (gradients) RF magnetic fields (B1) Acoustical noise Psychological distress (claustrophobia) Contrast 25

26 Safety (continued) -The patient is also exposed to: Metal implant accident (foreign materials in the body) Cryogen accident Projectile acident 26

27 Safety (continued) The technologist, health care providers and other individuals are exposed to: Static magnetic fields: few Gauss to 3.0 T Cryogen accident Projectile accident 27

28 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 28

29 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. 29

30 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 30

31 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) 31

32 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 32

33 Mean Cardiac Stimulation Threshold Mean Respiratory Stimulation Threshold Mean Painful Nerve Stimulation Threshold Mean Peripheral Nerve Stimulation Threshold Stimulation Threshold Levels 3600 T/s 900 T/s 90 T/s 60 T/s T/S = Tesla per second 0-20 T/s = clinical mode >20 T/s = First or Second Controlled Modes* * Limited by IRB or Human Ethical Committee 45 T/s Typical Operating Range 20 T/s 0 T/s MR Safety Guide 2003 by General Electric Company, Inc. 33

34 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. 34

35 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 35

36 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 o C FDA: 0.4 W/kg whole body 3.2 W/kg head 8.0 W/kg peak 36

37 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 ( β and Ca channel blockers), anesthesia 37

38 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 38

39 Auditory safety Safety (continued) 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 39

40 Safety (continued) -FDA Safety Guidelines for MR Devices -Acoustic noise level International standard: 140 db relative to 20 mpa 40

41 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 41

42 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) 42

43 -Psychological Distress Safety (continued) -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 43

44 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 44

45 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 MCGHI MRI scan rooms have oxygen monitors that automatically activate emergency exhaust systems in the event the oxygen level drops below 18% (An Alarm Sounds) 45

46 Cryogen Safety There has been one fatality by asphyxiation related to a liquid nitrogen leak in MRI 46

47 Safety (continued) -Experiences with higher magnetic fields (4 T) A few 7T systems Several 4 T systems Numerous 3T systems Bioeffects: magnetophosphenes vertigo nausea headaches metallic taste 47

48 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 48

49 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 49

50 Pregnancy Can MRI Adversely Affect a Fetus? Emanuel Kanal, M.D. University of Pittsburgh Director, Pittsburgh NMR Institute women surveyed: 823 had at least 1 pregnancy (not necessarily during MRI employment), data on 1421 pregnancies Miscarriage Premature delivery Infertility Low birth weight Offspring gender 50

51 Pregnancy Can MRI Adversely Affect a Fetus? -Compared to a similar group of pregnancies in women that had not worked in MRI -Smoking, alcohol usage and age were considered -No statistically significant differences in the five areas studied RT Image; January 6, 1992; Vol. 5, No. 1 51