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1、CT 成像原理介紹Computed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsX-Ray Discovery X-ray was discovered by a German scientist Roentgen 100 years ago.This made people for the first time be able to view the anatomy structure o
2、f human body without operation77 But its superimposed 77 And we couldnt view soft tissuel 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 scanner in 1971History of Computed Tomography Early 1970s1963-Alan Corma
3、ck developed a mathematical method of reconstructing images from x-ray projections For the first time we could view:CT Broke the Barrier-Tomographic or“Slice”anatomy-Density difference77 But its time consuming77 And resolution needs to be improvedConcept of X-ray AttenuationAn X-ray beam passing thr
4、oughthe body is attenuated(loses itsenergy)by:AbsorptionScatteringIncident X-rayTransmitted raySCATTERED X-SCATTERED X-RAYSRAYSBODYTISSUEAbsorption by the tissue is proportional to the densityLessdensetissueMoredensetissueMORE ATTENUATIONLESS ATTENUATIONHow does CT Work?How does CT Work?Recon.&postp
5、ro.Data acquisitionX-ray generationX-ray goes through collimator thereforepenetrate only an axial layer of the object,called sliceHow does CT Work?How does CT Work?Patient is placed in the center of the measurement fieldX-ray is passed through the patients slice from many direction along a 360 pathT
6、he transmitted beams are captured by the detectors which digitizes these signalsThese digitized signals called raw data are sent to a computer which create the CT imageHow does CT Work?How does CT Work?The object slice is divided into small volume elements called voxels.Each voxel is assigned a valu
7、e which is dependent on the average amount of attenuationHow is CT Image generated?The attenuation values are transferred to the computer where they are coded&used to create a slice imageHow is CT Image generated?CT Generations&Design“Generation”is used to label CT tube-detector designs3rd Generatio
8、n Design3rd Generation DesignRotating tube&detectorRotating tube&detector4th Generation Design4th Generation DesignFixed ring detectorFixed ring detectorSlip-ring TechnologyPower is transmitted through parallel sets of conductive ringsinstead of electrical cables Continuous Gantry Rotation Prerequis
9、ite for Spiral CTNon Slip-ring ScannerNon Slip-ring ScannerSlip-ring ScannerSlip-ring ScannerComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsWhat is Spiral Scan?-just 4“C”Continuously rotating tube/detector systemC
10、ontinuously generating X-rayContinuously table feedContinuously data acquisitionReconstruction of arbitraryslices(either contiguous or overlapping)within thescanned volume Distance between the slices is called IncrementABVolume DataContinuous data acquisitionIncrementSlice ThicknessIncrement=Slice T
11、hicknessIncrement=Slice Thickness No Overlap No Overlap No Gaps No GapsContiguous Image ReconstructionIncrementOverlapSliceThicknessOverlapping Image ReconstructionIncrement Slice ThicknessIncrement Slice ThicknessIncrement Slice Thickness Gaps between slices Gaps between slices Images are further a
12、part Images are further apart Less images created Less images createdImage Reconstruction with GapsIncrementSlice ThicknessStandard CT/Slice ImagingStandard CT/Slice ImagingDeep InspirationDeep InspirationShallow InspirationShallow Inspiration Misregistration Misregistration due to different due to
13、different respiratory levels between slices respiratory levels between slicesPartial Volume EffectPartial Volume Effect Unable to resconstruct Unable to resconstruct images at images at arbitrary position arbitrary position Slice imaging is slow Slice imaging is slow Scan the whole region of Scan th
14、e whole region of interest in one breath hold interest in one breath 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 a
15、rbitrary position within the scanned volume within the scanned volume No gaps since radiation always No gaps since radiation always transmits the whole volume transmits the whole volumeSpiral CT/Volume ImagingSpiral CT/Volume ImagingComputed Tomography CT Basics Principle of Spiral CT Scan Parameter
16、&Image Quality Optimizing Injection Protocols Clinical ApplicationsScan 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 Interp
17、olation Process Increment(mm)Increment(mm)Table Speed&Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch =Table Feed per rotation Collimation10mm 10mm P1P110mm P210mm P23030s s30s30sMore Coverage inMore Coverage inthe same time with the same time with extended Pitch!extend
18、ed Pitch!Pitch 2 covers Pitch 2 covers 2x2xdistance as distance as Pitch 1Pitch 1Scan Range=300mm10mm P110mm P110 mm/s10 mm/s10mm P210mm P220 mm/s20 mm/s30s15sCover the same volume in shorter time with extended PitchCover the same volume in shorter time with extended PitchTo reduce artifacts due to
19、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 imagesimagesInterpolation AlgorithmWide AlgorithmWide AlgorithmSlim AlgorithmSlim Algorithm2 x 360 2 x 360=720=
20、720raw dataraw data2(180+52)2(180+52)=464=464raw dataraw dataWide algorithm produces a broader image thicknessWide algorithm produces a broader image thicknessWide algorithm uses more raw data=less image noiseWide algorithm uses more raw data=less image noisePITCH 2PITCH 2PITCH 1PITCH 1Pitch 2 scann
21、ing produces a broader image thicknessPitch 2 scanning produces a broader image thicknessPitch 2 scanning does not increase image noisePitch 2 scanning does not increase image noise30%increase in 30%increase in image thicknessimage thicknesswith Pitch 2 with Pitch 2 Slice Sensitivity Profile(SSP)SSP
22、 describes the effective slice thickness of an image and to what extent anatomy within that slice contribute 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
23、x-ray beam=slice profileSlice Profile(SP)Effective slice thickness of an image Slice ProfileResolutionFactors influencing Slice Profile Collimation Collimation Pitch Pitch Interpolation algorithm(360 or 180)Interpolation algorithm(360 or 180)Factors influencing SSPCollimator width collimation=SSPSpi
24、ral CTTable speed or PitchInterpolation Algorithm=mathematical process required to reconstruct axial images from the spiral volume data setPitch&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 116WIDE720 degreeMore
25、 photonsSLIM464 degreeLess photons SSP Spatial resolutionSSPSpatialresolutionSmoother imageNoisier imageSlim-AdvantagesImproved Z ResolutionReduced partial volume artifactsSlim+extended PitchLonger coverageSame coverage with shorter scan time or thinner slicesLess radiation doseWide-AdvantagesNoise
26、ReductionSmoother imageUseful for scanning huge patientOnly for scanning at Pitch OneSlice Profile Comparison Lesion smaller than 1cm SCAN RANGE=150mm 10/10/10 10/10/10(15s)(15s)5/10/5 5/10/5(15s)(15s)Slice Profile=10mm Slice Profile=6.5mmOptimizing the Scanning ParametersSmallest Possible Effective
27、 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 DurationScan DurationDepends on the scan length&patients breath-hold complianceTable SpeedTable SpeedPitch FactorPitch Factor1 Pitch
28、2 to cover the whole volume in one breath-holdComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsPeripheral vein eg.antecubital vein 19-20 gauge needle or IV catheterVolumeFlow RateConcentrationInjection Protocols300
29、mg I/ml300 mg I/mlnon-ionic contrastnon-ionic contrast2-5 ml/s 2-5 ml/s cardiac output cardiac output80-150 ml80-150 ml patients weight®ion of interestpatients weight®ion of interestSiteScan DelayDelay between injection initiation&the Delay between injection initiation&the start of the scan se
30、quencestart of the scan sequenceTailoring Scan&Injection ProtocolsInjection Duration must be Injection Duration must be equal to or greater thanequal to or greater than Scan Time Scan Time5050100100150150200200250250TimesTimesHUHUCONTRASTCONTRAST5050100100150150200200250250TimesTimesHUHUCONTRASTCONT
31、RASTNaClNaClBolus Duration scan timeBolus Duration scan timeInsufficient,inhomogeneousInsufficient,inhomogeneous opacificationopacificationBolus Duration=scan timeBolus Duration=scan timeUniform,maximum opacificationUniform,maximum opacificationEnhancement Curve of the Target RegionEnhancement Curve
32、 of the Target Region5050100100150150200200250250TimesTimesHUHUOptimal WindowOptimal WindowEarlyEarlyTime-density curveTime-density curveof the target regionof the target regionLateLateContrast Bolus TimingDetermines optimal scan delay for spiral CTA sequenceCONTRASTCONTRASTNaClNaClTest Bolus Proced
33、ureImaging VolumeImaging Volumefor 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 i
34、mages are acquired every 2s 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 curveDy
35、namic EvaluationTime-density curve Scan Delay Peak Enhancement Time Dynamic ScansROI placed in the AortaComputed Tomography CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical Applications Arterial Phase Arterial Phase Venous Phase Venous PhaseDual P
36、hase Liver ExamLiver MetastasesSingle 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 certain axisSagittalSagittalCoronalCoronalOblique recon.of AortaOblique recon.of AortaCT Ang
37、iographySpine 3D image:AVMSpine 3D image:AVMMax.IntensityMax.IntensityProjectionProjectionSurface ShadedSurface ShadedDisplay(3D)Display(3D)Femoral Arteries CT AngiogramFemoral Arteries CT Angiogram3D Post-processing3D BronchoscopyLesion in the right upper lobe branchLesion in the right upper lobe b
38、ranchColour Segmentation 3D Colour Segmentation 3D Volume Rendering TechniqueTransparent imageTransparent imageTransparent&color imageTransparent&color imageSolid ImageSolid ImageVirtual Endoscopy Real Time Fly through Real Time Fly through Reverse Perspective Reverse Perspective Axial Image reference Axial Image reference High Resolution High ResolutionBronchoscopyBronchoscopy