Computed and digital radiography notes

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Chapter 15- COMPUTED RADIOGRAPH?■*“ Acassette,similartothecassetteusedinconventional radiography, encases a phosphorplate rather than film.vWhenx-raysstrikethephosphorplate,electrons are crapped in a high energy state Thelatentimageisstoredinthegrainsofthephosphor in the plate.vThe phosphor plate is then fed into a computerreader where it is scanned by a laser light Afteramplification and numbering, the light signal isdigitized through a series of steps, which createsa visible image.Thepatientispositionedthesameasinconventional radiography.COMPUTED RADIOGRAPHIC' IMAGE RECEPTORvPhotostimulable Luminescence>Some materials such asbarium fluorohalidewitheuropium(EaFEriEu orEaFLEu)emitsomelightpromptlyinthewaythatascintillator does following x-ray exposure.>Theyalsoemitlightsometimelaterwhenexposedtoadifferentlightsource.Suchaprocessiscalledphotostimulableluminescence (FSL).>The atoms of barium fluorobromide have atomicnumbers of 56, 9, and 35, respectively, with K-shell electron bindittg energies of 37, 5, and 12keV, respectively.>ManyComptonandphotoelectricx-rayinteractionsoccurwithouter-shellelectrons,sending them into an excited, metastable state.DigitalImagingbeganivithcomputedradiography(CT)andMagneticresonanceimaging (MRI).1981,Fujiintroduced digital radiography withthefirstcommercialcomputedradiographyimaging systemMedical imaging is complemented bymultipleforms of DR in addition to CR.Much of the informatian relevant to CR appliesalso toDR becauseCRis a formof DRCR& Screen-film RadiographySIMILARITIES■Both modalities use as the image receptor an x-ray-serisitiveplatethatisencasedinaprotective cassette.■The two techniques can be used interchangeablywith any x-ray imaging system■Bothproducealatentimage,althoughinadifferent formthatmustbemade visible viaprocessingDIFFERENCEScreen-film Radiography, intensifying screen isa scintillator that emits light in response to an x-ray interaction.■CR, the response to x-ray interaction is seen astrapped electrons in a higher energy metastablestate.CONCEPT OF COMPUTED RADIOGRAPHYThe latent image inCR is held on a phosphorplate.

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When these electrons return to the ground state.,visible light is emitted.>SPSs aremechanically stable,electrostaticallyprotectedandfashionedtooptimizetheintensityofstimulatedlightSomeSPSsincorporatephosphorsgrownasLinearfilaments chat enhance the absorption of x-raysand limit the spread of stimulated emission,IMAGING PLATEThe PSP screen is housed in a rugged cassette thatappears similar to a screen-film cassette.*Over time, these metastable electrons return totheground state on their own. However, thisreturn to the ground state can be accelerated orstimulated by exposing the phosphor to intenseinfraredlightfromalaser—hencethetermphotostimulableluminescencefromaphotostimulable phosphor [PSP].*ThePSP.bariumfluorohalide,isfashionedsimilarly to a radiographic intensifying screen.LIGHT STIMULATION-EMISSIONSequence of events engaged in producing a PSL:1.EXPOSEWhen an x-ray beam exposesaPhotostimulablePhosphor[PSP],theenergytransferresultsinexcitationofelectronsintoametastablestate. Approximately 50% oftheseelectronsreturntotheirgroundstateimmediately,resultinginprompt emission of fight2STJMEZLA77OMThe finely focused beamof infrared lightwithabeamdiameter of 50 toIDO pm is directed at thePSP.Aslaserbeamintensityincreases,sodoes the intensity of theemitted signalNote thatasdielaserbeampenetrates,itspreads.TheamountofspreadincreaseswithPSPthickness. Thediameter ofthe spatial resolution of die (STORAGE PHOSPHOR SCREENS [STS]Screens for latent image formation through metastableelectrons.Appearswhitebecause the small PSPpartides scatters liglitexcessively.-calledturbid>PSPparticlesarerandomlypositionedthroughout a binder.e laser beam determinesimaging system

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