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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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.