CT+成像原理介紹

1、CT+成像原理介紹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 ope

2、rationBut its 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 mathematica

3、l method of reconstructing 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

4、:AbsorptionScatteringIncident 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 th

5、e object, called slice Patient 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 comp

6、uter which create the CT imageThe 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 Gener

7、ations & Design“Generation” is used 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 Op

8、timizing Injection Protocols Clinical 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 D

9、istance between the slices is called IncrementABVolume DataContinuous data acquisitionIncrementSlice ThicknessContiguous Image ReconstructionIncrementOverlapSliceThicknessOverlapping Image ReconstructionImage Reconstruction with GapsIncrementSlice Thickness CT Basics Principle of Spiral CT Scan Para

10、meter & 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 & Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch = Table Feed per rotat

11、ion CollimationT Ta ab bl le e F Fe ee ed d C Co 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

12、special reconstruction process called INTERPOLATIONInterpolation AlgorithmCT+成像原理介紹Slice 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

13、 & 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 Profile (SP)Effective slice thickness of an image Slice ProfileResolutionCT+成像原理介紹Factors influencing SSPCollimator width collimation = SSPSpira

14、l CTTable speed or PitchInterpolation Algorithm= mathematical process required to reconstruct axial images from the spiral volume data setCT+成像原理介紹Pitch & Slice ProfileCT+成像原理介紹Slim 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

15、116CT+成像原理介紹WIDE720 degreeMore photonsSLIM464 degreeLess photons SSP Spatial resolutionSSPSpatialresolutionSmoother imageNoisier imageCT+成像原理介紹Slim - AdvantagesImproved Z ResolutionReduced partial volume artifactsSlim + extended PitchLonger coverageSame coverage with shorter scan time or thinner sli

16、cesLess radiation doseCT+成像原理介紹Wide - AdvantagesNoise ReductionSmoother imageUseful for scanning huge patientOnly for scanning at Pitch OneSlice ProC 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 i

17、t 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 ne ed d1 11 16 63 30 0 Lesion smaller than 1cm SCAN RANGE = 150mm Slice Profile = 10mm Slice Profile = 6.5mmOptimizing the Scanning Par

18、ametersSmallest Possible Effective Slice ThicknessDepends on the scan length & patients breath-hold compliance1 Pitch 2 to cover the whole volume in one breath-hold 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 catheterTailoring Scan & Injection ProtocolsDetermines optimal scan delay for spir