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2.X‐rayplanarradiographyandCT(1)

Lecture1,2

MedicalImagingSystems

JaeGwan Kim

jaekim@gist.ac.kr ,X2220

DepartmentofBioMedical ScienceandEngineering

Gwangju InstituteofSciencesandTechnologyCopyright.Mostfigures/tables/textsinthislecturearefromthetextbook“IntroductiontoMedicalImaging:Physics,EngineeringandClinicalApplicationsbyNadineBarrieSmithAndrewWebb2011”andthismaterialisonlyforthosewhotakethisclassandcannotbedistributedtoanyonewithoutthepermissionfromthelecturer.

Contents

1. Introduction

2. X‐rayenergyspectrum

3. InteractionofX‐rayswiththebody

1) Photoelectricattenuation

2) Comptonscattering

4. X‐raylinearandmassattenuationcoefficient

5. Instrumentationforplanarradiography

– X‐raytube

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Overview

Black Air

Dark Gray

Fat

Light  Gray

Soft tissues/ Water

White Calcification/bone

Whiter Metal/constrast

Introduction

• X‐rayswerefirstobservedanddocumentedin1895byWilhelmConradRoentgen,aGermanscientistwhofoundthemquitebyaccidentwhenexperimentingwithvacuumtubes

• Aweeklater,hetookanX‐rayphotographofhiswife'shandwhichclearlyrevealedherweddingringandherbones.Thephotographelectrifiedthegeneralpublicandarousedgreatscientificinterestinthenewformofradiation.Roentgencalledit"X"toindicateitwasanunknowntypeofradiation.

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Introduction

• X‐rayplanarradiography– Bonefracture– Presenceofmassinlung– Presenceofkidneystones– Diseasesofthegastrointestinaltract

• ThebasisofX‐rayscancomesfromthedifferentialabsorptionofX‐raysbyvarioustissues.(highabsorptionbyboneandcalcifications)

• X‐raysdirectedtowardthepatientsanddetectedbyoppositepaneldetectorwhichisplacedbelowthepatient.

Introduction

• X‐raysaredetectedandconvertedintolight,andthenintovoltagewhichwillbedigitized.

• X‐raysarealsoscatteredastheypassthroughthebody,and‘anti‐scattergrid’canreducethebackgroundsignal

• Specializedapplications– X‐rayfluoroscopy(GItractwithcontrasts)– Digitalmammography(lowerdosethanstandardforhighsofttissuecontrast)

– Digitalsubtractionangiography(vasculatureimaging)

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Introduction

Planar radiography setup Anti‐scatter grid Chest X‐ray

Introduction

• Afull3Dimagesfromparticularregionofbodyarerequiredtodiagnosebetter.(Headimagingfortraumalocationandsize) ComputedTomography

• Problem:CTusesmuchhigherradiationdose

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X‐raySpectrum

HighEnergyElectronwithMetal

• Twopossibilitieswhenelectronsstrikethemetal

– Bremsstrahlung(brakingradiationinGerman):convertingkineticenergytoelectromagneticradiationduetodeceleration(byprotonsinnucleus)andchangeofmomentum,emitscontinuousenergyspectrumofX‐rayphotons

– Characteristicradiation(=X‐rayfluorescence):incidentelectronejectstheinnerelectron(K,L,M…shell)andoutershellelectrontransitionstotheinnershellandemitsanX‐ray

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HighEnergyElectronwithMetal

• TheenergyofX‐raybeam:weightedaverageofallthedifferentenergies,andistypically2/3ofthekVp value.

X‐ray energy 

Relative number of X‐ray 

Characteristic X‐rays

Low energy X‐rays are absorbed by X‐ray housing itself

PeriodicTable

http://en.wikipedia.org/wiki/Periodic_table

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HighEnergyElectronwithMetal

• Fortungsten,bindingenergiesoftheK,L,andMshellsare69.5,10.2~12.1,and1.9~2.8keV,respectively.

• ForMolybdenum,theyare20,2.5~2.8,and0.4~0.5keV,respectively.

Depends on the metal, therefore, it is characteristic

Refer to http://www.webelements.com/for the binding energy values

InteractionofX‐rayswiththebody

• ToacquirehighSNRandCNRimages,

– SufficientX‐rayspassthroughthebodyforhighSNR

– X‐rayabsorptionshouldbesufficientlydifferentamongtissuesforhighCNR

– BackgroundX‐raysscatteredfromunknownanglesneedtoberemoved

• TwomajormechanismsthatX‐rayinteractswithtissue

– PhotoelectricinteractionwithdifferentX‐rayattenuation

– Comptonscattering:X‐raysarebeingdeflectedfromitsoriginaltrajectory contributestorandombackgroundsignal

– Othermechanismsincludingcoherentscatteringareminorandnotconsideredforclinicalradiography

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InteractionofX‐rayswiththebody

• Photoelectriceffect(Hertzeffect,light emitselectroninamaterial)– TissueabsorbsX‐rays,providesthecontrastinX‐rayimages1. ElectronsatK‐ orL‐shellareemittedfrommatterafterabsorbing

energyfromX‐ray2. Electronsfromahigherenergylevelfillsthe‘hole’withtheemission

ofa‘characteristic’X‐raywhichisveryweak3. ThenetresultisthattheincidentX‐rayiscompletelyabsorbedand

doesnotreachthedetector

Electron binding energyCarbon(Z=6) 1s: 284.2eV,  Nitrogen(Z=7) 1s: 409.9eV, 2s: 37.3eVOxygen(Z=8) 1s: 543.1eV, 2s: 41.6eV Calcium(Z=20) 1s: 4038.5eV, 2s: 438.4eV, 3s: 44.3eV

InteractionofX‐rayswiththebody

• Photoelectriceffect(Hertzeffect,light emitselectroninamaterial)– TissueabsorbsX‐rays,providesthecontrastinX‐rayimages

(photoelectron: energy is the difference between incident X-ray and binding energy of electron)

(1) (2) (3)

This characteristic X-ray energy is difference between binding energy of two electron, ~keV, and absorbed after travelling ~1mm in tissue

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InteractionofX‐rayswiththebody

• Photoelectriceffect– Theprobabilityofphotoelectricinteraction(Ppe)is

∝

• E:incidentX‐rayenergy• Zeff:theeffectiveatomicnumberofthetissue(tissue~7.4,lipid~6.9,bone

~13.8(duetoCalcium))• ρ:tissuedensity:(tissue:lipid:bone=1:0.9:1.85)• AsincidentX‐rayenergyincreases,thecontrastdecreases.

InteractionofX‐rayswiththebody

• Comptonscattering– InteractionbetweenanincidentX‐rayandalooselyboundelectroninanoutershellofanatomintissue

– ScatteringthatX‐rayorGammarayphotoninteractinmatter

– Inelasticscattering:energytransferstoejectedelectronandtherestenergycanbefoundinscatteredX‐ray

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InteractionofX‐rayswiththebody

• StandardComptonequation

– 1

– λistheinitialwavelength,λ'isthewavelengthafterscattering,h isthePlanckconstant,mo istherestmassoftheelectron,c isthespeedoflight,and isthescatteringangle. (assumption:eachX‐rayphotoninteractedwithonlyoneelectron)

λ

λ'

InteractionofX‐rayswiththebody

• Relevantlossinenergyis

∆ , ,

Therefore,theenergyofthescatteredX‐rayis

,,

1 , 1

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InteractionofX‐rayswiththebody

• Fromthisgraph,wecanseethatevenwith90degreescatteringangle,thescatteredX‐rayenergyreducesalittle.

• Andthus,mostofscatteredX‐raywillpassthroughthebody.• TheComptonscatteringprobabilityis

– Independentofatomicnumber– Proportionaltothetissueelectrondensity– WeaklydependentontheincidentX‐rayenergy

• LowX‐rayenergy:photoelectriceffectdominates

• HighX‐rayenergy:Comptonscatteringcontributesmore

X‐rayattenuationintissue

•– N:numberofX‐raystransmitted– No:incidentX‐rays– μ:tissuelinearattenuationcoefficient– x:tissuethickness

•

• Massattenuationcoefficient(μ/ρ)ismeasuredinunitsofcm2g‐1.

1⁄

g⁄

water

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• Atlowenergy,thereisabigdifferencebetweenboneandothers.

• However,thedifferencedecreasesasX‐rayenergyincreases.

• K‐edge:atanenergyjusthigherthantheK‐shellbindingenergyofacertainatom(calciumforbone=4keV),photoelectricinteractionincreasesgreatlybyafactorof5to8.

• Halfvaluelayer(HVL):

– ThetissuethicknessthatreducestheintensityofX‐raytothehalf

– HVL=(ln 2)/μ

– Ex)HVLformuscle:3.9cm,bone:2.3cmat100keVWith30keV(digitalmammography),muscleis1.8cmandboneis0.4cm.At100keV,3.9/2.3=1.69,andat30keV,1.8/0.4=4.5morecontrast

X‐rayattenuationintissue

LightMatterInteractions

• Lowenergyphenomena(afeweV~1MeV)

– Photoelectriceffect

• Midenergyphenomena(511keV)

– Thomsonscattering(elastic,when ≪ )

– Comptonscattering(inelastic)

• Highenergyphenomena(>1.022MeV)

– Pairproduction:thecreationofanelementaryparticleanditsantiparticle,normallyfromaphoton(oranotherneutralboson),ex)electronandpositron,muon andanti‐muon,tauandanti‐tau

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LightMatterInteractions

• DominatingtissueinteractionprocessdependingonX‐rayenergy

http://epswww.unm.edu/xrd/xrdclass/02‐Rad‐Safety.pdf

X‐RAYGENERATION

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X‐raytube

• Highenergyofelectronshitthesurfaceofametaltarget

• X‐rayproductionsteps

1) Anegativelychargedcathode:asmallhelixofthintungstenwire,~2200oCelectronsstarttoleave(thermionicemission)

• Anegativelycharged‘focusingcup’focuselectronbeamsfromcathodetoanode

Cathode:negativechargeflowsin

Anode:positivechargeflowsin

X‐raytube

• X‐rayproductionsteps

2) Anodeisapositivelychargedmetaltargetand25~140kVisappliedbetweencathodeandanode(acceleratingvoltage,kVp)

• Tubecurrent(no.ofelectronstravelingbetweenthecathodeandanode)is50~300mA(1mA=6.24x1015 electrons/s)

• Therefore,tubevoltage(kVp),tubecurrent(mA),andexposureduration(s)areparametersthatusercanselect

3) StrikingelectronsproduceX‐rayfromanodeandanodeshouldbeabletostandthehightemperature

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X‐raytube

• HigheratomicnumberofthemetalproduceX‐raymoreefficiently.

• Themostcommonmetalistungsten,atomicnumber74andmeltingpointis3370oC.

• Evenforthetungsten,only~1%ofelectronenergyconvertstoX‐ray andtherestgoesawayfortheheat.

• Tungstentargetisabout0.7mmthicknessandrotatesat~3000rpmtoreducethelocalheating.

• Inpractice,atungsten‐rhenium(Re)(2‐10%)alloyisusedforextrastability

• Forthedigitalmammography,molybdenumisusedasananodeinsteadoftungstensinceituseslowenergyX‐ray.

X‐raytube

vacuum

(cooling,electric isolation)

J. Anthony Seibert, JNMT, 32 (3) 139‐147, 2004

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X‐raytube

• X‐raygeneratorandx‐raytubecomponentsareillustrated.– Thex‐raygeneratorprovidesoperatorcontroloftheradiographictechniques,includingtubevoltage(kVp,kilovoltspeak),tubecurrent(mA),andexposureduration,anddeliverspowertothex‐raytube.

– Thex‐raytubeprovidestheenvironment(evacuatedx‐raytubeinsertandhigh‐voltagecablesockets),sourceofelectrons(cathode),sourceofx‐rays(anode),inductionmotortorotatetheanode(rotor/stator),transformeroilandexpansionbellowstoprovideelectricalandheatbuild‐upprotection,andthetubehousingtosupporttheinsertandprovideprotectionfromleakageradiation.

J. Anthony Seibert, JNMT, 32 (3) 139‐147, 2004

X‐raytube

http://www.waybuilder.net/sweethaven/MedTech/Dental/DentalRad/lessonMain.asp?iNum=fra0102

http://www.dentalxraywebsite.com/category/dental‐x‐ray‐tube/

Toshiba X‐ray Tube History 1915‐2005

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X‐raytube

• ManyX‐raytubeshavetwocathodefilamentsofdifferentlength

– alongone:highercurrent/lowerresolution

– ashortone:lowercurrent/higherresolution

Focusing cup

X‐raytube

• Anodeisbeveledatananglebetween8and17o (normally12‐15o)

• Thesmallerangle(θ) producesthesmallerfocalsize(f)

f =Fsin θ• frangesfrom0.3mmfordigitalmammographyandtobetween0.6and1.2mmforplanarX‐rayandCT

• BevelanglealsoaffectsthecoverageareaCoverage=2(source‐patientdistant)xtan θ(eg. 2(1m)*tan15o=0.53m)

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X‐raytube

• Heeleffect:X‐raybeamismoreintenseatthe‘cathode‐end’thanatthe‘anode‐end’

• ItisfromtheabsorptionofX‐raytothetargetitself.(moreX‐rayabsorptionattheanodeside)

• Therefore,signalintensityatcathodesideishigherthananodesideinplanarradiographyandcanbecorrectedbyimageprocessingalgorithms.

• However,inpracticeitdoesnotaffectthediagnosticqualityoftheimagessignificantly.

X‐raytube

• 3parameterstocontrol1. Acceleratingvoltage(kVp)

• ~25kVfordigitalmammographyand~140kVforboneandchestX‐ray

2. Tubecurrent(mA)• 50~400mAforplanarX‐ray• ~1000mAforCT

3. Exposuretime(sec)

• Powerrating:themaximumpowerdissipatedinanexposuretimeof0.1s

– Ex)10kWpowerrating:kVp of125kVwith1Atubecurrentfor~78ms

– X‐raytubeoutputismainlylimitedbyanodeheating

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Practice

• Ifthethicknessofthechestis20cm,whatpercentageofX‐raysaretransmittedthroughthechestatanincidentX‐rayenergyof70keVassumingHVLvaluesof3.5and1.8cmformuscleandbone,respectively,andthebonethicknesstobe4cmandthetissuethickness16cm?

Answer) Forthemuscle,μ =(ln 2)/3.5≈0.2cm‐1. For16cmoftissue:N/No =exp(‐0.2*16)=0.04. Forthebone,μ =(ln 2)/1.8≈0.4cm‐1. For4cmofbone:N/No =exp(‐0.4*4)=0.2. Therefore,theoverallpercentageis

100X(0.2X0.04)=0.8%