第六章基于射線透射的技術(shù)及應(yīng)用透射衰減對(duì)厚度及密度的測量射線透射成像射線照相射線CT基礎(chǔ)知識(shí)回顧I0Ixx線性吸收系數(shù)X射線測厚儀測厚儀的發(fā)展上世紀(jì)50-60年代-----------同位素測厚儀上世紀(jì)70年代前期---------雙束光測厚儀上世紀(jì)70年代后期---------單束光測厚儀現(xiàn)如今：激光測厚儀、渦流測厚儀、電容式測厚儀、超聲波測厚儀、磁感應(yīng)測厚儀等。X射線測厚儀的工作原理按照X射線測厚儀的工作原理可把X射線測厚儀分為兩類熒光X射線測厚儀透射X射線測厚儀透射X射線測厚儀原理X射線穿過薄膜時(shí)，將有一部分射線被吸收，被吸收部分與薄膜厚度成一定函數(shù)關(guān)系，通過被測量吸收強(qiáng)度可計(jì)算出薄膜厚度。若一X射線出射強(qiáng)度為U，有如下關(guān)系：其中U0為發(fā)射源與探測器之間只有空氣時(shí)探測到的信號(hào)強(qiáng)度。這是一個(gè)近似關(guān)系，用一個(gè)系數(shù)來修正能譜硬化及散射造成的影響。透射X射線測厚儀原理A為一個(gè)未知的常數(shù)μ是被測材質(zhì)的吸收系數(shù)通過上面的函數(shù)關(guān)系可以算出d，即薄膜厚度。由于A與μ是未知常數(shù)，因此進(jìn)行計(jì)算時(shí)需要提前知道這兩個(gè)數(shù)，即進(jìn)行標(biāo)準(zhǔn)式樣的制備與測量。透射X射線測厚儀的構(gòu)造操作終端：進(jìn)行數(shù)據(jù)的設(shè)置，輸出結(jié)果，一般一臺(tái)PC機(jī)（裝有設(shè)備程序）就能搞定主電控柜：對(duì)儀器進(jìn)行電氣控制（使用專業(yè)電器控制設(shè)備）冷卻器：給X光源降溫C形架：對(duì)樣品進(jìn)行照射與數(shù)據(jù)采集。高壓電源：給X射線管提供高電壓，以激發(fā)X射線X射線管

：圖中為電子槍電離室

：對(duì)X射線強(qiáng)度進(jìn)行測定前放：對(duì)電信號(hào)進(jìn)行放大，將電信號(hào)傳入計(jì)算器。X射線熒光膜厚測量優(yōu)點(diǎn)：此方法可以做到無標(biāo)準(zhǔn)樣測量，同時(shí)可以對(duì)薄膜成分進(jìn)行分析。鋁輪轂的鍍層X射線測厚儀參數(shù)與適用范圍（舉例）熒光X射線測厚儀韓國MicroPioneerXRF-2000

原子序可測量厚度范圍22-250.1-0.8μm26-400.05-35μm43-520.1-100μm

72-820.05-5μm一般熒光X射線測厚儀適用于：多層金屬薄膜測厚印刷電路板，金屬層測厚各種涂層，覆蓋層測厚XRF2000可測六層.誤差大約如下:第一層5%.第二層10%.第三層15%.第四層20%.第五層25%.第六層為底材

兩種X射線測厚儀的優(yōu)缺點(diǎn)分析熒光X射線測厚儀優(yōu)點(diǎn)：

可脫離標(biāo)準(zhǔn)樣

可測量覆蓋層

可測量多層式樣

缺點(diǎn)：

無法在生產(chǎn)時(shí)同步測量

后幾層測量精度差透射X射線測厚儀優(yōu)點(diǎn)：

可以在生產(chǎn)時(shí)同步測厚缺點(diǎn)：

需要標(biāo)準(zhǔn)式樣密度測量極限值指示和料位測量極限值；容器充許的填充材料的最大料位。測量極限值只是一個(gè)容器裝置的填充過程。料位測量:對(duì)在即裝又卸的過程的監(jiān)測。利用γ輻射和繼電器型儀表來處理，確定信息含量的電磁繼電器隨輻射強(qiáng)度的變化而吸合和釋放。使用源的活度決定于γ繼電器的靈敏度、容器的直徑、壁厚、材料以及允許開關(guān)時(shí)間。:

60Co,

137Cs。

小容器和指示管中常用β源。射線成像技術(shù)成像（Imaging）？相關(guān)知識(shí)介紹數(shù)字圖像成像質(zhì)量分辨率對(duì)比度噪聲圖像的數(shù)學(xué)描述圖像質(zhì)量分辨對(duì)比噪聲如何評(píng)價(jià)分辨率（Resolution）1280×1024表示像素（成像單元）的數(shù)目（采樣數(shù)）；用于數(shù)字圖像，數(shù)字顯示或數(shù)字化像探測器；5.0lp/mm線對(duì)（linepair）：黑線及其相鄰的白色間隔；最常用0.1mm能分辨的最小間距（1/(線對(duì)×2)）；可用于描述不銳度點(diǎn)擴(kuò)散函數(shù)PSF；調(diào)制傳遞函數(shù)MTFPSF描述成像系統(tǒng)對(duì)單點(diǎn)的響應(yīng)。MTF描述系統(tǒng)對(duì)不同頻率響應(yīng)的振幅分量。系統(tǒng)的分辨率最終圖像為物分布與系統(tǒng)內(nèi)所有相關(guān)元件的點(diǎn)擴(kuò)散函數(shù)的卷積系統(tǒng)的調(diào)制傳遞函數(shù)為各相關(guān)元件調(diào)制傳遞函數(shù)的乘積對(duì)比度多用于均勻本底的圖像樣品與空白處對(duì)比度樣品厚度變化造成的對(duì)比度自上而下灰度變化分別為：連續(xù)，1level/4pixel,2level

/8pixel,4level/16pixel,

8level/32pixel,16level/64pixel。最黑為0，最亮為255。人眼一般能分辨2～4%的亮度變化(CT)。能夠識(shí)別的亮度差異為反差靈敏度。對(duì)比度調(diào)節(jié)數(shù)字處理中對(duì)比度調(diào)節(jié)有助于人眼的觀看，但調(diào)節(jié)不當(dāng)會(huì)造成部分有價(jià)值信息不可見通過對(duì)比度調(diào)節(jié)是否能看到極小的厚度差異？噪聲來源統(tǒng)計(jì)漲落散射干擾探測器不均勻電子學(xué)熱噪聲主要的噪聲每像素的統(tǒng)計(jì)漲落N為每個(gè)像素上的計(jì)數(shù)，K為比例系數(shù)。K正比于：中子注量、像素面積及探測效率。IS為散射中子造成的本底。探測器不均勻?yàn)橄到y(tǒng)噪聲，可以通過修正消除。電子學(xué)熱噪聲可以通過降溫及降低工作頻率等手段降低到可以忽略的水平。噪聲的影響原圖2%漲落4%漲落8%漲落假設(shè)大于漲落的灰度變化可分辨，則可分辨的條件為：射線照相第一張X射線照片(正片)醫(yī)學(xué)X射線照片（負(fù)片）ProjectionImagingSID:SourcetoImage-receptorDistanceSOD:SourcetoObjectDistanceOID:ObjecttoImage-receptorDistanceProjectionRadiographyProjectionimagingacquisitionofa2Dimageofpatient’s3DanatomytransmissionimagingprocedureTheopticaldensity(OP)(measureoffilmdarkening)atanylocationonthefilmcorrespondstotheattenuationcharacteristicsofthepatientatthatlocationBasicGeometricPrinciplesSimilartriangles(geometry)3anglesofone=3anglesoftheothera/A=b/B=c/C=h/Hd/D=e/E=f/F=g/GSimilartrianglesareencounteredduringimagemagnificationandwhenevaluatingimageunsharpnesscausedbyfocalspotsizeandpatientmotionMagnification(M)occursbecausex-raybeamdivergesfromfocalspottoimageplaneforapointsource,M=I/O=SID/SODlargestwhenSODsmall(patientnearthetube)Foraextendedsource(focalspot),Penumbra半影orblur

(f)edgegradientblurringduetofinitesizeoffocalspot(F)f/F=OID/SODf/F=(SID-SOD)/SODf/F=(SID/SOD)-1f=F(M-1)forblurincreaseswithFandMThesmallerthefocalspotsize,thelesstheblureffectandthebettertheresolutioninanimageHowever,asmallfocalspotcannotsustainhightubecurrents,whicharerequiredforshortexposuretimesthatareusefulinreducingpatientmotion.ThisisduetoitsinabilitytodissipateheatrapidlyenoughtoavoiddamageTypicallywehaveanx-raytubewithdualortriplefocalspotssothatthebestperformancefortheprocedurecanbeused,balancingheatloadingofthefocalspotwiththerequirementsforresolutionandmagnificationInrealimagingsituations,thesizeofthefocalspotisthelimitingfactortotheamountofmagnificationthatcanbeused.InverseSquareLawTheinversesquarelawstatedsimplysaysthatofthenumberofx-raysstrikinganarea,theintensityisinverselyrelatedtothesquareofthedistancefromthesourcewhereIistheintensityatapointandDisthedistancefromthesource.

Whataretheimplicationsofthisforradiography?Asthedistancefromthesourceofxraysincreases,thebeamdivergessothattheconstantnumberofphotonsbeingproducedatthefocalspotisdistributedoverlargerareasTherewillbefewerx-raysreachinganygivenspotonthedetector,resultinginanoverallincreaseinthenoisewithintheimageInordertocompensateforthiseffect,morex-rayswouldneedtobeproducedasthedistanceincreases,inordertomaintainthemeannumberofx-rayphotonsperunitareaatsamelevelasatcloserpositionsBalancingtheEffectsSource-to-ObjectDistance(SOD)AsSODincreases,itcausesthex-raybeamtoappeartocomefromasmallerfocalspot,yieldingasharperimagefromlessfocalspotblur.However,theinversesquarelawhasaneffectastheSODincreasesandmustbecompensatedforbyincreasingthetubecurrentformorex-rayoutput.AshorterSODyieldsmorex-raysandreducednoise,butattheexpenseofdecreasedsharpnessduetoincreasedblurfromthefocalspotObject-to-DetectorDistance(ODD)OIDincreasesintroducemagnification,yieldingbettereffectivespatialresolutionattheimagereceptoratthecostofmoreblurduetotheincreasingeffectoffocalspotsize.AstheODDisdecreased,themagnificationgoesdown,butsodoesthefocalspotblureffect,sotheimageissharper.Insituationswheremagnificationimagingisdesirable,asmallfocalspotinadualfocalspotx-raytubewillpermitusablemagnification.ContrastandDoseinRadiographyThecontrastofaspecificradiographicstudydependsontherequirementsofthestudy,totalexposuretime,radiationdose,sizeofsample

andsoon…ThekVp(quality)andmAs(quantity)areadjustedbythetechnologisttoadjustthesubjectcontrastTechniquechartPhototimer(automatictechnique)Differentbodyhabitus體質(zhì)LowerkVpbeam(orlessfiltration),providelessx-raypenetrationandlessscatteredradiationleadingtohighercontrast.

Techniquesusingsofterbeamsfurtherrequiremorephotonsinordertopenetratethebodypart;asthesephotonsarealsomorehighlyabsorbedbythebody,thepatientdosewillbegreater.

HigherkVpbeam(ormorefiltration),providemorex-raypenetration,andmorescatteredradiation.

ThisleadstolowercontrastbutalsolowerdoseForagivenbodypart,theselectionofproperbeamenergyrequiresacarefulbalancebetweentheneededcontrast/latitude,andthepatientdoseKVpandContrastElbowphantomimagesacquiredattheproper55kVp(left)andnotablyhigher,110kVp(right)leadingtoreducedbonecontrastmAsandContrastPhantomchestradiograph(left)atstandard,photo-timedmAssetting(top)andat25%reducedmAs(bottom).

Theimages(top&bottom)aremagnifiedandcontrastenhancedfromamediastinumregionoftheimageonthelefttoillustratetheincreasedquantumnoiseinthereduced-mAsimage.ScatteredRadiationinProjectionRadiographyScatteredradiationviolatesthebasicprincipleofprojectionradiography,thatisprojectionradiographyassumesthatphotonstravelinastraightlinefromthex-raysource(x-raytubefocalspot)totheimagereceptorThescatteredphotonifdetectedbyfilmcausesfilmdarkeningbutprovidesnousefulinformationtotheimageScatteredphotonsaretheresultofComptoninteractionswithinthebody.TheresultofaComptoninteractionisaredirectedlower-energyphotonScatter-to-Primaryratio(S/P)Areaofcollimatedx-rayfieldObjectthicknessEnergyofx-raybeamAsFOVisreduced,scatterisreducedLargerpatientscreatemorescatterScatterseeninallmodalities(Comptonscatter)ScatterradiationcauseslossofcontrastIntheabsenceofscatter,fortwoadjacentareastransmittingphotonfluencesofAandB,thecontrastis:C0=[A-B]/AInthepresenceofscatter:C=C0x[1/(1+S/P)]S/P↑→contrast↓1/(1+S/P):contrastreductionfactorTheantiscattergridisusedtocleanupscatterBetweenobjectanddetectorUsesgeometryto↓scatterThinleadseptaseparatedbyaluminumorcarbonfiber,alignedwithfocalspotGridratio(GR)=H/W=septaheight/interspacewidth↑GR→↓S/P↑GR→↑doseAirGapsAirgap:↓S/P,but↑M,↓FOVand↓MTF(unlessverysmallfocalspotused)Notusedallthatofteninradiographyexceptchestradiography,usedinmammographySystemGeometryHistoricalDevelopmentofCT(略)FormationofaCTimage–Reconstruction(略)ConvolutionisamathematicalfilteringstepusedtoremoveblurfromthedataThisfigureshowsthedataevolutionfromacquisition,reconstruction,andimagedisplayTomographicReconstruction

Preprocessing&RawDataEachrayisatransmissionmeasurementthroughtheobjectalongaline,wherethedetectormeasuresanx-rayintensity,It

I0

=unattenuatedintensityofx-raybeamIt

=I0

e-μt

t=thicknessofpatientalongtherayμ

=theaveragelinearattenuationcoefficientalongtherayln(I0

/It)=

μtforeachrayThisisthepreprocessingstepperformedbeforeimagereconstructionImageprimarilydependsonthepatient’sanatomiccharacteristicsTomographicReconstruction(略)AlgebraicproceduresapproachisusedforimagereconstructionBysolvingasystemoflinearequationsforseveralprojections,thevalueoftheattenuationcoefficient,μcanbecomputedTomographicReconstruction(略)ACTreconstructionalgorithmisusedtoproducetheCTimageFilteredbackprojectionismostwidelyusedinclinicalCTscannersSimplebackprojectionmethodbuildsuptheCTimageinthecomputerbyessentiallyreversingtheacquisitionstepsDuringbackprojection,the

μvalueforeachrayisinessencesmearedalongthesamepathintheimageofthepatientAreasofhighattenuationreinforceeachotherandareasoflowattenuationreinforceeachotherbuildinguptheimageinthecomputerFormationofaCTimage–FilteredbackprojectionSimplebackprojectionproducesanimagethatissomewhatblurredRawdatamustfirstbefilteredusingamathematicalfilter,orkernelThemathematicalfilteringstepinvolvesconvolvingtheprojectiondatawithaconvolutionkernelFormationofaCTimage–FilteredbackprojectionDirectbackprojectionofattenuationprofilesresultsinunsharpimagesConvolutionofattenuationprofilesbeforebackprojection,removestheblurringImageReconstructionVariousconvolutionfilterscanbeusedtoemphasizedifferentcharacteristicsintheCTimage,offeringtradeoffsbetweenspatialresolutionandnoiseBonealgorithm-finedetailbutwithincreasednoiseSofttissuefilters-smoothing,whichdecreasesimagenoisebutalsodecreasesspatialresolutionThechoiceofthebestfiltertousewiththereconstructionalgorithmdependsontheclinicaltaskCTImageApixel(pictureelement)isthebasic2DelementofthedigitalimageEachpixeldisplaysbrightnessinformationconcerningthepatient’sanatomythatislocatedinthecorrespondingvoxel(volumeelement)ThepixelwidthandheightareequaltothevoxelwidthandheightThevoxelhasathirddimensionthatrepresentstheslicethicknessoftheCTscanTherowsandcolumnscompriseamatrixMatrixsizesare512x512,1024x1024etcThetechnologistselectsthefieldofview(FOV).Pixelsize=FOV/matrixsizeThegrayscalerangeforeachpixelis12-bits(0-4095)Spatialresolutionimproveswithalargermatrix(smallerpixels)orsmallerFOVWhatIsBeingMeasured?TheCTreconstructionprocessresultsina2Dmatrixoffloatingpointnumbersinthecomputerwhichrangefromnear0.0to1.0ThesenumberscorrespondtotheaveragelinearattenuationcoefficientofthetissuecontainedineachvoxelTheCTimagesarenormalizedandtruncatedtointegervaluesthatencompass4096values,between-1000and3095(typically)CTnumbersarerescaledlinearattenuationcoefficientsCTNumbersorHounsfieldUnitsμmistheattenuationcoefficientforthevoxel,μwistheattenuationcoefficientofwaterandHUmistheCTnumber(orHounsfieldunit)thatcomprisesthefinalclinicalCTimageAir=-1000,softtissuesrangefrom-300(lung)to-90(fat),water=0,whitematter=30,graymatter=40,muscle=50,denseboneandareasfilledwithhighcontrastagentrangeupto+3000CTNumbersorHounsfieldUnitsWhatdoCTnumberscorrespondtophysicallyinthepatient?CTnumbersandhenceCTimagesderivetheircontrastmainlyfromthephysicalpropertiesoftissuethatinfluenceComptonscatterThelinearattenuationcoefficienttrackslinearlywithdensityoftissueandplaysthedominantroleinformingcontrastinmedicalCTCTnumbersarequantitative,pulmonarynodulesthatarecalcifiedaretypicallybenign,andamountofcalcificationcanbedeterminedfromthemeanCTnumberofthenoduleCTisalsoquantitativeintermsoflineardimensionsandcanbeusedtoaccuratelyaccesstumorvolumeorlesiondiameterWhydoCTsliceimagesofferhighercontrast?CTimagesofferhighcontrastduetotheimagingprinciplewherethecontrastdependsonlocaldifferencesinattenuationunlikeconventionalradiographywheresignalisasumofallsignalcontributionsalongarayfromx-raysourceDigitalImageDisplay:Window/Level（略）Computermonitorsandlaserimagersforprintinghaveabout8bitsofdisplayfidelity(28=256)The12-bitCTimagesmustbereducedto8bitstoaccommodatemostimagedisplayhardwareThewindowwidth(W)determinesthecontrastoftheimage,withnarrowerwindowsresultingingreatercontrastThelevel(L)istheCTnumberatthecenterofthewindowP1=L–?WP2=L+?WDigitalImageDisplay:Window/Level（略）ArtifactsBeamHardeningLower-energyx-raysareattenuatedtoagreaterextentthanhigher-energyx-raysasx-rayspassthroughapatientShapeofthespectrumbecomesmo