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1、MRI Safety,Jerry Allison,Ph.D.,Caveat,This lecture emphasizes MRI safety for high field(1.5T and 3.0T)superconducting magnets such as those used at MCGHI.,MRI Safety,Projectile AccidentsForeign Materials in the BodyOther Stuff,Projectile Accidents,The MRI magnets are ALWAYS on(24 hours/day,365 days/
2、year)There is a STRONG fringe magnetic field around the magnetsThe fringe magnetic field is confined to the scan room,4,Projectile Accidents,The fringe magnetic field can cause ferrous metal objects to fly into the bore of the magnet with great force and speedFerrous metal objects contain iron(steel
3、)The closer you are to the bore of the magnet,the stronger the fringe field,5,As you approach the magnet,the fringe magnetic field gets STRONGER,6,7,http:/,8,http:/,9,http:/,10,http:/,11,My Favorite Horror StoryHeres one I heard from an Oxford Magnets engineer which was later independently verified
4、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 deal
5、t 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
6、.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!,http:/,Projectile Accidents,The fringe magnetic field can be deadlyValhalla,N.Y.Westchester County Medical CenterJuly 27,2001Six year old Michael Colombini was fatal
7、ly injured“when an oxygen canister was turned into a guided missile by the powerful MRI magnet”,12,Fatal Accident Details,Michael Colombini was in the bore of a GE SignaAn anesthesiologist was in the scan room with the patientThe built-in oxygen delivery system failedA nurse heard the anesthesiologi
8、st calling for oxygen,13,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,Foreign Materials in the Body,Foreign material in t
9、he body is a concern in MRIForeign material is any material in the body other than the flesh and bones that we are born withWide variety of foreign material is intentionally and unintentionally present in patients,health care providers and others,15,Foreign Materials in the Body,The presence of cert
10、ain foreign materials in the body can be fatal in an MRI magnetDeaths have occurred due to the presence of aneurysm clips and cardiac pacemakers in patients undergoing MRISight has been lost by movement of ferrous metal in the eye resulting in vitreous hemorrhage(2.0 mm x 3.5 mm iron fragment 0.35 T
11、)Prostheses are usually securely anchored to bone and are usually non-ferrous.,16,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,th
12、e Exclusion Zones are the MRI scan rooms,17,Fringe Diagram of Active Shield1.5 T*,*Siemens Symphony,18,16 6”,26 4”,5 Gauss,Safety(continued),-At least two deaths have resulted from MRI Scans on patients with pacemakers.,19,Foreign Materials in the Body,Heating of metals and implants has not been sig
13、nificant(even for large prostheses)There are resources for assessing safety of foreign materials in MRIF.G.Shellock.Reference manual for Magnetic Resonance Safety:2003 Edition Salt Lake City:Amirsys Inc.2003.,20,Signage,FDA Guidance for the Submission Of Premarket Notifications for Magnetic Resonanc
14、e 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.,21,22,Other Safety Is
15、suesStatic Magnetic FieldsTime Varying Magnetic FieldsRF Magnetic FieldsAuditory SafetyFDA Safety GuidelinesContrast Agents,M R I Physics Course,23,Other Safety IssuesPsychological DistressCryogen SafetyExperiences at Higher Magnetic FieldsPregnancy,M R I Physics Course,24,Safety,Why worry about bio
16、logical and safety issues?-The patient is exposed to:Static magnetic field B0:3.0 T Time varying magnetic fields(gradients)RF magnetic fields(B1)Acoustical noise Psychological distress(claustrophobia)Contrast,25,Safety(continued),-The patient is also exposed to:Metal implant accident(foreign materia
17、ls in the body)Cryogen accident Projectile acident,26,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,Safety(continued),Biological effects of static magnetic fields-Augment
18、ation 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,Safety(continued),Biological effects of static magnetic fields“
19、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,Safety(continued),Biological effects of time varying(gradient)magnetic fields-Faradays Law:Induction of ele
20、ctric 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,Safety(continued),Biological effects of time varying(gradient)magnetic fields-Nerve stimulation:electrical stimulat
21、ion 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 electrica
22、l stimulation of the retina(not experienced in imagers having field strengths less than 4.0 T),31,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 MRIne
23、wer“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,Stimulation Threshold Leve
24、ls,Mean CardiacStimulation ThresholdMean RespiratoryStimulation ThresholdMean Painful NerveStimulation ThresholdMean Peripheral NerveStimulation Threshold,T/S=Tesla per second0-20 T/s=clinical mode 20 T/s=First or Second Controlled Modes*Limited by IRB or Human Ethical Committee,33,3600 T/s,900 T/s,
25、90 T/s,60 T/s,45 T/s Typical Operating Range,20 T/s,0 T/s,MR Safety Guide 2003 by General Electric Company,Inc.,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 pat
26、ients 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.,34,MR Safety Guide 2003 by General Electric Company,Inc.,Safety(continued),Biological eff
27、ects 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,Safety(continued),Biological effects of RF magnetic fields-SAR depends
28、 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 FDA:0.4 W/kg whole body 3.2 W/kg head 8.0 W/kg peak,36,Safety(continued),Biological effects of RF magnetic fields-Simi
29、lar 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,37
30、,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,Safety(continued),Auditory safet
31、yActivation 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
32、a high-field MRI system(1.5T and 3.0T).Noise attenuating ear-plugs or head phones are routinely used in MRI,39,Safety(continued),-FDA Safety Guidelines for MR Devices-Acoustic noise levelInternational standard:140 dB relative to 20 mPa,40,Safety(continued),-Contrast agents-Gd-DTPA is significantly s
33、afer 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,Safety(continued),-Psychological Distress(before,during and after MRI)ClaustrophobiaAnxietyDepressionPanic Disorders-RF coils need to be ne
34、ar the body for high signal to noise-Usually transient(2 cases of claustrophobia persisted requiring psychiatric treatment),42,Safety(continued),-Psychological Distress-Countermeasures Briefing Friend or relative in scan room Physical or verbal contact with patient Prone Mirrors,prism glasses,video
35、goggles Blindfold Relaxation strategies Psychological desensitization techniques,43,Cryogen Safety,Superconducting MRI magnets use cryogens(1970 liters of liquid helium)Magnets can QUENCH resulting in conversion of the liquid helium to gaseous helium1970 liters of liquid helium becomes 53,000 cubic
36、feet of gaseous helium in about 1 minute,44,Cryogen Safety,Magnets are designed to dump the gaseous helium directly outdoors through a quench pipeIf the quench pipe fails,gaseous helium could be dumped into the scan room,displacing the oxygenMCGHI MRI scan rooms have oxygen monitors that automatical
37、ly activate emergency exhaust systems in the event the oxygen level drops below 18%(An Alarm Sounds),45,Cryogen Safety,There has been one fatality by asphyxiation related to a liquid nitrogen leak in MRI,46,Safety(continued),-Experiences with higher magnetic fields(4 T)A few 7T systemsSeveral 4 T sy
38、stemsNumerous 3T systems Bioeffects:magnetophosphenes vertigo nausea headaches metallic taste,47,Pregnancy,MRI is not considered to be hazardous to the fetusLimited studies have examined teratogenic effectsCells that are reproducing rapidly,as in the first trimester,are susceptible to damage from a
39、variety of physical agents,48,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 b
40、e 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 radiationShould only be performed if the result is expected to be an important factor
41、in the management of the patient,49,Pregnancy,Can MRI Adversely Affect a Fetus?Emanuel Kanal,M.D.University of PittsburghDirector,Pittsburgh NMR Institute-1915 women surveyed:823 had at least 1 pregnancy(not necessarily during MRI employment),data on 1421 pregnancies Miscarriage Premature delivery I
42、nfertility Low birth weight Offspring gender,50,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,
