磁共振安全信息簡介

CaveatThislectureemphasizesMRIsafetyforhighfield(1.5Tand3.0T)superconductingmagnetssuchasthoseusedatMCGHI.MRISafetyProjectileAccidentsForeignMaterialsintheBodyOtherStuffProjectileAccidentsTheMRImagnetsareALWAYSon(24hours/day,365days/year)ThereisaSTRONGfringemagneticfieldaroundthemagnetsThefringemagneticfieldisconfinedtothescanroom3ProjectileAccidents

ThefringemagneticfieldcancauseferrousmetalobjectstoflyintotheboreofthemagnetwithgreatforceandspeedFerrousmetalobjectscontainiron(steel)Thecloseryouaretotheboreofthemagnet,thestrongerthefringefield4Asyouapproachthemagnet,thefringemagneticfieldgetsSTRONGER56/flying_objects.html#7/flying_objects.html#8/flying_objects.html#9/flying_objects.html#10MyFavoriteHorrorStoryHere'soneIheardfromanOxfordMagnetsengineerwhichwaslaterindependentlyverifiedbyatechnologistwhohadbeenhiredtoworkatthissite.Abrandnewmagnethadjustfinishedbeinginstalledintoabrandnewbuilding.Alloftheacceptancetestinghadbeencompletedandthemagnetwastobeturnedovertothecustomertheverynextday.Therewasonlyoneminorproblemtobedealtwithfirst.Oneofthesprinklersinthescanroomhadatinyleak.Awelderwasbroughtintofixtheleak,butsomebodyforgottotellhimthatthemagnetwasatfield.So.....inwalksthiswelderwithhisacetylenetorchsystem.Histankfliesintothemagnet,thevalvebreaksoff,sparksandcatchesfire.Sincehewasintheretofixaleakinthesprinklersystem,ithadbeenturnedofffirst.Thebrandnewbuildingburnedtotheground!/flying_objects.html#ProjectileAccidentsThefringemagneticfieldcanbedeadlyValhalla,N.Y.WestchesterCountyMedicalCenterJuly27,2001SixyearoldMichaelColombiniwasfatallyinjured“whenanoxygencanisterwasturnedintoaguidedmissilebythepowerfulMRImagnet”11FatalAccidentDetailsMichaelColombiniwasintheboreofaGESignaAnanesthesiologistwasinthescanroomwiththepatientThebuilt-inoxygendeliverysystemfailedAnurseheardtheanesthesiologistcallingforoxygen12FatalAccidentDetailsThenursedeliveredaSTEELoxygencylindertotheanesthesiologistthinkingitwassafe(ALUMINUM)Within3to6feetofthemagnet,theSTEELcylinderflewintothebore,fatallyinjuringthepatient13ForeignMaterialsintheBodyForeignmaterialinthebodyisaconcerninMRIForeignmaterialisanymaterialinthebodyotherthanthefleshandbonesthatwearebornwithWidevarietyofforeignmaterialisintentionallyandunintentionallypresentinpatients,healthcareprovidersandothers14ForeignMaterialsintheBodyThepresenceofcertainforeignmaterialsinthebodycanbefatalinanMRImagnetDeathshaveoccurredduetothepresenceofaneurysmclipsandcardiacpacemakersinpatientsundergoingMRISighthasbeenlostbymovementofferrousmetalintheeyeresultinginvitreoushemorrhage(2.0mmx3.5mmironfragment@0.35T)Prosthesesareusuallysecurelyanchoredtoboneandareusuallynon-ferrous.15ForeignMaterialsintheBodyFringemagneticfieldsstrongerthan5Gausscanaffectcardiacpacemakers.Toprotectpatientsandothershavingpacemakersthereisa5GaussExclusionZonearoundMRImagnets.AtMCGHI,theExclusionZonesaretheMRIscanrooms16FringeDiagramofActiveShield

1.5T**SiemensSymphony1716’6”26’4”5GaussSafety

(continued)-AtleasttwodeathshaveresultedfromMRIScansonpatientswithpacemakers.18ForeignMaterialsintheBodyHeatingofmetalsandimplantshasnotbeensignificant(evenforlargeprostheses)ThereareresourcesforassessingsafetyofforeignmaterialsinMRIF.G.Shellock.ReferencemanualforMagneticResonanceSafety:2003EditionSaltLakeCity:AmirsysInc.2003.19SignageFDAGuidancefortheSubmissionOfPremarketNotificationsforMagneticResonanceDiagnosticDevicesstates:"Thecontrolledaccessareashouldbelabeled"Danger-HighMagneticField"atallentries."Theterm"warning"doesnotconveytheimportanceofasituationthatmaynotonlybepotentiallyhazardous,buthasbeenresponsibleforseriousinjuriesanddeaths.2021OtherSafetyIssuesStaticMagneticFieldsTimeVaryingMagneticFieldsRFMagneticFieldsAuditorySafetyFDASafetyGuidelinesContrastAgentsMRIPhysicsCourse22OtherSafetyIssuesPsychologicalDistressCryogenSafetyExperiencesatHigherMagneticFieldsPregnancyMRIPhysicsCourse23SafetyWhyworryaboutbiologicalandsafetyissues?-Thepatientisexposedto:StaticmagneticfieldB0:<3.0TTimevaryingmagneticfields(gradients)RFmagneticfields(B1)AcousticalnoisePsychologicaldistress(claustrophobia)Contrast24Safety

(continued)-Thepatientisalsoexposedto:Metalimplantaccident(foreign materialsinthebody)CryogenaccidentProjectileacident

25Safety

(continued)Thetechnologist,healthcareprovidersandotherindividualsareexposedto:Staticmagneticfields:fewGaussto3.0TCryogenaccidentProjectileaccident26Safety

(continued)Biologicaleffectsofstaticmagneticfields-AugmentationoftheTwaveduringpeakflowintheaorticarch-Noevidenceofmutageniceffects(invitroorinvivo)-Noevidenceofaneffectonorgandevelopmentinvivo-Nopronouncedthreshold-Notrendssuggestiveofadoseresponsecurve27Safety

(continued)Biologicaleffectsofstaticmagneticfields“Ingeneral,thereisnoconclusiveevidenceforirreversibleorsignificantbiologicaleffectsrelatedtoacute,short-termexposureofhumanstostaticmagneticfieldstrengthsupto2.0T.”28Safety

(continued)Biologicaleffectsoftimevarying(gradient)magneticfields-Faraday’sLaw:Inductionofelectricfieldsandcurrentsintissues(dependsontissueconductivity,gradientstrength,risetime,waveshape,polarity,repetitiontimeTRanddurationofthescan)-Eddycurrentformation29Safety

(continued)Biologicaleffectsoftimevarying(gradient)magneticfields-Nervestimulation:electricalstimulationofperipheralandcutaneousnervesshouldprecedeanystimulationofventricularfibrillation.Guidelinesfortimevaryingmagneticfieldsaredesignedtopreventanyperceptiblenervestimulation(withasafetyfactoroftimesthree)-Magnetophosphenes:perceptiblelightflashescausedbyelectricalstimulationoftheretina(notexperiencedinimagershavingfieldstrengthslessthan4.0T)30Safety

(continued)Biologicaleffectsoftimevarying(gradient)magneticfields“…guidelinesarebelievedtoprovideawidemarginofsafetywithrespecttoexposuretogradientmagneticfieldsduringMRI…newer“fastscanning”techniquesthatmayrequiremorecomplexapplicationsofgradientmagneticfieldsmayeasilyexceedtheserecommendedlevelsandmustbethoroughlyevaluatedforpotentialhealthhazards.”-Timevaryingmagneticfieldsareusedtostimulatebonehealing31StimulationThresholdLevelsMeanCardiacStimulationThresholdMeanRespiratoryStimulationThresholdMeanPainfulNerveStimulationThresholdMeanPeripheralNerveStimulationThresholdT/S=Teslapersecond0-20T/s=clinicalmode>20T/s=FirstorSecondControlledModes**LimitedbyIRBorHumanEthicalCommittee323600T/s900T/s90T/s60T/s45T/sTypicalOperatingRange20T/s0T/sMRSafetyGuide?2003byGeneralElectricCompany,Inc.Duetotherapidrateofchangeofthemagneticfields(dB/dt)usedduringsomeEPIscans,apercentageofpatientsmayexperienceatinglingortouchsensation.ThismeansapercentageofpatientsmayexperiencePeripheralNerveStimulation(PNS)at45T/s.Ifthissensationisbothersomeoruncomfortabletothepatient,stopthescan.Changetoanon-EPIpulsesequencetocontinuescanningthepatient.33MRSafetyGuide?2003byGeneralElectricCompany,Inc.Safety

(continued)BiologicaleffectsofRFmagneticfields-RFmagneticfieldsareoscillatingmagneticfields-Thesefieldsproduceheatingoftissue-Energydeposited(degreeofheating)ismeasuredbytheSpecificAbsorptionRate(SAR)inunitsofWatts/kilogram34Safety

(continued)BiologicaleffectsofRFmagneticfields-SARdependsuponflipangle,TR,tissuedensity,RFpulsedurationandfrequency,RFcoilradius,tissueconductivityandstaticmagneticfield.-Guidelinesrestrictriseincoretemperatureto1oC

FDA:0.4W/kgwholebody3.2W/kghead8.0W/kgpeak35Safety

(continued)BiologicaleffectsofRFmagneticfields-Similartoultrasoundsinceaslongastheenergydepositedisbelowestablishedlimits,minimalheatingoftissuesoccurs.-Extraprecautionsarenecessaryforpatientswithcompromisedthermoregulation:fever,diabetes,obesity,cardiovasculardisease,oldage,medicated(andCachannelblockers),anesthesia

36Safety

(continued)BiologicaleffectsofRFmagneticfields-Thermalburns-Electricalconductorscanabsorbenergyandbecomeextremelyhot-Avoidhavinganycablewireorleadincontactwiththepatient-Surfacecoilcablescanbeplacedinsleevestopreventburns37Safety

(continued)AuditorysafetyActivationofgradientmagneticfieldsproducessignificantvibrationsinthegradientcoils.MRIacousticalnoisehasbeenshowntoproducereversiblehearingimpairmentandcouldpotentiallyproducepermanentdamage.HearingprotectionisrecommendedforallpatientsundergoinganMRIprocedureonahigh-fieldMRIsystem(1.5Tand3.0T).Noiseattenuatingear-plugsorheadphonesareroutinelyusedinMRI38Safety

(continued)-FDASafetyGuidelinesforMRDevices-AcousticnoiselevelInternationalstandard:140dBrelativeto20mPa

39Safety

(continued)-Contrastagents-Gd-DTPAissignificantlysaferthaniodinebasedcontrastagentsusedinX-rayprocedures(x10)-Severereactionsdooccur-Pregnancy:Gd-DTPAmayhavepoorclearancefromamnioticfluid40Safety

(continued)-PsychologicalDistress(before,duringandafterMRI)ClaustrophobiaAnxietyDepressionPanicDisorders-RFcoilsneedtobenearthebodyforhighsignaltonoise-Usuallytransient(2casesofclaustrophobiapersistedrequiringpsychiatrictreatment)41Safety

(continued)-PsychologicalDistress-CountermeasuresBriefingFriendorrelativeinscanroomPhysicalorverbalcontactwithpatientProneMirrors,prismglasses,videogogglesBlindfoldRelaxationstrategiesPsychologicaldesensitizationtechniques42CryogenSafetySuperconductingMRImagnetsusecryogens(1970litersofliquidhelium)MagnetscanQUENCHresultinginconversionoftheliquidheliumtogaseoushelium1970litersofliquidheliumbecomes53,000cubicfeetofgaseousheliuminabout1minute43CryogenSafetyMagnetsaredesignedtodumpthegaseousheliumdirectlyoutdoorsthroughaquenchpipeIfthequenchpipefails,gaseousheliumcouldbedumpedintothescanroom,displacingtheoxygenMCGHIMRIscanroomshaveoxygenmonitorsthatautomaticallyactivateemergencyexhaustsystemsintheeventtheoxygenleveldropsbelow18%(AnAlarmSounds)44CryogenSafety

TherehasbeenonefatalitybyasphyxiationrelatedtoaliquidnitrogenleakinMRI45Safety

(continued)-Experienceswithhighermagneticfields(4T) Afew7Tsystems Several4TsystemsNumerous3Tsystems

Bioeffects:

magnetophosphenesvertigonauseaheadachesmetallictaste46Pregnancy

MRIisnotconsideredtobehazardoustothefetusLimitedstudieshaveexaminedteratogeniceffectsCellsthatarereproducingrapidly,asinthefirsttrimester,aresusceptibletodamagefromavarietyofphysicalagents

47Pre