CT成像原理介绍课件.ppt

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1、CT 成像原理介紹CT 成像原理介紹Computed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsComputed Tomography CT BasicsX-Ray Discovery X-ray was discovered by a German scientist Roentgen 100 years ago.This made people for the first time b

2、e able to view the anatomy structure of human body without operation77 But its superimposed 77 And we couldnt view soft tissueX-Ray Discovery X-ray was disl My name is Godfrey Hounsfieldl I work for the Central Research Labs.of EMI,Ltd in Englandl I developed the the first clinically useful CT scann

3、er in 1971History of Computed Tomography Early 1970s1963-Alan Cormack developed a mathematical method of reconstructing images from x-ray projections My name is Godfrey Hounsfiel For the first time we could view:CT Broke the Barrier-Tomographic or“Slice”anatomy-Density difference77 But its time cons

4、uming77 And resolution needs to be improved For the first time we could vConcept of X-ray AttenuationAn X-ray beam passing throughthe body is attenuated(loses itsenergy)by:AbsorptionScatteringIncident X-rayTransmitted raySCATTERED X-SCATTERED X-RAYSRAYSBODYTISSUEAbsorption by the tissue is proportio

5、nal to the densityLessdensetissueMoredensetissueMORE ATTENUATIONLESS ATTENUATIONConcept of X-ray AttenuationAHow does CT Work?How does CT Work?Recon.&postpro.Data acquisitionX-ray generationHow does CT Work?Recon.&postX-ray goes through collimator thereforepenetrate only an axial layer of the object

6、,called sliceHow does CT Work?How does CT Work?X-ray goes through How does CTPatient is placed in the center of the measurement fieldX-ray is passed through the patients slice from many direction along a 360 pathThe transmitted beams are captured by the detectors which digitizes these signalsThese d

7、igitized signals called raw data are sent to a computer which create the CT imageHow does CT Work?How does CT Work?Patient is placed in the centeThe object slice is divided into small volume elements called voxels.Each voxel is assigned a value which is dependent on the average amount of attenuation

8、How is CT Image generated?The object slice is divided inThe attenuation values are transferred to the computer where they are coded&used to create a slice imageHow is CT Image generated?The attenuation values are traCT Generations&Design“Generation”is used to label CT tube-detector designs3rd Genera

9、tion Design3rd Generation DesignRotating tube&detectorRotating tube&detector4th Generation Design4th Generation DesignFixed ring detectorFixed ring detectorCT Generations&Design“GeneraSlip-ring TechnologyPower is transmitted through parallel sets of conductive ringsinstead of electrical cables Conti

10、nuous Gantry Rotation Prerequisite for Spiral CTNon Slip-ring ScannerNon Slip-ring ScannerSlip-ring ScannerSlip-ring ScannerSlip-ring TechnologyPower is tComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsComputed Tom

11、ography CT BasicsWhat is Spiral Scan?-just 4“C”Continuously rotating tube/detector systemContinuously generating X-rayContinuously table feedContinuously data acquisitionWhat is Spiral Scan?-just 4Reconstruction of arbitraryslices(either contiguous or overlapping)within thescanned volume Distance be

12、tween the slices is called IncrementABVolume DataContinuous data acquisitionReconstruction of arbitraryABVIncrement Slice ThicknessIncrement=Slice ThicknessIncrement=Slice Thickness No Overlap No Overlap No Gaps No GapsContiguous Image ReconstructionIncrementSlice ThicknessIncrIncrement Overlap Slic

13、eThicknessOverlapping Image ReconstructionIncrement Slice ThicknessIncrement Slice ThicknessIncrement Slice Thickness Gaps between slices Gaps between slices Images are further apart Images are further apart Less images created Less images createdImage Reconstruction with GapsIncrement Slice Thickne

14、ssIncrement Slice ThicknessImaStandard CT/Slice ImagingStandard CT/Slice ImagingDeep InspirationDeep InspirationShallow InspirationShallow Inspiration Misregistration due to different Misregistration due to different respiratory levels between slices respiratory levels between slicesPartial Volume E

15、ffectPartial Volume Effect Unable to resconstruct images at Unable to resconstruct images at arbitrary position arbitrary position Slice imaging is slow Slice imaging is slowStandard CT/Slice ImagingDee Scan the whole region of Scan the whole region of interest in one breath hold interest in one bre

16、ath hold Reconstruction of overlapping Reconstruction of overlapping images without additional dose images without additional dose Retrospective reconstruction Retrospective reconstruction of slices in arbitrary position of slices in arbitrary position within the scanned volume within the scanned vo

17、lume No gaps since radiation always No gaps since radiation always transmits the whole volume transmits the whole volumeSpiral CT/Volume ImagingSpiral CT/Volume Imaging Scan the whole region of ReComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection P

18、rotocols Clinical ApplicationsComputed Tomography CT BasicsScan ParametersX-ray Tube Voltage(kVp)X-ray Tube Current(mA)Scan Time(s)Slice thickness or Collimation(mm)Table Speed(Table Speed(mm/rot)mm/rot)or Feed per 360 rotationor Feed per 360 rotation Pitch Pitch Interpolation Process Interpolation

19、Process Increment(mm)Increment(mm)Scan ParametersX-ray Tube VoltTable Speed&Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch =Table Feed per rotation CollimationTable Speed&Pitch Table Spee10mm 10mm P1P110mm P210mm P23030s s30s30sMore Coverage inMore Coverage inthe same

20、time with the same time with extended Pitch!extended Pitch!Pitch 2 covers Pitch 2 covers 2x2xdistance as distance as Pitch 1Pitch 110mm P110mm P230s30sMore CoverScan Range=300mm10mm P110mm P110 mm/s10 mm/s10mm P210mm P220 mm/s20 mm/s30s15sCover the same volume in shorter time with extended PitchCove

21、r the same volume in shorter time with extended PitchScan Range=300mm10mm P110mm To reduce artifacts due to table motion during spiral scanning,we use a special reconstruction process called INTERPOLATION Interpolation Interpolation Converts volume data into slice Converts volume data into slice ima

22、gesimagesInterpolation AlgorithmTo reduce artifacts due to tabWide AlgorithmWide AlgorithmSlim AlgorithmSlim Algorithm2 x 360 2 x 360=720=720raw dataraw data2(180+52)2(180+52)=464=464raw dataraw dataWide algorithm produces a broader image thicknessWide algorithm produces a broader image thicknessWid

23、e algorithm uses more raw data=less image noiseWide algorithm uses more raw data=less image noiseWide AlgorithmSlim AlgoritPITCH 2PITCH 2PITCH 1PITCH 1Pitch 2 scanning produces a broader image thicknessPitch 2 scanning produces a broader image thicknessPitch 2 scanning does not increase image noiseP

24、itch 2 scanning does not increase image noise30%increase in 30%increase in image thicknessimage thicknesswith Pitch 2 with Pitch 2 PITCH 2PITCH 1Pitch 2 scanningSlice Sensitivity Profile(SSP)SSP describes the effective slice thickness of an image and to what extent anatomy within that slice contribu

25、te to the signalSSPRESOLUTIONAll points within the slice contribute equally&points outside of the slice do not contribute to the image at all.Image signalIdeal SSPZ-axis(mm)Collimation=width of x-ray beam=slice profileSlice Sensitivity Profile(SSSlice Profile(SP)Effective slice thickness of an image

26、 Slice ProfileResolutionFactors influencing Slice Profile Collimation Collimation Pitch Pitch Interpolation algorithm(360 or 180)Interpolation algorithm(360 or 180)Slice Profile(SP)Effective slFactors influencing SSPCollimator width collimation=SSPSpiral CTTable speed or PitchInterpolation Algorithm

27、=mathematical process required to reconstruct axial images from the spiral volume data setFactors influencing SSPCollimaPitch&Slice ProfilePitch&Slice ProfileSlim vs Wide SSP ComparisonSlice Profile Slim%Broaden Wide%BradenPitch One 5.0 mm 0 6.3 mm 26Pitch Two 6.5 mm 30 10.8 mm 116Slim vs Wide SSP C

28、omparisonSWIDE720 degreeMore photonsSLIM464 degreeLess photons SSP Spatial resolutionSSPSpatialresolutionSmoother imageNoisier imageWIDESLIM SSPSSPSmoother imagSlim-AdvantagesImproved Z ResolutionReduced partial volume artifactsSlim+extended PitchLonger coverageSame coverage with shorter scan time o

29、r thinner slicesLess radiation doseSlim-AdvantagesImproved Z Wide-AdvantagesNoise ReductionSmoother imageUseful for scanning huge patientOnly for scanning at Pitch OneWide-AdvantagesNoise ReductiSlice ProSlice Pro Lesion smaller than 1cm SCAN RANGE=150mm 10/10/10 10/10/10(15s)(15s)5/10/5 5/10/5(15s)

30、(15s)Slice Profile=10mm Slice Profile=6.5mmOptimizing the Scanning Parameters Lesion smaller than 1cm SSmallest Possible Effective Slice ThicknessScan Length Scan Length(mm)(mm)Scan Duration Scan Duration(s)(s)Table Table Speed Speed(mm/s)(mm/s)SmallestSmallestCollimation Collimation(mm)(mm)Scan Dur

31、ationScan DurationDepends on the scan length&patients breath-hold complianceTable SpeedTable SpeedPitch FactorPitch Factor1 Pitch 2 to cover the whole volume in one breath-holdSmallest Possible Effective SlComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing I

32、njection Protocols Clinical ApplicationsComputed Tomography CT BasicsPeripheral vein eg.antecubital vein 19-20 gauge needle or IV catheterVolumeFlow RateConcentrationInjection Protocols300 mg I/ml300 mg I/mlnon-ionic contrastnon-ionic contrast2-5 ml/s 2-5 ml/s cardiac output cardiac output80-150 ml8

33、0-150 ml patients weight®ion of interestpatients weight®ion of interestSiteScan DelayDelay between injection initiation&the Delay between injection initiation&the start of the scan sequencestart of the scan sequencePeripheral vein eg.antecubitaTailoring Scan&Injection ProtocolsInjection Durati

34、on must be Injection Duration must be equal to or greater thanequal to or greater than Scan Time Scan Time5050100100150150200200250250TimesTimesHUHUCONTRASTCONTRAST5050100100150150200200250250TimesTimesHUHUCONTRASTCONTRASTNaClNaClBolus Duration scan timeBolus Duration scan timeInsufficient,inhomogen

35、eousInsufficient,inhomogeneous opacificationopacificationBolus Duration=scan timeBolus Duration=scan timeUniform,maximum opacificationUniform,maximum opacificationEnhancement Curve of the Target RegionEnhancement Curve of the Target RegionTailoring Scan&Injection Pro5050100100150150200200250250Times

36、TimesHUHUOptimal WindowOptimal WindowEarlyEarlyTime-density curveTime-density curveof the target regionof the target regionLateLateContrast Bolus TimingDetermines optimal scan delay for spiral CTA sequenceCONTRASTCONTRASTNaClNaCl50100150200250TimesHUOptimalTest Bolus ProcedureImaging VolumeImaging V

37、olumefor spiral CTAfor spiral CTA 10-20 ml of contrast is injected 10-20 ml of contrast is injected at the chosen rate for spiral at the chosen rate for spiral After a delay of 8-10s,low-dose,After a delay of 8-10s,low-dose,single-level axial images are single-level axial images are acquired every 2

38、s at the starting acquired every 2s at the starting point of the imaging volume point of the imaging volume Dynamic scans at Dynamic scans at this positionthis position Dynamic Evaluation to generate Dynamic Evaluation to generate a Time-density curve a Time-density curveTest Bolus ProcedureImaging

39、VoDynamic EvaluationTime-density curve Scan Delay Peak Enhancement Time Dynamic ScansROI placed in the AortaDynamic EvaluationTime-densityComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsComputed Tomography CT Basic

40、s Arterial Phase Arterial Phase Venous Phase Venous PhaseDual Phase Liver ExamLiver Metastases Arterial Phase Venous PhaseDuSingle Plane Imaging with Multiplanar Results2D reconstruction based on a serial 2D reconstruction based on a serial of axial images along a certain axisof axial images along a

41、 certain axisSagittalSagittalCoronalCoronalOblique recon.of AortaOblique recon.of AortaSingle Plane Imaging with MultCT AngiographySpine 3D image:AVMSpine 3D image:AVMMax.IntensityMax.IntensityProjectionProjectionSurface ShadedSurface ShadedDisplay(3D)Display(3D)Femoral Arteries CT AngiogramFemoral

42、Arteries CT AngiogramCT AngiographySpine 3D image:3D Post-processing3D BronchoscopyLesion in the right upper lobe branchLesion in the right upper lobe branchColour Segmentation 3D Colour Segmentation 3D 3D Post-processing3D BronchoscVolume Rendering TechniqueTransparent imageTransparent imageTranspa

43、rent&color imageTransparent&color imageSolid ImageSolid ImageVolume Rendering TechniqueTranVirtual Endoscopy Real Time Fly through Real Time Fly through Reverse Perspective Reverse Perspective Axial Image reference Axial Image reference High Resolution High ResolutionBronchoscopyBronchoscopyVirtual Endoscopy Real Time FThe END