A2Physics物理出國(guó)英語(yǔ)MedicalImagingPart2Xrayimaging

1.WhatistheprinciplebehindXrayproductioninanXraytube?

XraysareproducedinanXraytubethroughaprocesscalledbremsstrahlungandcharacteristicXrayemission.InanXraytube,aheatedcathodeemitselectronsthroughthermionicemission.Theseelectronsareacceleratedtowardsapositivelychargedanodebyahighvoltageappliedbetweenthecathodeandtheanode.Whenthehighspeedelectronsstriketheanodetarget,twomainprocessesoccur.

Bremsstrahlung:Astheelectronsaredeflectedbythestrongelectricfieldsoftheatomicnucleiintheanodematerial,theydeceleraterapidly.Accordingtothelawsofelectromagnetism,whenachargedparticleacceleratesordecelerates,itemitselectromagneticradiation.Inthiscase,thedecelerationofelectronsresultsintheemissionofacontinuousspectrumofXrayswitharangeofenergies.

CharacteristicXrayemission:Ifanelectronwithsufficientenergyknocksoutaninnershellelectronfromanatomintheanodematerial,anelectronfromahigherenergylevelwillfillthevacancy.TheenergydifferencebetweenthetwolevelsisemittedasacharacteristicXrayphotonwithaspecificenergycorrespondingtotheenergylevelsinvolvedinthetransition.

2.HowdoestheintensityofXraysvarywiththetubecurrentinanXraytube?

TheintensityofXraysisdirectlyproportionaltothetubecurrent.ThetubecurrentistheflowofelectronsfromthecathodetotheanodeintheXraytube.Whenthetubecurrentisincreased,moreelectronsareemittedfromthecathodeperunittime.SinceeachelectronthatstrikestheanodecanpotentiallycontributetotheproductionofXrays(eitherthroughbremsstrahlungorcharacteristicXrayemission),agreaternumberofelectronshittingtheanodemeansmoreXrayphotonsareproduced.So,ifthetubecurrentisdoubled,thenumberofelectronshittingtheanodepersecondisdoubled,andtheintensityoftheXraybeam(thenumberofXrayphotonsperunitareaperunittime)isalsoapproximatelydoubled.

3.Explaintheconceptofhalfvaluelayer(HVL)inXrayattenuation.

ThehalfvaluelayerisameasureofthepenetratingpowerofanXraybeamandtheattenuationpropertiesofamaterial.WhenanXraybeampassesthroughamaterial,itsintensitydecreasesduetoabsorptionandscattering.ThehalfvaluelayerisdefinedasthethicknessofaparticularmaterialthatreducestheintensityofanXraybeamtohalfofitsoriginalvalue.

Mathematically,if(I_0)istheinitialintensityoftheXraybeamand(I)istheintensityafterpassingthroughathickness(x)ofthematerial,and(x=HVL),then(I=frac{I_0}{2}).DifferentmaterialshavedifferentHVLsforagivenXraybeam.DensematerialssuchasleadhavearelativelysmallHVL,meaningthatathinlayerofleadcansignificantlyreducetheintensityofXrays.Incontrast,lessdensematerialslikeairhaveaverylargeHVL.TheHVLdependsontheenergyoftheXraysandtheatomicnumberanddensityoftheattenuatingmaterial.

4.WhatarethefactorsaffectingthecontrastinanXrayimage?

Differencesintissuedensity:TissueswithdifferentdensitiesattenuateXraysdifferently.Forexample,boneisverydenseandattenuatesXraysstrongly,soitappearswhiteonanXrayimage.SofttissuessuchasmuscleandfathavelowerdensitiesandattenuateXraysless,appearingasshadesofgray.AirfilledcavitieslikethelungsattenuateXraysverylittleandappearblack.

Xrayenergy:TheenergyoftheXraybeamaffectscontrast.LowerenergyXraysaremoreeasilyabsorbedbytissues,resultinginhighercontrastimages.However,theymaynotpenetratethickordensetissueswell.HigherenergyXrayspenetratemorebutcanreducecontrastastheyarelessselectivelyabsorbedbydifferenttissues.

Scatteredradiation:ScatteredXrayscanreducecontrast.WhenXraysinteractwithtissues,somearescatteredindifferentdirections.Thesescatteredrayscanreachthedetectorandaddauniformbackgroundfogtotheimage,makingitmoredifficulttodistinguishbetweendifferenttissues.Antiscattergridsareoftenusedtoreducetheamountofscatteredradiationreachingthedetectorandimprovecontrast.

5.HowareantiscattergridsusedtoimproveXrayimagequality?

AntiscattergridsareplacedbetweenthepatientandtheXraydetector.Theyconsistofaseriesofthinleadstripsseparatedbylowabsorptionmaterialsuchasaluminumorplastic.TheleadstripsareorientedparalleltotheprimaryXraybeam.

PrimaryXraysthattravelinastraightlinefromtheXraysourcethroughthepatientandtowardsthedetectorpassthroughthegapsbetweentheleadstripsandreachthedetector.However,scatteredXrays,whichtravelinrandomdirections,aremorelikelytobeabsorbedbytheleadstrips.Byremovingasignificantportionofthescatteredradiation,antiscattergridsreducethebackgroundfogontheXrayimage.Thisresultsinanincreaseincontrast,makingiteasiertodistinguishbetweendifferenttissuesandimprovingtheoverallqualityoftheXrayimage.

6.DescribetheprocessofdigitalradiographyinXrayimaging.

Indigitalradiography,insteadofusingtraditionalfilmbaseddetectors,digitaldetectorsareemployed.Therearetwomaintypesofdigitaldetectors:computedradiography(CR)anddirectdigitalradiography(DR).

Computedradiography:Aphotostimulablephosphorplateisusedasthedetector.WhenXraysinteractwiththephosphorplate,theycauseelectronsinthephosphormaterialtobeexcitedtoahigherenergystate.Theseelectronsaretrappedinmetastablestates.AftertheXrayexposure,thephosphorplateisscannedbyalaserbeam.Thetrappedelectronsreleasetheirenergyasvisiblelight,whichisthendetectedbyaphotodetector.Thedetectedlightisconvertedintoanelectricalsignal,whichisdigitizedandprocessedtoformanimage.

Directdigitalradiography:Thisusesflatpaneldetectors.Thesedetectorsaremadeupofanarrayofdetectorelements,eachcontainingaphotodiodeandathinfilmtransistor.WhenXraysinteractwithascintillatorlayeronthedetector,thescintillatorconvertstheXraysintovisiblelight.Thephotodiodesthenconvertthevisiblelightintoanelectricalcharge,whichisstoredinacapacitor.Thethinfilmtransistorsareusedtoreadoutthechargesfromeachdetectorelement,andthedataisdigitizedandprocessedtocreateanXrayimage.Digitalradiographyoffersadvantagessuchasfasterimageacquisition,theabilitytomanipulatetheimage(e.g.,adjustingcontrastandbrightness),andeasierstorageandtransmissionofimages.

7.WhatistheroleoffiltrationinanXraybeam?

FiltrationisusedtomodifytheXraybeamspectrum.InanXraytube,theXraybeamproducedhasawiderangeofenergies.LowenergyXraysareoftenoflittleuseinmedicalimagingbecausetheyaremostlyabsorbedbythepatient'sskinandsuperficialtissues,contributingtothepatient'sradiationdosewithoutaddingmuchusefulinformationtotheimage.

Filtrationinvolvesplacingamaterial(usuallyaluminum)inthepathoftheXraybeam.ThelowenergyXraysarepreferentiallyabsorbedbythefilter.Thisresultsina“hardening”oftheXraybeam,meaningthattheaverageenergyoftheXrayphotonsinthebeamincreases.ByremovingthelowenergyXrays,filtrationreducesthepatient'sradiationdosewhilemaintainingthediagnosticqualityoftheimage.ItalsohelpstoimprovethecontrastoftheimagebyreducingtheamountofscatteredradiationproducedbythelowenergyXrays.

8.ExplainthedifferencebetweenafluoroscopyandaconventionalXrayimage.

Imageacquisition:AconventionalXrayimageisastaticimage.ItisobtainedbyexposingthepatienttoasingleburstofXrays,andtheresultingimageisasnapshotoftheinternalstructuresatthatmoment.Incontrast,fluoroscopyisarealtimeimagingtechnique.ItinvolvescontinuousorpulsedXrayexposure,andtheresultingimagesaredisplayedinrapidsuccessiononamonitor,allowingtheobservertoviewthemovementofinternalorgansortheflowofcontrastagentsinrealtime.

Application:ConventionalXraysarecommonlyusedfordiagnosingconditionssuchasfractures,lungdiseases,anddentalproblemswhereastaticimageoftheinternalstructureissufficient.Fluoroscopyisusedforproceduressuchasmonitoringthemovementofthedigestivetractduringabariumswalloworenema,guidingtheplacementofcathetersorothermedicaldevices,andobservingthebeatingheartduringcardiaccatheterization.

Radiationdose:SincefluoroscopyinvolvescontinuousorrepeatedXrayexposure,theradiationdosetothepatientisgenerallyhighercomparedtoasingleconventionalXray.However,modernfluoroscopysystemsaredesignedtominimizethedosewhilestillprovidinghighqualityrealtimeimages.

9.HowdoestheuseofcontrastagentsenhanceXrayimaging?

ContrastagentsaresubstancesthatareintroducedintothebodytoincreasethedifferenceinXrayattenuationbetweendifferenttissuesorstructures.TheyworkbyeitherincreasingordecreasingtheabsorptionofXraysintheareaofinterest.

Therearetwomaintypesofcontrastagents:positivecontrastagentsandnegativecontrastagents.Positivecontrastagents,suchasbariumsulfateandiodinebasedcompounds,haveahighatomicnumberanddensity.Whentheyareintroducedintoabodycavityorbloodvessel,theystronglyattenuateXrays.Forexample,inabariumswallow,bariumsulfateisswallowed,anditcoatstheesophagus,stomach,andintestines.OnanXrayimage,theareasfilledwithbariumappearwhite,allowingtheradiologisttoclearlyvisualizetheshapeandfunctionoftheseorgans.

Negativecontrastagents,suchasairorcarbondioxide,haveaverylowdensityandattenuateXraysverylittle.Theyareusedtooutlinestructuresbycreatingacontrastbetweenthegasfilledareaandthesurroundingtissues.Forexample,inapneumoperitoneum,airisintroducedintotheabdominalcavity,anditcanhelptovisualizetheoutersurfaceoftheabdominalorgans.

10.WhatarethesafetyconsiderationsinXrayimaging?

Patientsafety:Themainconcernforpatientsistheradiationdose.Xraysareionizingradiation,whichmeanstheycancausedamagetolivingcellsandincreasetheriskofcancer.Therefore,theprincipleofALARA(AsLowAsReasonablyAchievable)isfollowed.Thisinvolvesusingthelowestpossibleradiationdosethatstillprovidesadiagnosticallyusefulimage.Factorssuchasadjustingthetubecurrent,voltage,andexposuretimearecarefullycontrolled.Shieldingdevicessuchasleadapronsareusedtoprotectsensitiveorgans(e.g.,thethyroid,breasts,andgonads)thatarenotintheareaofinterest.

Operatorsafety:Xrayoperatorsarealsoatriskofradiationexposure.Theymustfollowstrictsafetyprotocols,suchasstandingbehindaleadlinedshieldduringXrayexposure.Theyarerequiredtowearpersonalradiationdosimeterstomonitortheircumulativeradiationdose.Regulartrainingonsafetyproceduresandequipmentoperationisessentialtominimizetheriskofaccidentalexposure.

Equipmentsafety:Xrayequipmentmustberegularlymaintainedandcalibratedtoensurethatitoperatessafelyandaccurately.Faultyequipmentcanleadtoincorrectradiationdosesorpoorqualityimages.Safetyinterlocksareinstalledontheequipmenttopreventaccidentalexposure,andemergencystopbuttonsareavailableincaseofanyproblemsduringoperation.

11.HowdoestheatomicnumberofamaterialaffectitsabilitytoattenuateXrays?

TheabilityofamaterialtoattenuateXraysisstronglyrelatedtoitsatomicnumber((Z)).TheprobabilityofXrayinteractions(suchasphotoelectricabsorptionandComptonscattering)withtheatomsinamaterialdependsontheatomicnumber.

Inphotoelectricabsorption,anXrayphotoniscompletelyabsorbedbyaninnershellelectronofanatom,ejectingtheelectronfromtheatom.Theprobabilityofphotoelectricabsorptionisapproximatelyproportionalto(Z^3).ThismeansthatmaterialswithahighatomicnumberhaveamuchhigherprobabilityofabsorbingXraysthroughthephotoelectriceffect.Forexample,leadhasahighatomicnumber((Z=82)),anditisveryeffectiveatabsorbingXrays.Incontrast,materialswithalowatomicnumber,likecarbon((Z=6))orhydrogen((Z=1)),haveamuchlowerprobabilityofphotoelectricabsorption.

InComptonscattering,anXrayphotoncollideswithanoutershellelectronandscattersinadifferentdirection.WhiletheatomicnumberhasalessdirecteffectonComptonscatteringcomparedtophotoelectricabsorption,higheratomicnumbermaterialsstilltendtohaveaslightlyhigherprobabilityofComptonscatteringduetothegreaternumberofelectronsperunitvolume.Overall,materialswithhigheratomicnumbersaremoreeffectiveatattenuatingXrays.

12.ExplaintheconceptofimagenoiseinXrayimaging.

ImagenoisereferstorandomfluctuationsintheintensityvaluesofanXrayimage.Itcandegradethequalityoftheimageandmakeitmoredifficulttodetectsmallorsubtlestructures.

ThereareseveralsourcesofimagenoiseinXrayimaging.Oneofthemainsourcesisquantumnoise,whichisrelatedtothestatisticalnatureofXrayphotondetection.Xraysaredetectedasdiscretephotons,andthenumberofphotonsreachingthedetectorinagivenareaandtimeintervalfollowsaPoissondistribution.Ifthenumberofdetectedphotonsislow,therewillbesignificantfluctuationsinthemeasuredintensity,resultinginvisiblenoiseintheimage.

Electronicnoisecanalsocontributetoimagenoise.Indigitaldetectors,theelectroniccomponentsusedtoconverttheXraysignalintoadigitalimagecanintroducerandomelectricalfluctuations.Thisnoisecanbereducedthroughproperdesignandsignalprocessingtechniques.

Scatteredradiationcanalsoaddtotheimagenoise.Asmentionedearlier,scatteredXrayscancreateauniformbackgroundfogontheimage,whichappearsasnoiseandreducesthecontrastandclarityoftheimage.

13.HowdoesthedistancebetweentheXraysource,thepatient,andthedetectoraffecttheXrayimage?

Sourcetopatientdistance(SSD):IncreasingtheSSDgenerallyresultsinamoreuniformXraybeamintensityacrossthepatient.AlongerSSDreducesthedivergenceoftheXraybeam,whichcanimprovethegeometricunsharpnessoftheimage.However,increasingtheSSDalsorequiresahighertubecurrentorlongerexposuretimetomaintainthesameimageintensityatthedetector,whichcanincreasethepatient'sradiationdose.

Patienttodetectordistance(PDD):AlargerPDDcanincreasegeometricunsharpness.Whenthepatientisfartherfromthedetector,theshadowsoftheinternalstructuresonthedetectorbecomemoreblurred.ThisisbecausetheXraysdivergeastheytravelfromthesourcethroughthepatienttothedetector.Tominimizegeometricunsharpness,thepatientshouldbeplacedasclosetothedetectoraspossible.

Sourcetodetectordistance(SDD):TheSDDaffectstheintensityoftheXraybeamatthedetectoraccordingtotheinversesquarelaw.TheintensityoftheXraybeamisinverselyproportionaltothesquareofthedistancefromthesource.IftheSDDisdoubled,theintensityoftheXraybeamatthedetectorisreducedtoonefourthofitsoriginalvalue.ThisrelationshipmustbeconsideredwhensettingtheexposureparameterstoensurethatasufficientnumberofXraysreachthedetectortoproduceagoodqualityimage.

14.WhatarethelimitationsofXrayimaging?

Softtissuedifferentiation:Xraysarenotveryeffectiveatdifferentiatingbetweendifferenttypesofsofttissues.Sincethedensitydifferencesbetweensofttissuessuchasmuscle,fat,andconnectivetissuearerelativelysmall,itcanbedifficulttodistinguishthesetissuesclearlyonanXrayimage.Forexample,inthediagnosisofsofttissuetumorsorearlystagesofttissueinfections,Xraysmaynotprovidesufficientinformation.

Depthinformation:Xrayimagesaretwodimensionalprojectionsofthreedimensionalstructures.Thismeansthatitcanbedifficulttodeterminetheexactdepthorlocationofastructurewithinthebody.Overlappingstructurescanalsomakeitchallengingtointerprettheimageaccurately.Forexample,inthechest,theheart,lungs,andgreatvesselsoverlap,anditmaybedifficulttodistinguishindividualstructuresinsomecases.

Radiationdose:Asmentionedearlier,Xraysareionizingradiation,andrepeatedorhighdoseexposurecanposearisktothepatient'shealth.ThislimitsthefrequencyofXrayexaminations,especiallyforpatientswhomayrequiremultipleimagingstudiesoverashortperiod.

15.HowdoesthekVp(kilovoltpeak)settinginanXraymachineaffecttheXraybeamandtheresultingimage?

ThekVpsettingdeterminesthemaximumenergyoftheXrayphotonsproducedintheXraytube.

Xraybeamcharacteristics:IncreasingthekVpincreasesthepenetrationpoweroftheXraybeam.HigherenergyXraysaremorelikelytopassthroughthepatient'sbodywithoutbeingabsorbed.Thisresultsina“harder”Xraybeamwithahigheraveragephotonenergy.ThespectrumoftheXraybeamalsoshiftstowardshigherenergies.

Imagecharacteristics:AhigherkVpsettinggenerallyreducesthecontrastoftheXrayimage.SincehigherenergyXraysarelessselectivelyabsorbedbydifferenttissues,thedifferencesinattenuationbetweendifferenttissuesbecomelesspronounced.Asaresult,theimageappearsmoregrayscale,withlessdistinctdifferencesbetweendifferentstructures.However,ahigherkVpcanbeusefulforimagingthickerordenserbodyparts,asitallowstheXraystopenetratethroughthetissueandreachthedetector.LowerkVpsettings,ontheotherhand,producea“softer”Xraybeamwithlowerenergyphotons.Thisincreasesthecontrastoftheimagebutmaynotbesufficientforimagingthickordensetissues.

16.WhatisthedifferencebetweenaPA(posteroanterior)andanAP(anteroposterior)Xrayprojection?

DirectionofXraybeam:InaPAprojection,theXraybeamtravelsfromtheposterior(back)ofthepatienttotheanterior(front)ofthepatient.ThepatientstandswiththeirbacktowardstheXraysourceandtheirfronttowardsthedetector.InanAPprojection,theXraybeamtravelsfromtheanteriortotheposteriorofthepatient.ThepatientstandswiththeirfronttowardstheXraysourceandtheirbacktowardsthedetector.

Imagecharacteristics:ThePAprojectionisgenerallypreferredforchestXrays.InaPAchestXray,theheartisclosertothedetector,whichreducesthemagnificationoftheheartshadow.Thisprovidesamoreaccuraterepresentationoftheheartsize.InanAPprojection,theheartisfartherfromthedetector,andthereismoremagnificationoftheheart,whichcanmaketheheartappearlargerthanitactuallyis.Additionally,inthePAprojection,thelungsarebettervisualizedastheXrayspassthroughlesssofttissuecomparedtotheAPprojection.

17.HowdoXraysinteractwithmatterthroughtheComptonscatteringprocess?

InComptonscattering,anXrayphotoncollideswithanoutershellelectroninanatom.TheXrayphotontransfersaportionofitsenergyandmomentumtotheelectron.

TheincidentXrayphotonhasenergy(E=hnu)(where(h)isPlanck'sconstantand(nu)isthefrequencyofthephoton).Whenitcollideswithanoutershellelectron(whichcanbeconsideredapproximatelyfreebecausethebindingenergyofoutershellelectronsismuchsmallercomparedtotheenergyoftheXrayphoton),theelectronisejectedfromtheatomwithkineticenergy(K_e),andascatteredXrayphotonisproducedwithenergy(E'=hnu').

ThechangeinthewavelengthoftheXrayphoton((Deltalambda=lambda'lambda))isgivenbytheComptonformula(Deltalambda=frac{h}{m_ec}(1costheta)),where(m_e)isthemassoftheelectron,(c)isthespeedoflight,and(theta)isthescatteringangleoftheXrayphoton.

ThescatteredXrayphotonhasalowerenergyandlongerwavelengththantheincidentphoton.ComptonscatteringisanimportantprocessinXrayimagingasitcontributestotheproductionofscatteredradiation,whichcanreducetheimagequalityandincreasethepatient'sradiationdose.

18.WhatistheroleoftheanodeangleinanXraytube?

TheanodeangleistheanglebetweenthesurfaceoftheanodetargetandthecentralaxisoftheXraytube.

Focalspotsize:Theanodeangleaffectstheeffectivefocalspotsize.Asmalleranodeangleresultsinasmallereffectivefocalspotsize.AsmallfocalspotsizeisdesirableforproducingsharpXrayimagesbecauseitreducesgeometricunsharpness.However,averysmallanodeanglecanleadtooverheatingoftheanodeduetotheconcentrationoftheelectronbeamonasmallarea.

Xraybeamintensitydistribution:TheanodeanglealsoinfluencesthedistributionoftheXraybeamintensity.Duetotheanodeheeleffect,theintensityoftheXraybeamishigheronthecathodesideandlowerontheanodeside.Asmalleranodeanglecanincreasetheanodeheeleffect.ThiseffectmustbeconsideredwhenpositioningthepatientandthedetectortoensurethattheareaofinterestreceivesanadequateanduniformXrayintensity.

19.HowcanthequalityofanXrayimagebeevaluated?

Contrast:Asmentionedearlier,contrastisanimportantfactor.Goodcontrastallowsforeasydifferentiationbetweendifferenttissues.Itcanbeevaluatedsubjectivelybylookingatthedifferencesinbrightnessbetweendifferentstructuresintheimage.Objectively,contrastcanbemeasuredusingthecontrasttonoiseratio(CNR),whichistheratioofthedifferenceinsignalintensitybetweentwotissuestothestandarddeviationofthebackgroundnoise.

Spatialresolution:Thisreferstotheabilityoftheimagingsystemtodistinguishbetweentwocloselyspacedobjects.Itcanbeevaluatedbyusingtestpatternswithdifferentlinespacings.Thesmallestlinespacingthatcanbeclearlyresolvedintheimageisameasureofthespatialresolution.HighresolutionXrayimagesarerequiredfordetectingsmallstructuressuchasfinebonefractures.

Noise:Asdescribedpreviously,imagenoisecandegradetheimagequality.Theamountofnoisecanbevisuallyassessedasagrainyorspeckledappearanceintheimage.Quantitatively,itcanbemeasuredasthestandarddeviationofthepixelvaluesinauniformareaoftheimage.

Artifacts:Artifactsareunwantedfeaturesintheimagethatcanbecausedbyvariousfactorssuchaspatientmovement,equipmentmalfunction,orscatteredradiation.Thepresenceofartifactscanmaketheimagedifficulttointerpret,andtheiridentificationandminimizationareimportantforevaluatingimagequality.

20.Whatisthepurposeofusingagridratioinantiscattergrids?

Thegridratioisdefinedastheratiooftheheightoftheleadstripsintheantiscattergridtothedistancebetweentheleadstrips.

Ahighergridratiomeansthattheleadstripsaretallerrelativetothespacingbetweenthem.Themainpurposeofusingagridratioistoimprovetheabilityoftheantiscattergridtoabsorbscatteredradiation.AhighergridratioismoreeffectiveatremovingscatteredXraysfromreachingthedetector.

However,usingahighergridratioalsohassomedrawbacks.ItrequiresahighertubecurrentorlongerexposuretimetomaintainthesameimageintensityatthedetectorbecausethegridalsoabsorbssomeoftheprimaryXrays.Thisincreasesthepatient'sradiationdose.Therefore,thechoiceofgridratiodependsonthethicknessanddensityofthepatient'sbodypartbeingimagedandthedesiredlevelofimagecontrast.Forthickeranddenserbodyparts,ahighergridratiomaybenecessarytoimprovethecontrast,butforthinnerbodyparts,alowergridratiomaybesufficientandcanreducetheradiationdose.

21.HowdoestheenergyofanXrayphotonrelatetoitsfrequencyandwavelength?

Theenergy(E)ofanXrayphotonisrelatedtoitsfrequency(nu)andwavelength(lambda)bythefollowingequations:

Theenergyfrequencyrelationshipisgivenby(E=hnu),where(h)isPlanck'sconstant((h=6.63times10^{34}Jcdots)).Thisequationshowsthattheenergyofaphotonisdirectlyproportionaltoitsfrequency.HigherfrequencyXrayphotonshavemoreenergy.

Therelationshipbetweenfrequencyandwavelengthis(nu=frac{c}{lambda}),where(c)isthespeedoflight((c=3times10^{8}m/s)).Substitutingthisintotheenergyfrequencyequation,weget(E=frac{hc}{lambda}).Thisequationshowsthattheenergyofaphotonisinverselyproportionaltoitswavelength.ShorterwavelengthXrayphotonshavehigherenergy.

22.Whataretheadvantagesofusingdigitalsubtractionangiography(DSA)inXrayimaging?

Enhancedvisualizationofbloodvessels:DSAisatechniqueusedtovisualizebloodvessels.ItinvolvestakingtwoXrayimages:aprecontrastimage(maskimage)andapostcontrastimage.Thepostcontrastimageisthensubtractedfromthemaskimage,leavingonlytheimageofthebloodvesselsfilledwiththecontrastagent.Thissubtractionprocesseliminatesthebackgroundstructuressuchasbonesandsofttissues,allowingforaclearanddetailedvisualizationofthebloodvessels.Itcanbeusedtodetectbloodvesselabnormalitiessuchasblockages,aneurysms,andarteriovenousmalformations.

Realtimeimaging:DSAcanproviderealtimeimagesofthebloodflowinthevessels.Thisisusefulforguidinginterventionalproceduressuchasangioplastyandstentplacement.Thephysiciancanmonitorthemovementofthecontrastagentthroughthevesselsandmakeimmediatedecisionsduringtheprocedure.

Lowcontrastagentdose:SinceDSAcanenhancethevisibilityofthecontrastfilledvessels,arelativelylowdoseofcontrastagentcanbeusedcomparedtotraditionalangiography.Thisreducestheriskofcontrastrelatedsideeffectsinthepatient.

23.ExplaintheprocessofphotoelectricabsorptioninXrayinteractionswithmatter.

Photoelectricabsor