【醫(yī)學(xué)英文課件】-Modern-Physics

L36—ModernPhysics[2]X-rays&gammaraysHowlasersworkMedicalapplicationsoflasersApplicationsofhighpowerlasersMedicalimagingtechniquesCATscansMRI’sModernphysicsideasarestrange!Electromagneticwavessometimesbehavelikeparticles-photons–discreet(quantized)packetsofenergyE=hf=hc/lTheelectronssometimesbehaveaswaves–matterwavesthatcanonlyexistinallowedorbitsElectronshaveawavelengthandcanexperiencediffraction!AAAThePhotonConceptabeamoflightwavesalsobehaveslikeabeamoflightparticlescalledPHOTONSPhotonsarelittlepacketsofelectro-magneticenergy

TheenergyisproportionaltothefrequencyorinverselyproportionaltothewavelengthEphoton=hf,butc=fsoEphoton=hc/,wherehisaconstantcalledPlanck’sconstant,andcisthespeedoflightblue

photonshavemoreenergythan

red

photonsEnergyisabsorbedoremittedindiscreetamountssodiumabsorptionlineX-raymachinesHowarex-raysproduced?electronguncoppertargetx-rayswhenelectronsthathavebeenacceleratedthroughabout50,000voltsslamintoapieceofcopper,someoftheelectronenergyisconvertedtox-raysx-raysareenergeticenoughtopenetratethroughsofttissueandthinmetalfoilsx-raytubeGammaraysextremelyenergeticphotonsconstantlybombardtheearthcosmicraysemittedbyradioactivematerialsxrayphotonsarea1000timesmoreenergeticthanvisiblelightphotonsgammarayphotonsare1,000,000moreenergeticthanvisiblelightphotonsLasersadevicethatcontrolsthewaythatenergizedatomsreleasephotons.LightAmplificationbyStimulatedEmissionofRadiationFirstwemustunderstandthedifferencebetweenincoherentandcoherentradiationOrdinarylightsources(lightbulbs,fluorescentlights,etc)produceincoherentlightlasersproducecoherentlightallatomsradiateinthesamemannerSpontaneousvsStimulated

Emission

Coherentradiationisproducedwhenanatomundergoesstimulatedemission.SpontaneousemissionoccurswhenanelectronmakesanunprovokedtransitiontoalowerenergylevelStimulatedemissionoccurswhenanincomingphotoninducestheelectrontochangeenergylevelsamplificationEi(largerenergy)Ef(smallerenergy)Spontaneous

emissionStimulatedemissionIncomingphotonphotonAHelium-Neon(HeNe)LaserMedicalApplicationsofLasersLasersurgerytocorrectfornearsightedness,andfarsightednessApplicationsofHighPowerLasersUsinglaserstoCutmetalsMultiplebeamsofapowerfullaserarefocusedonatinypelletcontainingfusionfuel.Thelaserenergycompressesthepelletproducingamini-hydrogenbombthatproducesenergyLaserFusionEnergyfuelpellet1mmdiameterpelletSolidStateLaserDiodessmallComeinavarietyofdifferentcolorsLasersDiodesDiodelasersusesemiconductormaterials(tinychipsofsilicon)asthelasingmediaWhencurrentflowsthroughthesiliconchipitemitsanintensebeamofcoherentlight.DiodelasersareusedtoreadtheinformationembeddedinthepitsinCD’sandDVD’s,andalsotoreadUPC’sinbarcodescannersandinlaserpointers!Laserspeedgunsthesearereplacing“radar”gunsthegunsendsoutaseriesofpulsesofinfraredlaserlightthatbounceoffthecarandreturntothegun.bymeasuringthetimeforthepulsetoreturnthedistancetothecarcanbemeasuredthespeedofthecarisdeterminedbytwoconsecutivemeasurementsofthedistancetimet1positionx1timet2positionx2HowdoesaCDburnerWork?/cd-burner4.htm

infraredlaserlightisappliedtoalayerofphotosensitivedyeontopoftheplasticthiscausesthedyetodarken(noburning!)byselectivelydarkeningparticularpointsalongtheCDtrack,andleavingotherareasofdyetranslucent,adigitalpatterniscreatedthatcanbereadbyastandardCDplayerMedicalImagingTechniquesx-raysCTandCATscans(ComputerizedTomography)MRI’s(MagneticResonanceImaging)X-raysveryshortwavelength(0.01–0.1nm)electromagneticwavesproducedwhenenergeticelectronsslamintoametaltargetabletopenetratesofttissue,butnotboneproducesatwodimensionalshadow

imagex-rayofHomer’sheadApineappleandabananaAshadowimagecanbemisleadingtwoshadowstakenfromdifferentanglesprovidesabetterpictureshadowstakenatmultipleanglesgivesamorecompletepicturethisiswhataCTorCATscandoesCATScansXrayimagesaretakenatmanydifferentanglespassingthroughthepatient.Someofthecutsoverlap.Afullthreedimensionalimagecanbereconstructedusingcomputers.

thisprocedureiscalledtomography.ComputerizedTomographyAcomputerizedtomographyorCTscanimageisformedbyanalyzingx-rayshadowimagestakenatmanydifferentanglesandpositionsanx-raysourceandanarrayofelectronicdetectorsrotatesaroundthepatientasthepatientslowlymovesthroughthering.Isthereabettermedicaldiagnostic?ACATscandoesagoodjobofimagingbones,butitdoesnotprovideasgoodanimageofsofttissueAlso,itrequiresthatthepatientreceivesabigdoseofx-rays,whichcanbeharmfulinthemselvesitisaninvasivediagnosticMagneticresonanceimaging(MRI)isabettermethodofimagingsofttissueMRIDEVICEMRIfindsthehydrogenatomsMRIworksbylocatingthehydrogenatomsinsidethebody.Sincethebodyismostlywater,therearelotsofhydrogenatomsthenucleusofahydrogenatomisasingleproton.Protonsbehaveliketinybarmagnetswithanorthpoleatoneendandasouthpoleattheotherend.Ifyouputabarmagnetinamagneticfield,itwilltrytoalignitselfwiththefield.AbarmagnetinamagneticfieldNSmagneticfieldNSSolenoidforproducingastrongmagneticfieldbypassingalargecurrentthroughasetofcoilsMRI–HowitworksUnlikeCT,MRIusesnoionizingradiation,butusesapowerfulmagneticfieldtoalignthenuclearmagnetizationof(usually)hydrogenatomsinwaterinthebody.Radiofrequency(RF)fieldsareusedtosystematicallyalterthealignmentofthismagnetization,causingthehydrogennucleitoproducearotatingmagneticfielddetectablebythescanner.Thissignalcanbemanipulatedbyadditionalmagneticfieldstobuildupenoughinformationtoallow