NOTEBOOK#5

DIGITALRADIOGRAPHY:DIRECTANDINDIRECTCONVERSIONTYPES

NancyLaFebruary16,2017

RTE142-RadiographicEquip/ImagingIINotebook#5

DRTypesOfImageReceptors;DirectandIndirectConversion

QuickOutline:

DigitalRadiography(DR)• Direct

- Directlyconvertincomingx-rayphotonsintoanelectronicdigitalsystemwith1. Amorphousselenium(a-Se)Photoconductorthatabsorbsx-rays

andconvertsthemintoelectrons,whicharethenstoredinafieldeffecttransistor(FET)ORthinfilmtransistor(TFT)forelectronicreadout

• Indirect(visiblelight)- 2systems;bothutilizingscintillatorstochangex-rayphotonstolightbutvaryin

thewaythelightisconvertedintoadigitalsignal1. Scintillatorstructured(needle/columnform,thalliumdopedcesium

iodideCsI[TI]crystal)/unstructured(rare-earthintensifyingscreenmadeofturbidfinepowderedgranulegadoliniumoxysulfide;Gd202Scrystal)phosphorarraysthatemitlightwhenstimulatedbyx-rayswithAmorphousSilicon(a-Si:H)PhotodiodelightdetectortocapturefluorescentlightandThinFilmTransistor(TFT)Arrayusedforelectronicreadout

2. Scintillatorstructured(needle/columnform,thalliumdopedcesiumiodideCsI[TI]crystal)/unstructured(rare-earthintensifyingscreenmadeofturbidfinepowderedgranulegadoliniumoxysulfide;Gd202Scrystal)phosphorarraysthatemitlightwhenstimulatedbyx-rayswithChargedCoupledDevice(CCD)photosensitivereceptorandelectronicsembeddedintoasubstratematerialinasiliconchip;couplingdevicethatactsascamerasORComplementaryMetalOxideSemiconductor(CMOS)containingpixelswithitsownamplifierthatisswitchedonandoffbycircuitrywithinthepixel;bothCCD/CMOSconvertslightphotonstoelectricalchargesforreadout

QuickChart:

DrawnInterpretations:

Ø DirectConversionAmorphousSeleniumFlatPanelImagingPlate

Ø IndirectAmorphousSiliconFlatPanelImagingPlateSystem

DrawnInterpretations(continued):

Ø IndirectMulti-CCDDetectorUnitandSimpleCMOSDetector

DetailedExplanation:

IndirectcaptureIR,conversionsystemsdirectlyconvertincomingx-raysphotonstoanelectronicdigitalsystemusinganamorphousselenium(a-Se)andafieldeffecttransistor(FET)orthinfilmtransistor(TFT)madeofsilicon.Ahigh-voltagechargeisappliedfromthetopsurfaceoftheseleniumlayer;thismaterialabsorbsx-raysandconvertsthemintoelectrons,whicharestoredinTFTdetector.Duringexposure,ionizingx-rayphotonsresultsinseleniumatomsfreeingelectronsforcollectionbytheelectrodesatthebottomoftheseleniumlayer.ThechargesarethencollectedataTFTandreadlinebylinetothecomputerforprocessing

Indirectcaptureisatwo-stepprocesswith2differenttypes;oneusesachargedcoupledevice(CCD)andtheotherusesaflatpanelTFTdetectorwithamorphoussiliconthatcannotdirectlyconvertx-raysintoanelectriccharge,butdoesworkasaphotodiode(lightdetector)tocapturefluorescentlightbutbothutilizesascintillatortoconvertthephotonstolightwiththedefiningdifferenceinthewaythelightisconvertedintoanelectricalsignal.Theamorphousseleniumrequiresascintillatorsuchasathalliumdopedcesiumiodide(CsI[TI])orarare-earthintensifyingscreenmadeofgadoliniumoxysulfide(Gd202S).Scintillatorsemitlightisotropicallyreducingspatialresolution.CsIismanufacturedasstructuredcrystalsinneedleorcolumnform.Whichsignificantlyreduceslightspreadandchannelslighttowardtheamorphoussiliconphotodiode.Gd202Sisaturbidphosphorwithcrystalsthatarefinepowderedparticles.

Aflatpanelthinfilmtransistorcollectselectricchargesproducedbyeithertheselenium/siliconasanarrayormatrixofpixel-sizedetectorelements(DEL).PixelsizeisrelatedtoDEL,whichaffectspixelpitchandresolution.Apixelisessentiallyanelementofthepicturemeasuredbyindividualmatrixboxeswhicharejustrectangularorsquaretableofnumbersthatrepresentthepixelintensitytobedisplayedonthemonitor.BitDepthreferstothenumberofbits(singleunitofdata)storedperpixelanddefinestheshadesofgrayavailableforeachpixel.Abyteismadeupof8bitsandistheamountofmemoryneededtostoreonealphanumericcharacter.Theseoverallaffectthedynamicrange(theabilityofanimagingsystemtorespondtovaryinglevelsofexposure)andexposurelatitude(amountoferrorthatcanbemadeinexposurefactorandstillresultinthecaptureofaqualityimage).

CCDismuchmoresimplethanTFTbasedflatpanelsysteminmodulardesignandismuchlesscostlyhoweverthegreatestdisadvantageliesinitsdemagnificationissuesresultinginreducedDQE(detectivequantumefficiency).CCDismadeupofaphotosensitivereceptorandelectronicsembeddedintoasubstratematerialinasiliconchip;couplingdevicethatactsascameras.TheCCDisaphotodetectorcapableof

convertingvisiblelightintoanelectricalchargeandstoringitinasequentialpatterntobereleasedlinebylinetotheanalogtodigitalconverterADC.TheelectricalsignalproducedbytheCCDisthensenttothecomputerforimageprocessing.Highlyefficientandsmall,CMOS(complementarymetaloxidesemiconductors)isasemiconductorthatisasolidchemicalelementorcompoundthatlikeCCDconvertslightintoelectronsandstoresthemBUTduetoitsmaterialdesign,thesemiconductor(antimony,arsenic,boron,carbon,germanium,selenium,silicon*mostcommonbasematerial,sulfur,tellurium)isdopedwithcommonimpurities(galliumarsenide,indiumantimonide,andoxidesofmostmetals)andturnedintoafullscaleconductorortheexcesselectronsbecomenegativeorpositivetransistors(N-typeextraelectronswithanegativecharge,P-typepositivechargecarriers)thatcanbecontrolledelectricallytoformgatestochangechargedirection.Therefore,similartoCCDintermsofconversionandstoring,CMOSdiffersinitsabilitytohaveeachpixelsamplifierswitchedonandoffconvertinglightphotonsaselectricalcharges,whicharethenfedtoanADCandarereleasedatreadout.