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WhiteandPharoah'sOralRadiology
PrinciplesandInterpretation
8THEDITION
SANJAYM.MALLYA,BDS,MDS,PhDDiplomate,AmericanBoardofOralandMaxillofacialRadiology;AssociateProfessorandChairSectionofOralandMaxillofacialRadiologySchoolofDentistryUniversityofCalifornia,LosAngelesLosAngeles,California
ERNESTW.N.LAM,DMD,MSc,PhD,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;ProfessorandtheDr.Lloyd&Mrs.KayChapmanChairinClinicalScience,AssociateDean,GraduateEducationOralandMaxillofacialRadiologyFacultyof
DentistryTheUniversityofTorontoToronto,Ontario,Canada
TableofContents
Coverimage
TitlePage
Copyright
Dedication
Contributors
Preface
AcknowledgmentsPartIFoundations
1Physics
Abstract
CompositionofMatter
NatureofRadiation
X-RayMachine
ProductionofXRays
FactorsControllingtheX-RayBeam
InteractionsofXRaysWithMatter
PhotoelectricAbsorption
Dosimetry
Bibliography
2BiologicEffectsofIonizingRadiation
Abstract
ChemicalandBiochemicalConsequencesofRadiationAbsorption
StochasticandDeterministicEffects
RadiotherapyInvolvingtheOralCavity
Bibliography
3SafetyandProtection
Abstract
SourcesofRadiationExposure
DentomaxillofacialRadiology:RiskandDoses
ImplementingRadiationProtection
Bibliography
PartIIImaging
4DigitalImaging
Abstract
AnalogVersusDigital
DigitalImageReceptors
Solid-StateDetectors
PhotostimulablePhosphor
DigitalDetectorCharacteristics
DigitalImageViewing
HardCopies
ImageProcessing
ImageAnalysis
ImageStorage
SystemsCompatibility
ClinicalConsiderations
Conclusion
Bibliography
5FilmImaging
Abstract
X-RayFilm
IntensifyingScreens
FormationoftheLatentImage
ProcessingSolutions
DarkroomandEquipment
Darkroom
ManualProcessingProcedures
Rapid-ProcessingChemicals
ChangingSolutions
AutomaticFilmProcessing
EstablishingCorrectExposureTimes
ManagementofRadiographicWastes
ImageCharacteristics
CommonCausesofFaultyRadiographs
MountingRadiographs
DuplicatingRadiographs
Bibliography
6ProjectionGeometry
Abstract
ImageSharpnessandResolution
ImageSizeDistortion
ImageShapeDistortion
ParallelingandBisecting-AngleTechniques
ObjectLocalization
EggshellEffect
Bibliography
7IntraoralProjections
Abstract
CriteriaofQuality
PeriapicalRadiography
BitewingRadiography
OcclusalRadiography
ImagingofChildren
MobileIntraoralRadiography
SpecialConsiderations
Bibliography
8CephalometricandSkullImaging
Abstract
SelectionCriteria
Technique
EvaluationoftheImage
CephalometricProjections
CraniofacialandSkullProjections
Conclusion
Bibliography
9PanoramicImaging
Abstract
PrinciplesofPanoramicImageFormation
PatientPositioningandHeadAlignment
ImageReceptors
PanoramicFilmDarkroomTechniques
InterpretingPanoramicImages
Bibliography
10ConeBeamComputedTomography
Abstract
PrinciplesofConeBeamComputedTomographicImaging
ComponentsofImageProduction
ClinicalConsiderations
ImageArtifacts
StrengthsandLimitations
Conclusions
Bibliography
11ConeBeamComputedTomography
Abstract
StagesinVolumetricDataDisplay
InterpretiveReport
Task-SpecificApplications
Conclusion
Bibliography
12RadiographicAnatomy
Abstract
GeneralPrinciplesofRadiologicEvaluation
Teeth
SupportingDentoalveolarStructures
MaxillaandMidfacialBones
Mandible
TemporomandibularJoint
BaseoftheSkull
Airway
RestorativeMaterials
Bibliography
13OtherImagingModalities
Abstract
MultidetectorComputedTomography
ComputedTomographicScanners
MagneticResonanceImaging
NuclearMedicine
Bibliography
14BeyondThree-DimensionalImaging
Abstract
Four-DimensionalImaging
Computer-GuidedTreatmentPlanning
Three-DimensionalPrinting
Bibliography
15DentalImplants
Abstract
ImagingTechniques
PreoperativeAssessmentandTreatmentPlanning
IntraoperativeImaging
Image-GuidedApplications
PostoperativeImagingandMonitoring
Bibliography
16QualityAssuranceandInfectionControl
Abstract
RadiographicQualityAssurance
InfectionControl
Bibliography
17PrescribingDiagnosticImaging
Abstract
RadiologicExaminations
GuidelinesforOrderingImaging
ImagingConsiderationsintheAbsenceofaPositiveFinding
SpecialConsiderations
ExamplesofUseoftheGuidelines
Bibliography
PartIIIInterpretation
18PrinciplesofRadiographicInterpretation
Abstract
AdequateDiagnosticImages
VisualSearchStrategies
DiagnosticReasoninginOralRadiology
AnalysisofAbnormalFindings
AnalyticorSystematicStrategy
WritingaDiagnosticImagingReport
Self-Test
Bibliography
19DentalCaries
Abstract
DiseaseMechanism
RoleofImagingintheDetectionofCariousLesions
ExaminationWithDigitalIntraoralSensors
ExaminationWithConventionalIntraoralFilm
DetectionofCariousLesions
AlternativeDiagnosticToolstoDetectDentalCaries
TreatmentConsiderations
Bibliography
SuggestedReadings
20PeriodontalDiseases
Abstract
DiseaseMechanism
AssessmentofPeriodontalDisease
ImagingModalitiesfortheAssessmentofPeriodontalDisease
AppearanceofNormalAnatomy
ImagingFeaturesofPeriodontalDiseases
ClassificationofthePeriodontalDiseases
OtherConditionsAffectingthePeriodontium
OtherModifiersofPeriodontalDisease
EvaluationofPeriodontalTherapy
DifferentialIntepretation
Bibliography
21DentalAnomalies
Abstract
DevelopmentalAbnormalities
AcquiredAbnormalities
Bibliography
22InflammatoryConditionsoftheJaws
Abstract
PeriapicalInflammatoryDisease
Osteomyelitis
Radiation-InducedChangestotheJaws
Medication-RelatedOsteonecrosisoftheJaws
DiagnosticImagingofSoftTissueInvolvement
Pericoronitis
Bibliography
23Cysts
Abstract
DiseaseMechanism
ClinicalFeatures
AppliedDiagnosticImaging
ImagingFeatures
OdontogenicCysts
NonodontogenicCysts
Pseudocysts
Healing
MandibularLingualBoneDepression
CystsOriginatinginSoftTissues
References
24BenignTumorsandNeoplasms
Abstract
DiseaseMechanism
ClinicalFeatures
AppliedDiagnosticImaging
ImagingFeatures
OdontogenicTumorsandNeoplasms
OdontogenicEpithelialNeoplasms
MixedEpithelialandMesenchymalOdontogenicTumorsandNeoplasms
MesenchymalOdontogenicTumors
NonodontogenicTumorsandNeoplasms
MesenchymalTumorsandNeoplasms
References
25DiseasesAffectingtheStructureofBone
Abstract
DiseaseMechanism
AppliedDiagnosticImaging
MetabolicBoneAbnormalities
Bibliography
26MalignantNeoplasms
Abstract
DiseaseMechanism
ClinicalFeatures
AppliedDiagnosticImaging
ImagingFeatures
Carcinomas
MetastaticDisease
Sarcomas
MalignanciesoftheHematopoieticSystem
OralandMaxillofacialImagingforCancerSurvivors
Bibliography
27Trauma
Abstract
AppliedRadiology
DentoalveolarTrauma
DentalFractures
PeriodontalTissueInjury
AlveolarProcessInjury
TraumaticInjuriestotheFacialBones
MonitoringtheHealingofFractures
Bibliography
28ParanasalSinusDiseases
Abstract
NormalDevelopmentandVariations
DiseasesAssociatedWiththeParanasalSinuses
IntrinsicDiseasesoftheParanasalSinuses
ExtrinsicDiseasesInvolvingtheParanasalSinuses
Bibliography
29CraniofacialAnomalies
Abstract
CleftLipandPalate
CraniofacialDysostosis(CrouzonSyndrome)
HemifacialMicrosomia
MandibulofacialDysostosis(TreacherCollinsSyndrome)
CleidocranialDysplasia
HemifacialHyperplasia
SegmentalOdontomaxillaryDysplasia
Bibliography
30TemporomandibularJointAbnormalities
Abstract
DiseaseMechanism
ClinicalFeatures
ImagingAnatomyoftheTemporomandibularJoint
ApplicationofDiagnosticImaging
TemporomandibularJointImagingModalities
AbnormalitiesoftheTemporomandibularJoint
Bibliography
31SoftTissueCalcificationsandOssifications
Abstract
HeterotopicCalcifications
HeterotopicOssifications
Bibliography
32SalivaryGlandDiseases
Abstract
SalivaryGlandDisease
DiagnosticImaging
ProjectionImaging
HighResolution
ConditionsAffectingtheSalivaryGlands
Space-OccupyingConditions
Bibliography
PartIVOtherApplications
33Forensics
Abstract
ScopeofForensicsinDentistry
NeedforIdentificationofHumanRemains
MethodsofBodyIdentification
UtilityofOralandMaxillofacialRadiologyforBodyIdentification
IdentificationofaSingleBody
RadiologicTechniquesinBodyIdentification
ForensicDentalIdentificationReport
ApplicationsofRadiologicImaginginMassDisasters
ApplicationofRadiologicImagingtoLong-TermUnidentifiedRemains
Bibliography
Index
Copyright
3251RiverportLaneSt.Louis,Missouri63043
WHITEANDPHAROAH'SORALRADIOLOGY,EIGHTHEDITIONISBN:978-0-323-54383-5Copyright©2019byElsevier,Inc.Allrightsreserved.
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NoticesKnowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroadenourunderstanding,changesinresearchmethods,professionalpractices,ormedicaltreatmentmaybecomenecessary.Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgeinevaluatingandusinganyinformation,methods,compounds,orexperimentsdescribedherein.Inusingsuchinformationormethodstheyshouldbemindfuloftheirownsafetyandthesafetyofothers,includingpartiesforwhomtheyhaveaprofessionalresponsibility.
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PrintedinChina
Lastdigitistheprintnumber:987654321
Dedication
Toourteachersandmentors,andourstudents,bothpastandpresent.
Contributors
MariamT.BaghdadyBDS,MSc,PhD,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;AssistantProfessor,OralandMaxillofacialRadiologyDepartmentofDiagnosticSciencesFacultyofDentistryKuwaitUniversitySafat,Kuwait;AssistantProfessor(affiliate)OralandMaxillofacialRadiologyFacultyofDentistryUniversityofTorontoToronto,Ontario,CanadaLaurieC.CarterDDS,PhDProfessorandDirectorofOralandMaxillofacialRadiology,Director,AdvancedDentalEducationDepartmentofOralDiagnosticSciencesSchoolofDentistryVirginiaCommonwealthUniversityRichmond,VirginiaEdwinChangDDS,MSc,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;OralandMaxillofacialRadiologyFacultyofDentistryUniversityofTorontoToronto,Ontario,CanadaFatimaM.JaduBDS,MSc,PhD,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;AssociateProfessorDepartmentofOralRadiologyKingAbdulazizUniversityFacultyofDentistryJeddah,SaudiArabiaErnestW.N.LamDMD,MSc,PhD,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;
ProfessorandtheDr.Lloyd&Mrs.KayChapmanChairinClinicalScience,AssociateDean,ClinicalEducationOralandMaxillofacialRadiologyFacultyofDentistryTheUniversityofTorontoToronto,Ontario,CanadaSanjayM.MallyaBDS,MDS,PhDDiplomate,AmericanBoardofOralandMaxillofacialRadiology;AssociateProfessorandChairSectionofOralandMaxillofacialRadiologySchoolofDentistryUniversityofCalifornia,LosAngelesLosAngeles,CaliforniaAndréMolDDS,MS,PhDDiplomate,AmericanBoardofOralandMaxillofacialRadiology;AssociateProfessorDepartmentofDiagnosticSciencesUniversityofNorthCarolinaatChapelHillSchoolofDentistryChapelHill,NorthCarolinaCarolAnneMurdoch-KinchDDS,PhDDiplomate,AmericanBoardofOralandMaxillofacialRadiology;TheDr.WalterH.SwartzProfessorofIntegratedSpecialCareDentistryAssociateDeanforAcademicAffairsSchoolofDentistryUniversityofMichiganAnnArbor,MichiganSusanneE.PerschbacherDDS,MSc,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;AssistantProfessorOralandMaxillofacialRadiologyFacultyofDentistryUniversityofTorontoToronto,Ontario,CanadaAnithaPotluriBDS,DMD,MDscDiplomate,AmericanBoardofOralandMaxillofacialRadiology;AssociateProfessorandChairDepartmentofDiagnosticSciences,DirectorofOralandMaxillofacialRadiologySchoolofDentalMedicineUniversityofPittsburghPittsburgh,PennsylvaniaArunaRameshBDS,DMD,MS
Diplomate,AmericanBoardofOralandMaxillofacialRadiology;ChairandAssociateProfessorDepartmentofDiagnosticSciencesTuftsUniversitySchoolofDentalMedicineBoston,MassachusettsWilliamC.ScarfeBDS,FRACDS,MSDiplomate,AmericanBoardofOralandMaxillofacialRadiology;ProfessorandDirectorDivisionofRadiologyandImagingScience,DepartmentofSurgical/HospitalDentistryUniversityofLouisvilleSchoolofDentistryLouisville,KentuckyAdityaTadinadaDDS,MS,MDSDiplomate,AmericanBoardofOralandMaxillofacialRadiology;AssistantProfessorOralandMaxillofacialRadiologySchoolofDentalMedicineUniversityofConnecticutFarmington,ConnecticutSotiriosTetradisDDS,PhDDiplomate,AmericanBoardofOralandMaxillofacialRadiology;SeniorAssociateDeanandProfessorUCLASchoolofDentistryLosAngeles,CaliforniaDanielP.TurgeonDMD,MSc,FRCD(C)Diplomate,AmericanBoardofOralandMaxillofacialRadiology;AssistantProfessorDépartementdeStomatologieFacultédeMédecineDentaireUniversitédeMontréalMontréal,Quebec,CanadaRobertE.WoodDDS,PhD,FRCD(C)Diplomate,AmericanBoardofForensicOdontology;Head,DepartmentofDentalOncologyPrincessMargaretHospital;AssociateProfessorOralandMaxillofacialRadiologyFacultyofDentistryUniversityofTorontoToronto,Ontario,Canada
Preface
Wetakeonourrolesastheneweditorsofthistextbookwithenthusiasmandenergy.Thepreviousseveneditions,undertheleadershipofProfessorsPaulW.Goaz,StuartC.White,andMichaelJ.Pharoah,presentedthescienceofdiagnosticoralandmaxillofacialradiologytodentalstudentsworldwideforoverthreedecades.Wehopethatourcontributionscontinuethistextbook'straditionofexcellenceandprovideourreaderswithexceptionaleducationalcontentthatiscurrentandscientificallybased.Thebookencompassesthefullscopeoforalandmaxillofacialradiologyforthedentalstudentandservesasacomprehensiveresourceforgraduatestudentsanddentalpractitioners.Radiologicimagingisanintegralcomponentofdiagnosisandtreatment
planningingeneralandspecialtydentalpractices.Dentistshaveaccesstoavarietyofimagingmodalities,eitherintheiroffices,oratimagingcentersandhospitals.Tooptimallyapplydiagnosticimaginginpatientcare,dentistsmustunderstandthebasicprinciplesofradiographicimageformationandinterpretation.Tothisend,thebookprovidesfoundationalknowledge,andrelatedguidelinesandregulationsforthesafeandeffectiveuseofx-rays,aswellasin-depthknowledgeonconventionalandadvancedimagingtechniquesusedtoevaluateoralandmaxillofacialdisease.Thisneweditionalsoprovidesustheopportunitytodiscussthelatestdevelopmentsinourfield.Withadvancesindigitaldentistry,informationfrommultipledigitalsourcesisbeingcombinedtoguidetreatmentplanningortofabricateappliancesandrestorations.Oralandmaxillofacialradiologyoftenformsthebackboneofsuchintegrateddata.Anewchapter—BeyondThree-DimensionalImaging—introducesadvancedapplicationsof3Dimaging,includingadditivemanufacturing.Sincethelastedition,severalprofessionalorganizationshavepublishedimagingguidelines,technicalreportsandpositionstatementsthatimpactthepracticeoforalandmaxillofacialradiology.Thiseditionhasbeenupdatedtoincorporatenewrecommendationsforqualityassuranceandupdatedguidelinesforuseofconebeamcomputedtomographyindentistry.
Dentistsmustbefamiliarwiththekeyradiographicfeaturesofdiseasesofthemaxillofacialregion.Thisbookprovidescomprehensivecoverageofradiographicmanifestationsandthedifferentialinterpretationofdiseasesaffectingtheteeth,jaws,paranasalsinuses,salivaryglands,andtemporomandibularjoints.Thechaptersemphasizethebiologicalfoundationsofdiseaseastheyrelatetotheirradiologicinterpretation.Toenhanceintegrationofbasicandclinicalsciences,weincludeanewchapterthatconsolidatesdiseasesaffectingthestructureofbone.Whereapplicable,radiographicappearancesofdiseaseareillustratedusingnotonlyconventional,2-dimensionalimagingbutalsoadvancedimaging,providingknowledgethatisapplicableingeneralandspecialtydentalpractices.Thebookalsoofferssupplementalresourcestoinstructorsviathecompanion
Evolvewebsite(http://evolve.elsevier.com),includingtestbanksandtheimagecollection.Ourgoalistomakethestudyoforalandmaxillofacialradiologystimulating
andexciting.SanjayM.MallyaBDS,MDS,PhD
ErnestW.N.LamDMD,MSc,PhD,FRCD(C)
Acknowledgments
Wethankourcolleagueswhohavecontributedaschapterauthors.Weappreciatetheirwillingnesstosharetheirexpertiseandknowledgewithourreaders.Thiseditionwelcomesfivenewauthors:Drs.EdwinChang,ArunaRamesh,AnithaPotluri,AdityaTadinada,andDanielTurgeon.
WethankMr.JohnHarveyforcreatingthenewillustrationsofthediseaseprocessesthatmaybefoundinthelatterchaptersofthebook.
WethankDrs.FrenyKarjodkar,MatheusOliveira,andNanditaShenoyforprovidingregion-specificinformationthataddstothebook'sglobalreach.Weappreciatetheeffortofindividualswhoassistedinproofreadingthechaptersduringtheproductionphase:Drs.KatyaArchambault,WilliamBoggess,KaranDharia,AkrivoulaSoundia,HollyVreeburg,MatthewWhiteley,andKayceeWalton.
Weareparticularlythankfultoourcolleaguesandstudents,andourreadersworldwide,whohavecontactedustosuggestimprovementsorwhentheyhaveuncoveredanerror.Amongtheseindividualsare:Drs.MansurAhmad,UlkemAydin,HannahDuong,RumpaGanguly,MohammedHusain,SungKim,ToreLarheim,PeterMah,MohadesehMarkazimoghadam,SusanWhite,MatheusOliviera,andKayceeWalton.
WethankthestaffteamfromElsevierwhosetirelesseffortshelpedkeepthebook'sauthorandeditorialteamontracktomeetproductionmilestones:CarolineDorey-Stein,KathyFalk,JenniferFlynn-Briggs,LuciaGunzel,AlexandraMortimer,RamkumarBashyam,andRachelMcMullen.
Finally,wethankDrs.StuartWhiteandMichaelPharoahforgenerouslysharingtheirvastexperienceastheformereditorsofthisbook.Theirfeedbackandadvicehavebeeninvaluable.
SanjayM.MallyaBDS,MDS,PhD
ErnestW.N.LamDMD,MSc,PhD,FRCD(C)
PARTIFoundations
OUTLINE
1Physics2BiologicEffectsofIonizingRadiation3SafetyandProtection
Physics
SanjayM.Mallya
AbstractThischapterprovidesbasicknowledgeonthenatureofradiation,theoperationofanx-raymachine,andtheinteractionsofx-radiationwithmatter,withanemphasisondiagnosticx-radiation.Thisfoundationalknowledgeisimportantforthesafeandeffectiveuseofx-raysindentistry.
Keywordselectromagneticradiation;x-raymachine;DCx-rayunit;photoelectricabsorption;comptonscatter;bremsstrahlungradiation;kilovoltage;milliamperage;beamfiltration;x-rayattenuation
Oneatomsaystoafriend,“IthinkIlostanelectron.”Thefriendreplies,“Areyousure?”“Yes,”saysthefirstatom,“I'mpositive.”Radiologicexaminationisanintegralcomponentofthedentist'sdiagnosticarmamentarium.Dentistsoftenmakeradiographicimagesofpatientstoobtainadditionalinformationbeyondthatavailablefromaclinicalexaminationortheirpatient'shistory.Informationfromtheseimagesiscombinedwiththeclinicalexaminationandhistorytomakeadiagnosisandformulateanappropriatetreatmentplan.Thischapterprovidesbasicknowledgeonthenatureofradiation,theoperationofanx-raymachine,andtheinteractionsofx-radiationwithmatter,withanemphasisondiagnosticx-radiation.Thisfoundationalknowledgeisimportantforthesafeandeffectiveuseofx-raysindentistry.
CompositionofMatterMatterisanythingthathasmassandoccupiesspace.Theatomisthebasicunitofallmatterandconsistsofanucleuscontainingprotonsandneutrons,andelectronsthatareboundtothenucleusbyelectrostaticforces.Theclassicviewoftheatom,theBohrmodel,considersthestructureofatomslikeasolarsystem,withnegativelychargedelectronsthattravelindiscreteorbitsaroundacentral,positivelychargednucleus(Fig.1.1A).Thecontemporaryview,thequantummechanicalmodel,assignselectronsintocomplexthree-dimensionalorbitalswithenergysublevels(seeFig.1.1B).
FIG.1.1 (A)SchematicviewoftheBohrmodeloftheoxygenatomshowinganucleuswithelectronsthattravelaroundthenucleusincircularorbits.(B)Schematicviewofthequantummechanicalmodeloftheoxygen
atom.Thecentralnucleusissurroundedbyanelectroncloudthatrepresentsprobabilityplotsofthelocationoftheelectroninacomplex
arrangement.
AtomicStructureNucleusInallatomsexcepthydrogen,thenucleusconsistsofpositivelychargedprotonsandneutralneutrons.Ahydrogennucleuscontainsasingleproton.Thenumberofprotonsinthenucleus,itsatomicnumber(Z),isuniquetoeachelement.Eachof118knownelementshasauniqueatomicnumber.Thetotalnumberofprotonsandneutronsinthenucleusofanatomisitsatomicmass(A).Theratioofneutronstoprotonsdeterminesthestabilityofthenucleusandisthebasisofradioactivedecay.
ElectronOrbitalsElectronsarenegativelychargedparticlesthatexistintheextranuclearspaceandareboundtothenucleusbyelectrostaticattraction.TheBohrmodelconsidersthatelectronsexistindiscreteorbitsor“shells”denotedasK,L,M,N,O,andP,withtheK-shellbeingclosesttothenucleus(seeFig.1.1A).Theshellsarealsodescribedbyaquantumnumber1,2,3…,with1beingthequantumnumberfortheK-shell.Eachshellcanholdamaximumof2n2electrons,wherenisthequantumnumberoftheshell.Thequantummechanicalmodeldescribestheelectronswithinthree-
dimensionalorbitals,orelectronclouds(seeFig.1.1B).Theelectronorbitalsaredescribedbasedontheirdistancefromthenucleus(principalquantumnumber ;n=1,2,3…)andtheirshape(designateds,p,d,f,g,h,andi).Onlytwoelectronsmayoccupyanorbital.Theelectronorbitalsinorderoffillingare1s,2s,2p,3s,3p,3d,4s,4p,4d,4f…andsoforth.TheBohrmodelandthequantummechanicalmodelbothprovideanadequatebasistoconceptuallyunderstanddiagnosticx-rayproductionandinteractions.Theenergyneededtoovercometheelectrostaticforcethatbindsanelectron
tothenucleusistermedtheelectronbindingenergy.Theelectronbindingenergyisrelatedtotheatomicnumberandtheorbitaltype.Elementswithalargeatomicnumber(highZ)havemoreprotonsintheirnucleusandthusbindelectronsinanygivenorbitalmoretightlythansmallerZelements.Withinagivenatom,electronsintheinnerorbitalsaremoretightlyboundthanthemoredistantouterorbitals.Electronbindingenergyistheconceptualbasistounderstandionization,whichoccurswhenmatterisexposedtox-rays.
IonizationWhenthenumberofelectronsinanatomisequaltothenumberofprotonsinitsnucleus,theatomiselectricallyneutral.Ifaneutralatomlosesanelectron,itbecomesapositiveion,andthefreeelectronbecomesanegativeion.Thisprocessofforminganionpairistermedionization.Toionizeanatom,sufficientexternalenergymustbeprovidedtoovercometheelectrostaticforces,andfreetheelectronfromthenucleus.High-energyparticles,x-rays,andultravioletradiationhavesufficientenergytodisplaceelectronsfromtheirorbitalsandionizeatoms.Suchradiationsarereferredtoasionizingradiations.Incontrast,visiblelight,infraredandmicrowaveradiations,andradiowavesdonothavesufficientenergytoremoveboundelectronsfromtheirorbitalsandare
nonionizingradiations.
NatureofRadiationRadiationisthetransmissionofenergythroughspaceandmatter.Itmayoccurintwoforms:(1)electromagneticand(2)particulate(Table1.1).Practicalapplicationsoftheseradiationsinhealthcarearelisted.
TABLE1.1ParticulateRadiation
Particle Symbol ElementaryChargea RestMass(amu)Alpha α +2 4.00154Beta+(positron) β+ +1 0.000549Beta−(electron) β− −1 0.000549Electron e− −1 0.000549Neutron n0 0 1.008665Proton p +1 1.007276
aElementarychargeof1equalsthatthechargeofaprotonortheoppositeofanelectron.
amu,Atomicmassunits,where1amu= themassofaneutralcarbon-12atom.
•Diagnosticimagingwithprojectionradiographyandcomputedtomographyusex-rays,acategoryofelectromagneticradiationthatisionizinginnature.•Magneticresonanceimaging(MRI,Chapter13)useselectromagneticradiationsofsignificantlylowerenergiesthanx-raysandatenergiesthatarenonionizing.•Someradiopharmaceuticalsusedindiagnosticnuclearmedicineemitparticulateradiation.Forexample,18F-fluorodeoxyglucose(18F-FDG)emitspositrons,akeystepinimagingwithpositronemissiontomography(PET;Chapter13).
•High-energyelectromagneticradiations(gammarays,γ)andhigh-energyparticulateradiations(electronbeamsandprotons)areusedincancertherapy.
ElectromagneticRadiationElectromagneticradiationisthemovementofenergythroughspaceasacombinationofelectricandmagneticfields.Itisgeneratedwhenthevelocityofanelectricallychargedparticleisaltered.γ-Rays,x-rays,ultravioletrays,visiblelight,infraredradiation(heat),microwaves,andradiowavesallareexamplesofelectromagneticradiation(Fig.1.2).γ-Raysoriginateinthenucleiofradioactiveatoms.Theytypicallyhavegreaterenergythanx-rays.Incontrast,x-raysareproducedoutsidethenucleusandresultfromtheinteractionofelectronswithlargeatomicnuclei,asinx-raymachines.Thehigher-energytypesofradiationintheelectromagneticspectrum—ultravioletrays,x-rays,andγ-rays—arecapableofionizingmatter.Somepropertiesofelectromagneticradiationarebestexplainedbyquantumtheory,whereasothersaremostsuccessfullydescribedbywavetheory.
FIG.1.2 Electromagneticspectrumshowingtherelationshipbetweenphotonwavelengthandenergyandthephysicalpropertiesofvarious
portionsofthespectrum.Photonswithshorterwavelengthshavehigherenergy.Photonsusedindentalradiography(blue)haveenergiesof10to120keV.Magneticresonance(MR)imagingusesradiowaves(orange).IR,
Infraredradiation;UV,ultravioletradiation.
Quantumtheoryconsiderselectromagneticradiationassmalldiscretebundlesofenergycalledphotons.Eachphotontravelsatthespeedoflightandcontainsaspecificamountofenergy,expressedwiththeunitelectronvolt(eV).
Thewavetheoryofelectromagneticradiationmaintainsthatradiationispropagatedintheformofwaves,similartothewavesresultingfromadisturbanceinwater.Suchwavesconsistofelectricandmagneticfieldsorientedinplanesatrightanglestooneanotherthatoscillateperpendiculartothedirectionofmotion(Fig.1.3).Allelectromagneticwavestravelatthevelocityoflight(c=3.0×108m/s)inavacuum.Wavesaredescribedintermsoftheirwavelength(λ,meters)andfrequency(ν,cyclespersecond,hertz).
FIG.1.3 Electricandmagneticfieldsassociatedwithelectromagneticradiation.
Boththeoriesareusedtodescribepropertiesofelectromagneticradiation.Quantumtheoryhasbeensuccessfulincorrelatingexperimentaldataontheinteractionofradiationwithatoms,thephotoelectriceffect,andtheproductionofx-rays.Wavetheoryismoreusefulforconsideringradiationinbulkwhenmillionsofquantaarebeingexamined,asinexperimentsdealingwithrefraction,reflection,diffraction,interference,andpolarization.Consideringthevalueofboththeoriestounderstandthepropertiesofelectromagneticradiationenergy,wavelength,andfrequencyareallusedtodescribetheseradiations.Inpracticaluse,high-energyphotonssuchasx-raysandγ-raysaretypicallycharacterizedbytheirenergy(eVs),medium-energyphotons(e.g.,visiblelightandultravioletwaves)aretypicallycharacterizedbytheirwavelength(nanometers),andlow-energyphotons(e.g.,AMandFMradiowaves)aretypicallycharacterizedbytheirfrequency(KHzandMHz).Box1.1showstherelationshipsbetweenphotonenergy,wavelength,and
frequency.
Box1.1
RelationshipBetweenEnergy(E)andWavelength(λ)ofElectromagneticRadiation
simplifiedas
Eisenergy(kiloelectronvolts,keV)histhePlanckconstant(6.626×10−34joule-secondsor4.13×10−15eV-s)cisthevelocityoflight=3×108m/sλiswavelength(nanometers,nm)
Keypoint:Inverserelationshipbetweenenergyandwavelengthofanelectromagneticradiation
ParticulateRadiationSmallatomshaveapproximatelyequalnumbersofprotonsandneutrons,whereaslargeratomstendtohavemoreneutronsthanprotons.Largeratomsareunstablebecauseoftheunequaldistributionofprotonsandneutrons,andtheymaybreakup,releasingα(alpha)orβ(beta)particlesorγ(gamma)rays.Thisprocessiscalledradioactivity.Whenaradioactiveatomreleasesanαoraβparticle,theatomistransmutedintoanotherelement.Anothertypeofradioactivityisγdecay,producingγ-rays.Theyresultaspartofadecaychainwhereanucleusconvertsfromanexcitedstatetoalowerlevelgroundstate;thisoftenhappensafteranucleusemitsanαorβparticleorafternuclearfissionorfusion.Examplesofradioactivedecaythatareimportantinhealthcarearelisted.
•Anunstableatomwithanexcessofprotonsmaydecaybyconvertingaprotonintoaneutron,aβ+particle(positron),andaneutrino.Positronsquicklyannihilatewithelectronstoformtwoγ-rays.ThisreactionisthebasisforPETimaging(seeChapter
13).•Anunstableatomwithanexcessofneutronsmaydecaybyconvertinganeutronintoaproton,aβ−particle,andaneutrino.β−particlesareidenticaltoelectrons.High-speedβ−particlesareabletopenetrateupto1.5cmintissue.β−particlesfromradioactiveiodine-131areusedfortreatmentofsomethyroidcancers.•αparticlesareheliumnucleiconsistingoftwoprotonsandtwoneutrons.Theyresultfromtheradioactivedecayofmanylargeatomicnumberelements.Becauseoftheirdoublepositivechargeandheavymass,αparticlesdenselyionizematterthroughwhichtheypassandpenetrateonlyafewmicrometersofbodytissues.Thislimitedrangehasprompteduseofalphaemitterssuchasradium-223intargetedradiationtherapyforbonemetastasis.
Thecapacityofparticulateradiationtoionizeatomsdependsonitsmass,velocity,andcharge.Therateoflossofenergyfromaparticleasitmovesalongitstrackthroughmatter(tissue)isitslinearenergytransfer(LET).Thegreaterthephysicalsizeoftheparticle,thehigheritscharge,andtheloweritsvelocity,thegreateritsLET.Forexample,αparticles,withtheirhighmasscomparedwithanelectron,highcharge,andlowvelocity,aredenselyionizing,losetheirkineticenergyrapidly,andhaveahighLET.β−particlesaremuchlessdenselyionizingbecauseoftheirlightermassandlowercharge;theyhavealowerLET.HighLETradiationsconcentratetheirionizationalongashortpath,whereaslowLETradiationsproduceionpairsmuchmoresparselyoveralongerpathlength.
X-RayMachineX-raymachinesproducex-raysthatpassthroughapatient'stissuesandstrikeadigitalreceptororfilmtomakearadiographicimage.Theprimarycomponentsofanx-raymachinearethex-raytubeanditspowersupply,positionedwithinthetubehead.Forintraoralx-rayunits,thetubeheadistypicallysupportedbyanarmthatisusuallymountedonawall(Fig.1.4).Acontrolpanelallowstheoperatortoadjustthedurationoftheexposure,andoftentheenergyandexposurerate,ofthex-raybeam.Anelectricalinsulatingmaterial,usuallyoil,surroundsthetubeandtransformers.Often,thetubeisrecessedwithinthetubeheadtoincreasethesource-to-objectdistanceandminimizedistortion(Fig.1.5;alsoseeChapter6).
FIG.1.4 Exampleofanintraoralwall-mountedx-rayunit,thePlanmecaProX.(CourtesyPlanmecaUSA,Inc.Roselle,Illinois.)
FIG.1.5 Tubeheadshowingarecessedx-raytube,componentsofthepowersupply,andoilthatconductsheatawayfromthex-raytube.Pathofusefulx-raybeam(blue)fromtheanode,throughtheglasswallofthex-raytube,oil,andfinallyanaluminumfilter.Thebeamsizeisrestrictedbythemetaltubehousingandcollimator.Low-energyphotonsarepreferentially
removedbythealuminumfilter.
X-RayTubeAnx-raytubeiscomposedofacathodeandananodesituatedwithinanevacuatedglassenvelopeortube(Fig.1.6).Toproducex-rays,electronsstreamfromthefilamentinthecathodetothetargetintheanode,wheretheenergyfromsomeoftheelectronsisconvertedintox-rays.
FIG.1.6 X-raytubewiththemajorcomponentslabeled.Thepathoftheelectronbeamisshowninyellow.X-raysproducedatthetargettravelinall
directions.Theusefulx-raybeamisshowninblue.
CathodeThecathode(Figs.1.7Band1.8)inanx-raytubeconsistsofafilamentandafocusingcup.Thefilamentisthesourceofelectronswithinthex-raytube.Itisacoiloftungstenwireapproximately2mmindiameterand1cmorlessinlength.Filamentstypicallycontainapproximately1%thorium,whichgreatlyincreasesthereleaseofelectronsfromtheheatedwire.Thefilamentisheatedtoincandescencewithalow-voltagesourceandemitselectronsatarateproportionaltothetemperatureofthefilament.
FIG.1.7 (A)Dentalstationaryx-raytubewithcathodeonleftandcopperanodeonright.(B)Focusingcupcontainingafilament(arrow)inthe
cathode.(C)Copperanodewithtungsteninset.Notetheelongatedactualfocalspotarea(arrow)onthetungstentargetoftheanode.([B]and[C],
CourtesyJohnDeArmond,TellicoPlains,Tennessee.)
FIG.1.8 Theangleofthetargettothecentralrayofthex-raybeamhasastronginfluenceontheapparentsizeofthefocalspot.Theprojected
effectivefocalspot(seenbelowthetarget)ismuchsmallerthantheactualfocalspotsize(projectedtotheleft).Thisprovidesabeamthathasasmall
effectivefocalspotsizetoproduceimageswithhighresolution,whileallowingforheatgeneratedattheanodetobedissipatedoverthelarger
area.
Thefilamentliesinafocusingcup(seeFig.1.7B;seealsoFig.1.8),anegativelychargedconcavemolybdenumbowl.Theparabolicshapeofthefocusingcupelectrostaticallyfocusestheelectronsemittedbythefilamentintoanarrowbeamdirectedatasmallrectangularareaontheanodecalledthefocalspot(seeFigs.1.7Cand1.8).Theelectronsmovetothefocalspotbecausetheyarebothrepelledbythenegativelychargedcathodeandattractedtothepositivelychargedanode.Thex-raytubeisevacuatedtopreventcollisionofthefast-movingelectronswithgasmolecules,whichwouldsignificantlyreducetheirspeed.Thevacuumalsopreventsoxidation,or“burnout,”ofthefilament.
AnodeTheanodeinanx-raytubeconsistsofatungstentargetembeddedinacopperstem(seeFigs.1.6and1.7C).Thepurposeofthetargetinanx-raytubeistoconvertthekineticenergyofthecollidingelectronsintox-rayphotons.Theconversionofthekineticenergyoftheelectronsintox-rayphotonsisaninefficientprocess,withmorethan99%oftheelectronkineticenergyconvertedtoheat.Thetargetismadeoftungsten,anelementthathasseveralcharacteristicsof
anidealtargetmaterial,includingthefollowing:
•Highatomicnumber(74),allowsforefficientx-rayproduction.•Highmeltingpoint(3422°C),towithstandheatproducedduringx-rayproduction.•Highthermalconductivity(173Wm−1K−1),todissipatetheheatproducedawayfromthetarget.•Lowvaporpressureattheworkingtemperaturesofanx-raytube,tohelpmaintainvacuuminthetubeathighoperatingtemperatures.
Thetungstentargetistypicallyembeddedinalargeblockofcopperwhichfunctionsasathermalconductortoremoveheatfromthetungsten,reducingtheriskofthetargetmelting.Thefocalspotistheareaonthetargettowhichthefocusingcupdirectsthe
electronsandfromwhichx-raysareproduced.Thesizeofthefocalspotisanimportanttechnicalparameterofimagequality—asmallerfocalspotyieldsasharperimage(seeChapter6).Alimitationtoreducingfocalspotsizeistheheatgenerated.Toovercomethislimitation,x-raytubesuseoneofthetwoanodeconfigurations.Stationaryanode:Inthisconfiguration,thetargetisplacedatanangletothe
electronbeam(seeFig.1.8).Typically,thetargetisinclinedapproximately20degreestothecentralrayofthex-raybeam.Whenviewedthroughtheaimingring,theareafromwhichthephotonsoftheusefulx-raybeamoriginateappearssmaller,makingtheeffectivefocalspotsmallerthantheactualfocalspotsize.Thisallowsproductionofx-raysfromalargerarea,allowingbetterheatdistributionwhilemaintainingtheimagequalitybenefitsofasmallfocalspot.IntheexampleshowninFig.1.8,theeffectivefocalspotisapproximately1mm×1mm,asopposedtotheactualfocalspot,whichisapproximately1mm×3mm.Thissmallereffectivefocalspotresultsinasmallapparentsourceofx-raysandanincreaseinthesharpnessoftheimage(seeFigs.6.1and6.2),withalargeractualfocalspotsizetoimproveheatdissipation.
Rotatinganode:Inthisdesign,thetungstentargetisintheformofabeveleddiskthatrotatesduringtheperiodofx-rayproduction(Fig.1.9).Asaresult,theelectronsstrikesuccessiveareasofthetargetdisk,distributingtheheatoverthisextendedareaofthedisk.However,atanygiventime,x-raysareproducedfromasmallspotonthetarget.X-raytubeswithrotatinganodecanbeusedwithlongerexposuresandwithhighertubecurrentsof100to500milliamperes(mA),whichis10to50timesthatpossiblewithstationarytargets.Thetargetandrotor(armature)ofthemotorliewithinthex-raytube,andthestatorcoils(whichdrivetherotoratapproximately3000revolutionsperminute)lieoutsidethetube.Suchrotatinganodesarenotusedinintraoraldentalx-raymachinesbutareoccasionallyusedincephalometricunits;areusuallyusedinconebeammachines;andarealwaysusedinmultidetectorcomputedtomographyx-raymachines,whichrequirehighradiationoutputforlonger,sustainedexposures.
FIG.1.9 X-raytubewitharotatinganodeallowsheatatthefocalspottospreadoutoveralargesurfacearea(darkband).Currentappliedtothestatorinducesrapidrotationoftherotorandtheanode.Thepathoftheelectronbeamisshowninyellow,andtheusefulx-raybeamisshownin
blue.
PowerSupplyThex-raytubeandtwotransformersliewithinanelectricallygroundedmetalhousingcalledtheheadofthex-raymachine.Theprimaryfunctionsofthepowersupplytransformersofanx-raymachineareto:
•Providealow-voltagecurrenttoheatthex-raytubefilament(Fig.1.10,filamenttransformer).
FIG.1.10 Schematicofdentalx-raymachinecircuitryandx-raytubewiththemajorcomponentslabeled.Theoperatorselectsthe
desiredkVpfromtheautotransformer.Thevoltageisgreatlyincreasedbythehigh-voltagestep-uptransformerandappliedtothex-raytube.ThekVpdialmeasuresthevoltageonthelow-voltagesideofthetransformerbutisscaledtodisplaythecorrespondingvoltageinthetubecircuit.Thetimerclosesthetubecircuitforthedesiredexposuretimeinterval.ThemAdialmeasuresthecurrentflowingthroughthetubecircuit.Thefilamentcircuitheatsthe
cathodefilamentandisregulatedbythemAselector.AC,Alternatecurrent.
•Generateahighpotentialdifferencetoaccelerateelectronsfromthecathodetothefocalspotontheanode(seeFig.1.10,high-voltagetransformer).
X-RayTubeControlsTubeCurrent(Milliamperes,mA)Duringx-rayproduction,electronsproducedatthefilamentareattractedtotheanode.Thisflowofelectronsfromthecathodetotheanodegeneratesacurrentacrossthex-raytubeandiscalledthetubecurrent.Themagnitudeofthiscurrentisregulatedbythemilliamperecontrol(seeFig.1.10,mAselector),which
adjuststheresistanceandthecurrentflowthroughthefilament,therebyregulatingthenumberofelectronsproduced.Formanyintraoraldentalx-rayunits,themAsettingisfixed,typicallyat7to10mA.SomeunitsoffertheflexibilityofaselectionofmAsettings,rangingfrom2to10mA.
TubeVoltage(Kilovoltage,kV)Ahighvoltageisrequiredbetweentheanodeandcathodetogiveelectronssufficientenergytogeneratex-rays.Thekilovoltpeak(kVp)selectoradjuststhehigh-voltagetransformertoboostthepeakvoltageoftheincominglinecurrent(110or220V).Typically,intraoral,panoramic,andcephalometricmachinesoperatebetween50and90kVp(50,000to90,000V),whereascomputedtomographicmachinesoperateat90to120kVp,andhigher.AlternatingCurrentX-rayGenerators:Foranincominglinewithalternating
current(AC),thepolarityofthelinecurrentalternates(60cyclespersecondinNorthAmerica;Fig.1.11A),andthepolarityofthex-raytubealternatesatthesamefrequency(seeFig.1.11B).Whenthepolarityofthevoltageappliedacrossthetubecausesthetargetanodetobepositiveandthefilamenttobenegative,theelectronsaroundthefilamentacceleratetowardthepositivetarget,andx-raysareproduced(seeFig.1.11C).Whenthevoltageacrossthecathodeandanodeishighest,theefficiencyofx-rayproductionishighest,andthustheintensityofx-raypulsespeaksatthecenterofeachcycle(seeFig.1.11C).
FIG.1.11 (A)Incomingalternatecurrentlinevoltage(110V,60cyclespersecondinthiscase).(B)VoltageattheanodevariesfromzerouptothekVpsetting(70kVpinthiscase).(C)Theintensityofradiationproducedattheanode(blue)isstronglydependentontheanodevoltageandishighestwhenthetubevoltageisatitspeak.(D)Incomingconstantpotential(110Vinthiscase)thatismaintainedthroughtheoperationcycle.(E)VoltageattheanodevariesfromzerouptothekVpsetting(70kVpinthiscase).Notethattheincreaseanddecreaseofthepotentialdifferenceatthestartandendofthecycleisrapid.Theintensityofradiationproducedattheanode(blue)ishigherwithconsiderablylessheterogeneityofphotonenergy.
(ModifiedfromJohnsHE,CunninghamJR.ThePhysicsofRadiology.3rded.Springfield,IL:CharlesCThomas;1974.)
Duringthefollowinghalf(ornegativehalf)ofeachcycle,thefilamentbecomespositive,andthetargetbecomesnegative(seeFig.1.11B).Atthese
times,theelectronsdonotflowacrossthegapbetweenthetwoelementsofthetube,andnox-raysaregenerated.Whenanx-raytubeispoweredwith60-cycleAC,60pulsesofx-raysaregeneratedeachsecond,eachhavingadurationofsecond.Thus,whenusingapowersupplywithAC,x-rayproductionislimitedtohalftheACcycle.Suchx-rayunitsarereferredtoasself-rectifiedorhalf-waverectified.Manyconventionaldentalx-raymachinesareself-rectified.ConstantPotential(DirectCurrent)X-rayGenerators:Somedentalx-ray
manufacturersproducemachinesthatreplacetheconventional60-cycleAC,half-waverectifiedpowersupplywithahigh-frequencypowersupplythatprovidesanalmostdirectcurrent(seeFig.1.11D).Thisresultsinanessentiallyconstantpotentialbetweentheanodeandcathode(seeFig.1.11E),andx-raysareproducedthroughtheentirecycle.Thisalmostconstantvoltageyieldsx-rayswithanarrowspectrumofenergies,andthemeanenergyofthex-raybeamproducedbythesex-raymachinesishigherthanthemeanenergyfromaconventionalhalf-waverectifiedmachineoperatedatthesamevoltage.Practicalimplicationswiththeuseofconstantpotentialintraoralx-rayunits
areasfollows:
•Becausex-rayproductionoccursduringtheentirevoltagecycle,constantpotentialunitsrequireshorterexposuretimestoproducethesamenumberofx-rayphotons,minimizingpatientmotion.•Theintensityofx-rayphotonsproducedismoreconsistentandreliable,especiallywithshortexposuretimes.Thisisofpracticalimportancewhenusingdigitalreceptorsthatrequirelessradiation.•WhenoperatedatthesamekVp,thex-raybeamproducedbyconstantpotentialunitshasahighermeanenergy,whichdecreasesradiographicimagecontrast.Tooffsetthiseffect,constantpotentialx-rayunitsaretypicallyoperatedataslightlylowerkVp,
typically60to65kVp.•Thenarrowerspectrumofenergies,withfewerlower-energyphotons,lowersthepatientradiationdoseby35%to40%,comparedwithconventionalACx-raygenerators.
TimerAtimerisbuiltintothehigh-voltagecircuittocontrolthedurationofthex-rayexposure(seeFig.1.10).Theelectronictimercontrolsthelengthoftimethathighvoltageisappliedtothetubeandthusthetimeduringwhichx-raysareproduced.However,beforethehighvoltageisappliedacrossthetube,thefilamentmustbebroughttooperatingtemperaturetoensureanadequaterateofelectronemission.Subjectingthefilamenttocontinuousheatingatnormaloperatingcurrentshortensitslife.Tominimizefilamentdamage,thetimingcircuitfirstsendsacurrentthroughthefilamentforapproximatelyhalfasecondtobringittotheproperoperatingtemperatureandthenappliespowertothehigh-voltagecircuit.Insomecircuitdesigns,acontinuouslow-levelcurrentpassingthroughthefilamentmaintainsitatasafelowtemperature,furthershorteningthedelaytopreheatthefilament.Forthesereasons,anx-raymachinemaybeleftoncontinuouslyduringworkinghours.Somex-raymachinetimersdisplaytheexposuretimeinfractionsofasecond.
Insomeintraoralunits,theexposuretimesarepresetfordifferentanatomicareasofthejaws.Insomeunits,theexposuretimeisexpressedasnumberofpulsesinanexposure(e.g.,3,6,9,15).Thenumberofpulsesdividedby60(thefrequencyofthepowersource)givestheexposuretimeinseconds.Asettingof30pulsesmeansthattherewillbe30pulsesofradiation,equivalenttoa0.5-secondexposure(Box1.2).
Box1.2
PracticalApplicationsofExposureControlsInmanyintraoralx-rayunits,themAsetting,
kVpsetting,orbothisfixed.IfthemAsettingisvariable,theoperatorshouldselectthehighestmAvalueavailableandoperatethemachineatthissetting;thisallowstheshortestexposuretimeandminimizesthechanceofpatientmovement.Iftubevoltagecanbeadjustedonanintraoralradiographicunit,theoperator
maychoosetooperateatafixedvoltage,typically65–70kVp.Thisprotocolsimplifiesselectingtheproperpatientexposuresettingsbyusingjustexposuretimeasthemeanstoadjustforanatomiclocationwithinthemouthandpatientsize.ThekVpsettingisoftenusedtocompensateforpatienttissuethickness,
particularlyforpanoramicandcephalometricradiography.Aruleofthumbistovarythesettingby2kVp/cmoftissuethickness.
TubeRatingandDutyCycleX-raytubesproduceheatatthetargetwhileinoperation.Theheatbuildupattheanodeismeasuredinheatunits(HU),whereHU=kVp×mA×seconds.Theheatstoragecapacityforanodesofdentaldiagnostictubesisapproximately20kHU.Heatisremovedfromthetargetbyconductiontothecopperanodeandthentothesurroundingoilandtubehousingandbyconvectiontotheatmosphere.Eachx-raymachinecomeswithatuberatingchartthatdescribesthelongest
exposuretimethetubecanbeenergizedforarangeofvoltages(kVp)andtubecurrent(mA)valueswithoutriskofdamagetothetargetfromoverheating.Thesetuberatingsgenerallydonotrestricttubeuseforintraoralradiography.Dutycyclerelatestothefrequencywithwhichsuccessiveexposurescanbemadewithoutoverheatingtheanode.Theintervalbetweensuccessiveexposuresmustbelongenoughforheatdissipation.Thischaracteristicisafunctionofthesizeoftheanode,theexposurekVpandmA,andthemethodusedtocoolthetube.Adutycycleof1:60indicatesthatonecouldmakea1-secondexposureevery60seconds.
ProductionofXRaysMosthigh-speedelectronstravelingfromthefilamenttothetargetinteractwithtargetelectronsandreleasetheirenergyasheat.Occasionally,theelectron'skineticenergyisconvertedintox-rayphotonsbytheformationofbremsstrahlungradiationandcharacteristicradiation.
BremsstrahlungRadiationBremsstrahlungphotonsaretheprimarysourceofradiationfromanx-raytube.Bremsstrahlungmeans“brakingradiation”inGerman,andthesephotonsareproducedbythesuddenstoppingorslowingofhigh-speedelectronsbytungstennucleiinthetargetasfollows:Mosthigh-speedelectronspassbytungstennucleiwithnearorwidemisses(Fig.1.12A).Intheseinteractions,theelectronisattractedtowardthepositivelychargednuclei,itspathisalteredtowardthenucleus,anditlosessomeofitsvelocity.Thisdecelerationcausestheelectrontolosekineticenergythatisgivenoffintheformofx-rayphotons.Thecloserthehigh-speedelectronapproachesthenuclei,thegreatertheelectrostaticattractionbetweenthenucleusandtheelectron,andtheresultingbremsstrahlungphotonshavehigherenergy.Theefficiencyofthisprocessisproportionaltothesquareoftheatomicnumberofthetarget;highZmetalsaremoreeffectiveindeflectingthepathoftheincidentelectrons,andthisisthebasisforselectionoftungsten(Z=74)asatargetmaterial.
FIG.1.12 Bremsstrahlungradiationisproducedmostoftenbythepassageofanelectronnearanucleus,whichresultsinelectronsbeingdeflectedanddecelerated(A)or,lessfrequently,bythedirecthitofan
electrononanucleusinthetarget(B).Forthesakeofclarity,thisdiagram
andothersimilarfiguresinthischaptershowonlythe1s,2s,or3sorbitals.
Occasionally,electronsfromthefilamentdirectlyhitthenucleusofatargetatom.Whenthishappens,allthekineticenergyoftheelectronistransformedintoasinglex-rayphoton(seeFig.1.12B).Theenergyoftheresultantphoton(inkeV)isnumericallyequaltotheenergyoftheelectron(i.e.,thevoltageappliedacrossthex-raytubeatthatinstant).Bremsstrahlunginteractionsgeneratex-rayphotonswithacontinuous
spectrumofenergy.Theenergyofanx-raybeamisusuallydescribedbyidentifyingthepeakoperatingvoltage(inkVp).Forexample,adentalx-raymachineoperatingatapeakvoltageof70kVpappliesavoltageofupto70kVpacrossthetube.Thistubethereforeproducesacontinuousspectrumofx-rayphotonswithenergiesrangingtoamaximumof70keV(Fig.1.13).Thereasonsforthiscontinuousspectrumareasfollows:
FIG.1.13 Spectrumofphotonsemittedfromanx-raymachineoperatingat70kVp.Thevastpreponderanceofradiationisbremsstrahlung(blue),
withaminoradditionofcharacteristicradiation.
•Thecontinuouslyvaryingvoltagedifferencebetweenthetargetandfilamentcausestheelectronsstrikingthetargettohavevaryinglevelsofkineticenergy.•Thebombardingelectronspassatvaryingdistances
aroundtungstennucleiandarethusdeflectedtovaryingextents.Asaresult,theygiveupvaryingamountsofenergyintheformofbremsstrahlungphotons.•Mostelectronsparticipateinmultiplebremsstrahlunginteractionsinthetargetbeforelosingalltheirkineticenergy.Consequently,anelectroncarriesdifferingamountsofenergyaftersuccessiveinteractionswithtungstennuclei.
CharacteristicRadiationCharacteristicradiationcontributesonlyasmallfractionofthephotonsinanx-raybeam.Itismadewhenanincidentelectronejectsaninnerelectronfromthetungstenatom.Whenthishappens,anelectronfromanouterorbitalisquicklyattractedtothevoidinthedeficientinnerorbital(Fig.1.14).Whentheouterorbitalelectronreplacesthedisplacedelectron,aphotonisemittedwithenergyequivalenttothedifferenceinthebindingenergiesofthetwoorbitals.Theenergiesofcharacteristicphotonsarediscretebecausetheyrepresentthedifferenceoftheenergylevelsofspecificelectronorbitalsandarecharacteristicofthetargetatoms.Theproductionofcharacteristicradiationhasnopracticalimplicationsfordentomaxillofacialradiography.