1、CT 成像原理介紹 CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsX-ray was discovered by a German scientist Roentgen 100 years ago.This made people for the first time be able toview the anatomy structure of human body without operationBut it
2、s superimposed 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 Cormack developed a mathematical method of reco
3、nstructing images from x-ray projections For the first time we could view:-Tomographic or“Slice”anatomy-Density differenceBut its time consumingAnd resolution needs to be improvedConcept of X-ray AttenuationAn X-ray beam passing throughthe body is attenuated(loses itsenergy)by:AbsorptionScatteringIn
4、cident X-rayTransmitted rayBODYTISSUEAbsorption by the tissue is proportional to the densityLessdensetissueMoredensetissueMORE ATTENUATIONLESS ATTENUATIONRecon.&postpro.Data acquisitionX-ray generationX-ray goes through collimator thereforepenetrate only an axial layer of the object,called slice Pat
5、ient is placed in the center of the measurement field X-ray is passed through the patients slice from many direction along a 360 path The transmitted beams are captured by the detectors which digitizes these signals These digitized signals called raw data are sent to a computer which create the CT i
6、mageThe object slice is divided into small volume elements called voxels.Each voxel is assigned a value which is dependent on the average amount of attenuationThe attenuation values are transferred to the computer where they are coded&used to create a slice imageCT Generations&Design“Generation”is u
7、sed to label CT tube-detector designsSlip-ring TechnologyPower is transmitted through parallel sets of conductive ringsinstead of electrical cables Continuous Gantry Rotation Prerequisite for Spiral CT CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clini
8、cal ApplicationsWhat is Spiral Scan?-just 4“C”Continuously rotating tube/detector systemContinuously generating X-rayContinuously table feedContinuously data acquisitionReconstruction of arbitraryslices(either contiguous or overlapping)within thescanned volume Distance between the slices is called I
9、ncrementABVolume DataContinuous data acquisitionIncrementSlice ThicknessContiguous Image ReconstructionIncrementOverlapSliceThicknessOverlapping Image ReconstructionImage Reconstruction with GapsIncrementSlice Thickness CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injecti
10、on Protocols Clinical ApplicationsScan ParametersX-ray Tube Voltage(kVp)X-ray Tube Current(mA)Scan Time(s)Slice thickness or Collimation(mm)Table Speed&Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch =Table Feed per rotation CollimationT Ta ab bl le e F Fe ee ed d C Co
11、ol ll li imma at ti io on n P 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 030s15sTo reduce artifacts due to table motion during spiral scanning,we use a special reconstruction process called INTERPOLATIONInterpolat
12、ion AlgorithmSlice Sensitivity Profile(SSP)SSP 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
13、signalIdeal SSPZ-axis(mm)Collimation=width of x-ray beam=slice profileSlice Profile(SP)Effective slice thickness of an image Slice ProfileResolutionFactors influencing SSPCollimator width collimation=SSPSpiral CTTable speed or PitchInterpolation Algorithm=mathematical process required to reconstruct
14、 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 photonsSLIM464 degreeLess photons SSP Spatial resolutionSSPSpatialresolutionSmoother imageNoi
15、sier imageSlim-AdvantagesImproved Z ResolutionReduced partial volume artifactsSlim+extended PitchLonger coverageSame coverage with shorter scan time or thinner slicesLess radiation doseWide-AdvantagesNoise ReductionSmoother imageUseful for scanning huge patientOnly for scanning at Pitch OneSlice Pro
16、C 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 oa ad de en
17、ne ed d1 11 16 63 30 0 Lesion smaller than 1cm SCAN RANGE=150mm Slice Profile=10mm Slice Profile=6.5mmOptimizing the Scanning ParametersSmallest Possible Effective Slice ThicknessDepends on the scan length&patients breath-hold compliance1 Pitch 2 to cover the whole volume in one breath-hold CT Basic
18、s Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsPeripheral vein eg.antecubital vein 19-20 gauge needle or IV catheterTailoring Scan&Injection ProtocolsDetermines optimal scan delay for spiral CTA sequenceTest Bolus ProcedureDynamic EvaluationTime-density curve Scan Delay Peak Enhancement Time Dynamic ScansROI placed in the Aorta CT Basics Principle of Spiral CT Scan Parameter&Image Quality Optimizing Injection Protocols Clinical ApplicationsCT Angiography3D Post-processing
