1、CT 成像原理介紹1 CT BasicsCT Basics Principle of Spiral CTPrinciple of Spiral CT Scan Parameter&Image QualityScan Parameter&Image Quality Optimizing Injection ProtocolsOptimizing Injection Protocols Clinical ApplicationsClinical Applications2X-ray was discovered by a German scientist Roentgen X-ray was di
2、scovered by a German scientist Roentgen 100 years ago.100 years ago.This made people for the first time be able toThis made people for the first time be able toview the anatomy structure of view the anatomy structure of human body without operation human body without operationBut its superimposed Bu
3、t its superimposed And we couldnt view soft And we couldnt view soft tissuetissue3l My name is Godfrey HounsfieldMy name is Godfrey Hounsfieldl I work for the Central Research I work for the Central Research Labs.of EMI,Ltd in England Labs.of EMI,Ltd in Englandl I developed the the first clinically
4、I developed the the first clinically useful CT scanner in 1971 useful CT scanner in 1971History of Computed Tomography Early Early 1970s1970s1963-Alan Cormack developed a 1963-Alan Cormack developed a mathematical method of reconstructing mathematical method of reconstructing images from x-ray proje
5、ctionsimages from x-ray projections4 For the first time we For the first time we could view:could view:-Tomographic or“-Tomographic or“SliceSlice”anatomyanatomy-Density difference-Density differenceBut its time consumingBut its time consumingAnd resolution needs to be And resolution needs to be impr
6、ovedimproved5Concept of X-ray AttenuationAn X-ray beam passing throughAn X-ray beam passing throughthe body is attenuated(loses the body is attenuated(loses itsitsenergy)by:energy)by:AbsorptionAbsorptionScatteringScatteringIncident X-rayIncident X-rayTransmitted rayTransmitted rayBODYBODYTISSUETISSU
7、EAbsorption by the tissue is proportional to the densityAbsorption by the tissue is proportional to the densityLessdensetissueMoredensetissueMORE ATTENUATIONMORE ATTENUATIONLESS ATTENUATIONLESS ATTENUATION6Recon.&postpro.Data acquisitionX-ray generation7X-ray goes through X-ray goes through collimat
8、or thereforecollimator thereforepenetrate only an axial penetrate only an axial layer of the object,layer of the object,called called sliceslice8Patient is placed in the Patient is placed in the center of the measurement center of the measurement fieldfieldX-ray is passed through the X-ray is passed
9、 through the patients slice from many patients slice from many direction along a 360direction along a 360 path pathThe transmitted beams are The transmitted beams are captured by the detectors captured by the detectors which digitizes these which digitizes these signalssignalsThese digitized signals
10、 These digitized signals called raw data are sent to called raw data are sent to a computer which create the a computer which create the CT imageCT image9The object slice is divided The object slice is divided into small volume elements into small volume elements called voxels.called voxels.Each vox
11、el is assigned a Each voxel is assigned a value which is dependent on value which is dependent on the average amount of the average amount of attenuationattenuation10The attenuation values are transferred to the computer The attenuation values are transferred to the computer where they are coded&use
12、d to create a slice imagewhere they are coded&used to create a slice image11CT Generations&Design“Generation”“Generation”is used to label CT tube-is used to label CT tube-detector designsdetector designs12Slip-ring TechnologyPower is transmitted through parallel sets of conductive Power is transmitt
13、ed through parallel sets of conductive ringsringsinstead of electrical cablesinstead of electrical cables Continuous Gantry RotationContinuous Gantry Rotation Prerequisite for Spiral CT Prerequisite for Spiral CT13 CT BasicsCT Basics Principle of Spiral CTPrinciple of Spiral CT Scan Parameter&Image
14、QualityScan Parameter&Image Quality Optimizing Injection ProtocolsOptimizing Injection Protocols Clinical ApplicationsClinical Applications14What is Spiral Scan?-just What is Spiral Scan?-just 4“C”4“C”C Continuously rotating tube/detector ontinuously rotating tube/detector systemsystemC Continuously
15、 generating X-rayontinuously generating X-rayC Continuously table feedontinuously table feedC Continuously data acquisitionontinuously data acquisition15Reconstruction of Reconstruction of arbitraryarbitraryslices(either slices(either contiguous or contiguous or overlapping)within theoverlapping)wit
16、hin thescanned volume scanned volume Distance between the Distance between the slices is called slices is called IncrementIncrementA AB BVolume DataVolume DataContinuous data Continuous data acquisitionacquisition16IncrementIncrementSlice ThicknessSlice ThicknessContiguous Image Reconstruction17Incr
17、ementIncrementOverlapOverlapSliceSlice ThicknessThicknessOverlapping Image Reconstruction18Image Reconstruction with GapsIncrementIncrementSlice ThicknessSlice Thickness192021 CT BasicsCT Basics Principle of Spiral CTPrinciple of Spiral CT Scan Parameter&Image QualityScan Parameter&Image Quality Opt
18、imizing Injection ProtocolsOptimizing Injection Protocols Clinical ApplicationsClinical Applications22Scan ParametersX-ray Tube Voltage X-ray Tube Voltage(kVp)(kVp)X-ray Tube Current X-ray Tube Current(mA)(mA)Scan Time Scan Time(s)(s)Slice thickness or Slice thickness or Collimation Collimation(mm)(
19、mm)23Table Speed&Pitch Table SpeedTable Speed is defined as distance is defined as distance traveled in mm traveled in mm per 360 rotationper 360 rotationPitchPitch =Table Feed per Table Feed per rotationrotation Collimation CollimationT Ta ab bl le e F Fe ee ed d C Co ol ll li imma at ti io on n P
20、Pi it tc ch h 1 10 0 mmmm/r ro ot t1 10 0 mmmm1 1.0 01 15 5 mmmm/r ro ot t1 10 0 mmmm1 1.5 52 20 0 mmmm/r ro ot t1 10 0 mmmm2 2.0 0242530s30s15s15s26To reduce artifacts due to table motion during spiral To reduce artifacts due to table motion during spiral scanning,scanning,we use a special reconstr
21、uction process called we use a special reconstruction process called INTERPOLATIONINTERPOLATIONInterpolation Algorithm272829Slice Sensitivity Profile(SSP)Slice Sensitivity Profile(SSP)SSP describes the SSP describes the effective slice effective slice thicknessthickness of an image and to what of an
22、 image and to what extent anatomy within that slice extent anatomy within that slice contribute to the signalcontribute to the signalSSPSSPRESOLUTIONRESOLUTIONAll points within the All points within the slice contribute slice contribute equally&points equally&points outside of the slice outside of t
23、he slice do not contribute to do not contribute to the image at all.the image at all.Image Image signalsignalIdeal Ideal SSPSSPZ-axis Z-axis(mm)(mm)Collimation=Collimation=width of x-ray width of x-ray beam=slice beam=slice profileprofile30Slice Profile(SP)Effective slice thickness of an Effective s
24、lice thickness of an image image Slice ProfileSlice ProfileResolutionResolution31Factors influencing SSPFactors influencing SSPCollimator widthCollimator width collimation=collimation=SSPSSPSpiral CTSpiral CTTable speed or PitchTable speed or PitchInterpolation AlgorithmInterpolation Algorithm=mathe
25、matical process required to reconstruct axial images from the spiral volume data set32Pitch&Slice ProfilePitch&Slice Profile33Slim vs Wide SSP ComparisonSlim vs Wide SSP ComparisonSlice Profile Slim%Broaden Wide%BradenSlice Profile Slim%Broaden Wide%BradenPitch One 5.0 mm 0 6.3 mm 26Pitch One 5.0 mm
26、 0 6.3 mm 26Pitch Two 6.5 mm 30 10.8 mm 116Pitch Two 6.5 mm 30 10.8 mm 11634WIDEWIDE720 degree720 degreeMore photonsMore photonsSLIMSLIM464 degree464 degreeLess photonsLess photons SSP SSP Spatial Spatial resolution resolutionSSPSSPSpatialSpatialresolutionresolutionSmoother Smoother imageimageNoisie
27、r Noisier imageimage35Slim-AdvantagesSlim-AdvantagesImproved Z ResolutionImproved Z ResolutionReduced partial volume Reduced partial volume artifactsartifactsSlim+extended PitchSlim+extended PitchLonger coverageLonger coverageSame coverage with shorter Same coverage with shorter scan time or thinner
28、 slicesscan time or thinner slicesLess radiation doseLess radiation dose36Wide-AdvantagesWide-AdvantagesNoise ReductionNoise ReductionSmoother imageSmoother imageUseful for scanning huge Useful for scanning huge patientpatientOnly for scanning at Only for scanning at Pitch OnePitch One37Slice Profil
29、e ComparisonC Co ol ll li imma at ti io on n5 5.0 0 mmmmWWi id de eI In nt te er rp po ol la at ti io on nS Sl li immI In nt te er rp po ol la at ti io on nP Pi it tc ch h 1 1.0 06 6.3 3 mmmm5 5.0 0 mmmm%B Br ro oa ad de en ne ed d2 26 60 0P Pi it tc ch h 2 2.0 01 10 0.8 8 mmmm 6 6.5 5 mmmm%B Br ro
30、oa ad de en ne ed d1 11 16 63 30 038 Lesion smaller than 1cmLesion smaller than 1cm SCAN RANGE=150SCAN RANGE=150mmmm Slice Profile=10Slice Profile=10mmmm Slice Profile=6.5Slice Profile=6.5mmmmOptimizing the Scanning Parameters39Smallest Possible Effective Slice ThicknessDepends on the scan length&De
31、pends on the scan length&patients patients breath-hold compliancebreath-hold compliance1 Pitch 21 Pitch 2 to cover the whole volume in one to cover the whole volume in one breath-holdbreath-hold40 CT BasicsCT Basics Principle of Spiral CTPrinciple of Spiral CT Scan Parameter&Image QualityScan Parame
32、ter&Image Quality Optimizing Injection ProtocolsOptimizing Injection Protocols Clinical ApplicationsClinical Applications41Peripheral vein eg.antecubital Peripheral vein eg.antecubital vein vein 19-20 gauge needle or IV catheter19-20 gauge needle or IV catheter42Tailoring Scan&Injection Protocols43D
33、etermines optimal scan delay for spiral CTA Determines optimal scan delay for spiral CTA sequencesequence44Test Bolus Procedure45Dynamic EvaluationTime-density curveTime-density curve Scan Delay Scan Delay Peak Peak Enhancement TimeEnhancement Time Dynamic ScansDynamic ScansROI placed ROI placed in the in the AortaAorta46 CT BasicsCT Basics Principle of Spiral CTPrinciple of Spiral CT Scan Parameter&Image QualityScan Parameter&Image Quality Optimizing Injection ProtocolsOptimizing Injection Protocols Clinical ApplicationsClinical Applications474849CT Angiography503D Post-processing51525354
