mri safety - Čvut, fjfi, katedra fyzikální...

MRI SafetyMRI SafetyJerry Allison, Ph.D.Jerry Allison, Ph.D.

CaveatCaveat

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

MRI SafetyMRI Safety

Projectile AccidentsProjectile Accidents

Foreign Materials in the BodyForeign Materials in the BodyOther StuffOther Stuff


• The MRI magnets are ALWAYS on (24 The MRI magnets are ALWAYS on (24 hours/day, 365 days/year)hours/day, 365 days/year)

• There is a STRONG fringe magnetic field There is a STRONG fringe magnetic field around the magnetsaround the magnets

• The fringe magnetic field is confined to the The fringe magnetic field is confined to the scan roomscan room

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• The fringe magnetic field can cause ferrous The fringe magnetic field can cause ferrous metal objects to fly into the bore of the metal objects to fly into the bore of the magnet with great force and speedmagnet with great force and speed

• Ferrous metal objects contain iron (steel)Ferrous metal objects contain iron (steel)• The closer you are to the bore of the The closer you are to the bore of the

magnet, the stronger the fringe fieldmagnet, the stronger the fringe field

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

STRONGERSTRONGER

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http://simplyphysics.com/flying_objects.html#http://simplyphysics.com/flying_objects.html#

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My Favorite Horror StoryMy Favorite Horror Story

Here's one I heard from an Oxford Magnets engineer which was later independently 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.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. 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 All of the acceptance testing had been completed and the magnet was to be turned over to the customer the very next day. over to the customer the very next day.

There was only one minor problem to be dealt with first.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 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. 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 So..... in walks this welder with his acetylene torch system. His tank flies into the magnet, the valve breaks off, sparks and catches fire. 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.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!The brand new building burned to the ground!



• The fringe magnetic field can be deadlyThe fringe magnetic field can be deadly• Valhalla, N.Y.Valhalla, N.Y.• Westchester County Medical CenterWestchester County Medical Center• July 27, 2001July 27, 2001• Six year old Michael Colombini was fatally Six year old Michael Colombini was fatally

injured “when an oxygen canister was injured “when an oxygen canister was turned into a guided missile by the powerful turned into a guided missile by the powerful MRI magnet”MRI magnet”

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Fatal Accident DetailsFatal Accident Details

• Michael Colombini was in the bore of a GE Michael Colombini was in the bore of a GE SignaSigna

• An anesthesiologist was in the scan room An anesthesiologist was in the scan room with the patientwith the patient

• The built-in oxygen delivery system failedThe built-in oxygen delivery system failed• A nurse heard the anesthesiologist calling A nurse heard the anesthesiologist calling

for oxygenfor oxygen

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Fatal Accident DetailsFatal Accident Details

• The nurse delivered a STEEL oxygen The nurse delivered a STEEL oxygen cylinder to the anesthesiologist thinking it cylinder to the anesthesiologist thinking it was safe (ALUMINUM)was safe (ALUMINUM)

• Within 3 to 6 feet of the magnet, the STEEL Within 3 to 6 feet of the magnet, the STEEL cylinder flew into the bore, fatally injuring cylinder flew into the bore, fatally injuring the patientthe patient

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Foreign Materials in the BodyForeign Materials in the Body

• Foreign material in the body is a concern in Foreign material in the body is a concern in MRIMRI

• Foreign material is any material in the body Foreign material is any material in the body other than the flesh and bones that we are other than the flesh and bones that we are born withborn with

• Wide variety of foreign material is Wide variety of foreign material is intentionally and unintentionally present in intentionally and unintentionally present in patients, health care providers and otherspatients, health care providers and others

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• The presence of certain foreign materials in the The presence of certain foreign materials in the body can be fatal in an MRI magnetbody can be fatal in an MRI magnet

• Deaths have occurred due to the presence of Deaths have occurred due to the presence of aneurysm clips and cardiac pacemakers in patients aneurysm clips and cardiac pacemakers in patients undergoing MRIundergoing MRI

• Sight has been lost by movement of ferrous metal Sight has been lost by movement of ferrous metal in the eye resulting in vitreous hemorrhage (2.0 in the eye resulting in vitreous hemorrhage (2.0 mm x 3.5 mm iron fragment @ 0.35 T)mm x 3.5 mm iron fragment @ 0.35 T)

• Prostheses are usually securely anchored to bone Prostheses are usually securely anchored to bone and are usually non-ferrous.and are usually non-ferrous.

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• Fringe magnetic fields stronger than 5 Fringe magnetic fields stronger than 5 Gauss can affect cardiac pacemakers.Gauss can affect cardiac pacemakers.

• To protect patients and others having To protect patients and others having pacemakers there is a 5 Gauss Exclusion pacemakers there is a 5 Gauss Exclusion Zone around MRI magnets. Zone around MRI magnets.

• At MCGHI, the Exclusion Zones are the At MCGHI, the Exclusion Zones are the MRI scan roomsMRI scan rooms

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Fringe Diagram of Active ShieldFringe Diagram of Active Shield1.5 T*1.5 T*

*Siemens Symphony*Siemens Symphony 18

16’ 6”16’ 6”

26’ 4”26’ 4”5 Gauss5 Gauss

Safety Safety (continued)(continued)

-At least two deaths have resulted from MRI Scans -At least two deaths have resulted from MRI Scans on patients with pacemakers.on patients with pacemakers.

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• Heating of metals and implants has not been Heating of metals and implants has not been significant (even for large prostheses)significant (even for large prostheses)

• There are resources for assessing safety of foreign There are resources for assessing safety of foreign materials in MRImaterials in MRI

• F.G. Shellock. F.G. Shellock. Reference manual for Magnetic Reference manual for Magnetic Resonance Safety: 2003 Edition Resonance Safety: 2003 Edition Salt Lake City: Salt Lake City: Amirsys Inc. 2003.Amirsys Inc. 2003.

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SignageSignage• FDA Guidance for the Submission Of Premarket FDA Guidance for the Submission Of Premarket

Notifications for Magnetic Resonance Diagnostic Notifications for Magnetic Resonance Diagnostic Devices states: Devices states:

• "The controlled access area should be labeled "Danger - "The controlled access area should be labeled "Danger - High Magnetic Field" at all entries." High Magnetic Field" at all entries."

• The term "warning" does not convey the importance of a The term "warning" does not convey the importance of a situation that may not only be potentially hazardous, but situation that may not only be potentially hazardous, but has been responsible for serious injuries and deaths.has been responsible for serious injuries and deaths.

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Other Safety IssuesOther Safety Issues

Static Magnetic FieldsStatic Magnetic FieldsTime Varying Magnetic FieldsTime Varying Magnetic Fields

RF Magnetic FieldsRF Magnetic FieldsAuditory SafetyAuditory Safety

FDA Safety GuidelinesFDA Safety GuidelinesContrast AgentsContrast Agents

M R I Physics CourseM R I Physics Course

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Other Safety IssuesOther Safety Issues

Psychological DistressPsychological DistressCryogen SafetyCryogen Safety

Experiences at Higher Magnetic FieldsExperiences at Higher Magnetic FieldsPregnancyPregnancy

M R I Physics CourseM R I Physics Course

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SafetySafety

Why worry about biological and safety issues?Why worry about biological and safety issues? -The patient is exposed to:-The patient is exposed to: Static magnetic field BStatic magnetic field B00: : << 3.0 T 3.0 T Time varying magnetic fields (gradients)Time varying magnetic fields (gradients) RF magnetic fields (BRF magnetic fields (B11)) Acoustical noiseAcoustical noise Psychological distress (claustrophobia)Psychological distress (claustrophobia) ContrastContrast

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-The patient is also exposed to: -The patient is also exposed to: Metal implant accident (foreign Metal implant accident (foreign

materials in the body)materials in the body) Cryogen accidentCryogen accident Projectile acident Projectile acident

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The technologist, health care providers and other The technologist, health care providers and other individuals are exposed to:individuals are exposed to:

Static magnetic fields: few Gauss to 3.0 TStatic magnetic fields: few Gauss to 3.0 T Cryogen accidentCryogen accident Projectile accident Projectile accident

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Biological effects of static magnetic fieldsBiological effects of static magnetic fields -Augmentation of the T wave during peak flow in -Augmentation of the T wave during peak flow in

the aortic archthe aortic arch -No evidence of mutagenic effects (in vitro or in -No evidence of mutagenic effects (in vitro or in

vivo)vivo) -No evidence of an effect on organ development in -No evidence of an effect on organ development in

vivovivo -No pronounced threshold-No pronounced threshold -No trends suggestive of a dose response curve -No trends suggestive of a dose response curve

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Biological effects of static magnetic fieldsBiological effects of static magnetic fields ““In general, there is no conclusive evidence for In general, there is no conclusive evidence for

irreversible or significant biological effects related to irreversible or significant biological effects related to acute, short-term exposure of humans to static acute, short-term exposure of humans to static magnetic field strengths up to 2.0 T.”magnetic field strengths up to 2.0 T.”

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Biological effects of time varying (gradient) Biological effects of time varying (gradient) magnetic fieldsmagnetic fields -Faraday’s Law: Induction of electric fields and -Faraday’s Law: Induction of electric fields and

currents in tissues (depends on tissue conductivity, currents in tissues (depends on tissue conductivity, gradient strength, risetime, waveshape, polarity, gradient strength, risetime, waveshape, polarity, repetition time TR and duration of the scan)repetition time TR and duration of the scan)

-Eddy current formation -Eddy current formation

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Safety Safety (continued)(continued) Biological effects of time varying (gradient) magnetic Biological effects of time varying (gradient) magnetic

fields fields -Nerve stimulation: electrical stimulation of peripheral and -Nerve stimulation: electrical stimulation of peripheral and

cutaneous nerves should precede any stimulation of cutaneous nerves should precede any stimulation of ventricular fibrillation. Guidelines for time varying magnetic ventricular fibrillation. Guidelines for time varying magnetic fields are designed to prevent any perceptible nerve fields are designed to prevent any perceptible nerve stimulation ( with a safety factor of times three)stimulation ( with a safety factor of times three)

-Magnetophosphenes: perceptible light flashes caused by -Magnetophosphenes: perceptible light flashes caused by electrical stimulation of the retina (not experienced in imagers electrical stimulation of the retina (not experienced in imagers having field strengths less than 4.0 T) having field strengths less than 4.0 T)

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Biological effects of time varying (gradient) Biological effects of time varying (gradient) magnetic fieldsmagnetic fields “…“…guidelines are believed to provide a wide margin guidelines are believed to provide a wide margin

of safety with respect to exposure to gradient of safety with respect to exposure to gradient magnetic fields during MRI…newer “fast scanning” magnetic fields during MRI…newer “fast scanning” techniques that may require more complex techniques that may require more complex applications of gradient magnetic fields may easily applications of gradient magnetic fields may easily exceed these recommended levels and must be exceed these recommended levels and must be thoroughly evaluated for potential health hazards.”thoroughly evaluated for potential health hazards.”

-Time varying magnetic fields are used to stimulate -Time varying magnetic fields are used to stimulate bone healing bone healing

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Stimulation Threshold LevelsStimulation Threshold Levels

Mean CardiacMean Cardiac

Stimulation ThresholdStimulation Threshold

Mean RespiratoryMean Respiratory

Stimulation ThresholdStimulation Threshold

Mean Painful NerveMean Painful NerveStimulation ThresholdStimulation Threshold

Mean Peripheral NerveMean Peripheral NerveStimulation ThresholdStimulation Threshold

T/S = Tesla per secondT/S = Tesla per second

0 - 20 T/s = clinical mode0 - 20 T/s = clinical mode

>>20 T/s = First or Second20 T/s = First or Second

Controlled Modes*Controlled Modes*

* Limited by IRB or * Limited by IRB or Human Ethical CommitteeHuman Ethical Committee

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3600 T/s3600 T/s

900 T/s900 T/s

90 T/s90 T/s

60 T/s60 T/s

45 T/s Typical Operating Range45 T/s Typical Operating Range

20 T/s20 T/s

0 T/s0 T/s

MR Safety Guide © 2003 by General Electric Company, Inc.MR Safety Guide © 2003 by General Electric Company, Inc.

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

34MR Safety Guide © 2003 by General Electric Company, Inc.MR Safety Guide © 2003 by General Electric Company, Inc.


Biological effects of RF magnetic fieldsBiological effects of RF magnetic fields -RF magnetic fields are oscillating magnetic fields-RF magnetic fields are oscillating magnetic fields -These fields produce heating of tissue-These fields produce heating of tissue -Energy deposited (degree of heating) is measured by -Energy deposited (degree of heating) is measured by

the Specific Absorption Rate (SAR) in units of the Specific Absorption Rate (SAR) in units of Watts/kilogramWatts/kilogram

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Biological effects of RF magnetic fields Biological effects of RF magnetic fields - SAR depends upon flip angle, TR, tissue density, RF - SAR depends upon flip angle, TR, tissue density, RF

pulse duration and frequency, RF coil radius, tissue pulse duration and frequency, RF coil radius, tissue conductivity and static magnetic field.conductivity and static magnetic field.

-Guidelines restrict rise in core temperature to 1 -Guidelines restrict rise in core temperature to 1 ooC C FDA: 0.4 W/kg whole bodyFDA: 0.4 W/kg whole body 3.2 W/kg head 3.2 W/kg head 8.0 W/kg peak8.0 W/kg peak

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Biological effects of RF magnetic fieldsBiological effects of RF magnetic fields -Similar to ultrasound since as long as the energy -Similar to ultrasound since as long as the energy

deposited is below established limits, minimal heating deposited is below established limits, minimal heating of tissues occurs. of tissues occurs.

-Extra precautions are necessary for patients with -Extra precautions are necessary for patients with compromised thermoregulation: fever, diabetes, compromised thermoregulation: fever, diabetes, obesity, cardiovascular disease, old age, medicated ( obesity, cardiovascular disease, old age, medicated ( ββ and Ca channel blockers), anesthesia and Ca channel blockers), anesthesia

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Biological effects of RF magnetic fields Biological effects of RF magnetic fields -Thermal burns-Thermal burns

-Electrical conductors can absorb energy and become -Electrical conductors can absorb energy and become extremely hotextremely hot

-Avoid having any cable wire or lead in contact with the -Avoid having any cable wire or lead in contact with the patientpatient

-Surface coil cables can be placed in sleeves to prevent -Surface coil cables can be placed in sleeves to prevent burnsburns

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• Auditory safetyAuditory safety• Activation of gradient magnetic fields produces significant Activation of gradient magnetic fields produces significant

vibrations in the gradient coils.vibrations in the gradient coils.• MRI acoustical noise has been shown to produce MRI acoustical noise has been shown to produce

reversible hearing impairment and could potentially reversible hearing impairment and could potentially produce permanent damage. produce permanent damage.

• Hearing protection is recommended for all patients Hearing protection is recommended for all patients undergoing an MRI procedure on a high-field MRI system undergoing an MRI procedure on a high-field MRI system

(1.5T and 3.0T)(1.5T and 3.0T)..• Noise attenuating ear-plugs or head phones are routinely Noise attenuating ear-plugs or head phones are routinely

used in MRI used in MRI

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-FDA Safety Guidelines for MR Devices -FDA Safety Guidelines for MR Devices -Acoustic noise level-Acoustic noise level

International standard: 140 dB relative to 20 mPa International standard: 140 dB relative to 20 mPa

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-Contrast agents -Contrast agents -Gd-DTPA is significantly safer than iodine based -Gd-DTPA is significantly safer than iodine based

contrast agents used in X-ray procedures (x 10)contrast agents used in X-ray procedures (x 10) -Severe reactions do occur-Severe reactions do occur -Pregnancy: Gd-DTPA may have poor clearance -Pregnancy: Gd-DTPA may have poor clearance

from amniotic fluidfrom amniotic fluid

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-Psychological Distress (before, during and after -Psychological Distress (before, during and after MRI)MRI)

ClaustrophobiaClaustrophobia AnxietyAnxiety DepressionDepression Panic DisordersPanic Disorders

-RF coils need to be near the body for high signal to -RF coils need to be near the body for high signal to noisenoise

-Usually transient (2 cases of claustrophobia -Usually transient (2 cases of claustrophobia persisted requiring psychiatric treatment)persisted requiring psychiatric treatment)

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-Psychological Distress-Psychological Distress -Countermeasures-Countermeasures BriefingBriefing Friend or relative in scan roomFriend or relative in scan room Physical or verbal contact with patientPhysical or verbal contact with patient ProneProne Mirrors, prism glasses, video gogglesMirrors, prism glasses, video goggles BlindfoldBlindfold Relaxation strategiesRelaxation strategies Psychological desensitization techniquesPsychological desensitization techniques

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Cryogen SafetyCryogen Safety

• Superconducting MRI magnets use cryogens Superconducting MRI magnets use cryogens (1970 liters of liquid helium)(1970 liters of liquid helium)

• Magnets can QUENCH resulting in conversion of Magnets can QUENCH resulting in conversion of the liquid helium to gaseous heliumthe liquid helium to gaseous helium

• 1970 liters of liquid helium becomes 53,000 cubic 1970 liters of liquid helium becomes 53,000 cubic feet of gaseous helium in about 1 minutefeet of gaseous helium in about 1 minute

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• Magnets are designed to dump the gaseous helium Magnets are designed to dump the gaseous helium directly outdoors through a quench pipedirectly outdoors through a quench pipe

• If the quench pipe fails, gaseous helium could be If the quench pipe fails, gaseous helium could be dumped into the scan room, displacing the oxygendumped into the scan room, displacing the oxygen

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

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• There has been one fatality by asphyxiation There has been one fatality by asphyxiation related to a liquid nitrogen leak in MRIrelated to a liquid nitrogen leak in MRI

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-Experiences with higher magnetic fields (4 T)-Experiences with higher magnetic fields (4 T) A few 7T systemsA few 7T systems

Several 4 T systemsSeveral 4 T systems Numerous 3T systemsNumerous 3T systems

Bioeffects:Bioeffects: magnetophosphenesmagnetophosphenes vertigovertigo nauseanausea headachesheadaches metallic tastemetallic taste

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PregnancyPregnancy MRI is not considered to be hazardous to the fetusMRI is not considered to be hazardous to the fetus Limited studies have examined teratogenic effectsLimited studies have examined teratogenic effects Cells that are reproducing rapidly, as in the first Cells that are reproducing rapidly, as in the first

trimester, are susceptible to damage from a variety trimester, are susceptible to damage from a variety of physical agentsof physical agents

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PregnancyPregnancy• FDA: “the safety of MR imaging when used to FDA: “the safety of MR imaging when used to

image the fetus and infant has not been established”image the fetus and infant has not been established”• British National Radiological Protection Board: “It British National Radiological Protection Board: “It

might be prudent to exclude pregnant women might be prudent to exclude pregnant women during the first three months of pregnancy”during the first three months of pregnancy”

• ““Particular caution should be exercised with the use Particular caution should be exercised with the use of MRI during the first trimester because there is a of MRI during the first trimester because there is a high spontaneous abortion rate (30%) with high spontaneous abortion rate (30%) with associated potential medicolegal implications.” associated potential medicolegal implications.”

• May be preferred over studies requiring ionizing May be preferred over studies requiring ionizing radiationradiation

• Should only be performed if the result is expected Should only be performed if the result is expected to be an important factor in the management of the to be an important factor in the management of the patientpatient 49

PregnancyPregnancy Can MRI Adversely Affect a Fetus?Can MRI Adversely Affect a Fetus?

Emanuel Kanal, M.D.Emanuel Kanal, M.D. University of PittsburghUniversity of Pittsburgh Director, Pittsburgh NMR InstituteDirector, Pittsburgh NMR Institute

-1915 women surveyed: 823 had at least 1 pregnancy (not -1915 women surveyed: 823 had at least 1 pregnancy (not necessarily during MRI employment), data on 1421 necessarily during MRI employment), data on 1421 pregnanciespregnancies

MiscarriageMiscarriage Premature deliveryPremature delivery InfertilityInfertility Low birth weightLow birth weight Offspring genderOffspring gender 50

PregnancyPregnancy

Can MRI Adversely Affect a Fetus? Can MRI Adversely Affect a Fetus? -Compared to a similar group of pregnancies in -Compared to a similar group of pregnancies in

women that had not worked in MRIwomen that had not worked in MRI -Smoking, alcohol usage and age were considered-Smoking, alcohol usage and age were considered -No statistically significant differences in the five -No statistically significant differences in the five

areas studiedareas studied RT Image; January 6, 1992; Vol. 5, No. 1RT Image; January 6, 1992; Vol. 5, No. 1

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