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1、CT 成像原理介紹2021/5/211Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications2021/5/212X-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
2、the anatomy structure of human body without operation77 But its superimposed 77 And we couldnt view soft tissue2021/5/213l 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 To
3、mography Early 1970s1963 - Alan Cormack developed a mathematical method of reconstructing images from x-ray projections2021/5/214 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 improved20
4、21/5/215Concept 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 proportional to the densityLessdensetissueMoredensetissueMORE ATTENUATI
5、ONLESS ATTENUATION2021/5/216How does CT Work?How does CT Work?Recon. & postpro.Data acquisitionX-ray generation2021/5/217X-ray goes through collimator thereforepenetrate only an axial layer of the object, called sliceHow does CT Work?How does CT Work?2021/5/218Patient is placed in the center of the
6、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 digitized signals called raw data are sent to a computer which create the CT imageHow does CT Work?How does CT Work?2
7、021/5/219The object slice is divided into small volume elements called voxels. Each voxel is assigned a value which is dependent on the average amount of attenuationHow is CT Image generated?2021/5/2110The attenuation values are transferred to the computer where they are coded & used to create a sli
8、ce imageHow is CT Image generated?2021/5/2111CT Generations & Design“Generation” is used to label CT tube-detector designs3rd Generation Design3rd Generation DesignRotating tube & detectorRotating tube & detector4th Generation Design4th Generation DesignFixed ring detectorFixed ring detector2021/5/2
9、112Slip-ring TechnologyPower is transmitted through parallel sets of conductive ringsinstead of electrical cables Continuous Gantry Rotation Prerequisite for Spiral CTNon Slip-ring ScannerNon Slip-ring ScannerSlip-ring ScannerSlip-ring Scanner2021/5/2113Computed Tomography CT Basics Principle of Spi
10、ral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications2021/5/2114What is Spiral Scan? - just 4“C”Continuously rotating tube/detector systemContinuously generating X-rayContinuously table feedContinuously data acquisition2021/5/2115Reconstruction of arbitraryslices
11、 (either contiguous or overlapping) within thescanned volume Distance between the slices is called IncrementABVolume DataContinuous data acquisition2021/5/2116Increment Slice ThicknessIncrement = Slice ThicknessIncrement = Slice Thickness No Overlap No Overlap No Gaps No GapsContiguous Image Reconst
12、ruction2021/5/2117Increment Overlap SliceThicknessOverlapping 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 Reconst
13、ruction with GapsIncrement Slice Thickness2021/5/2119Standard CT / Slice ImagingStandard CT / Slice ImagingDeep InspirationDeep InspirationShallow InspirationShallow Inspiration Misregistration Misregistration due to different due to different respiratory levels between slices respiratory levels bet
14、ween 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 slow2021/5/2120 Scan the whole region of Scan the whole region of interest in one breath hold interest
15、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 arbitrary position within the scanned volume within the
16、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 Imaging2021/5/2121Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Pr
17、otocols Clinical Applications2021/5/2122Scan 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 Process I
18、ncrement (mm) Increment (mm)2021/5/2123Table Speed & Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch = Table Feed per rotation Collimation2021/5/212410mm 10mm P1P110mm P210mm P23030s s30s30sMore Coverage inMore Coverage inthe same time with the same time with extended P
19、itch!extended Pitch!Pitch 2 covers Pitch 2 covers 2x2xdistance as distance as Pitch 1Pitch 12021/5/2125Scan 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 Pitch202
20、1/5/2126To 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 imagesimagesInterpolation Algorithm2021/5/2127Wide AlgorithmWide AlgorithmSl
21、im 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 thicknessWide algorithm uses more raw data = less image noiseWide algorithm uses more raw data =
22、less image noise2021/5/2128PITCH 2PITCH 2PITCH 1PITCH 1Pitch 2 scanning 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 thicknessimag
23、e thicknesswith Pitch 2 with Pitch 2 2021/5/2129Slice 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 n
24、ot contribute to the image at all .Image signalIdeal SSPZ-axis (mm)Collimation = width of x-ray beam =slice profile2021/5/2130Slice Profile (SP)Effective slice thickness of an image Slice ProfileResolutionFactors influencing Slice Profile Collimation Collimation Pitch Pitch Interpolation algorithm (
25、360 or 180) Interpolation algorithm (360 or 180)2021/5/2131Factors influencing SSPCollimator width collimation = SSPSpiral CTTable speed or PitchInterpolation Algorithm= mathematical process required to reconstruct axial images from the spiral volume data set2021/5/2132Pitch & Slice Profile2021/5/21
26、33Slim 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 1162021/5/2134WIDE720 degreeMore photonsSLIM464 degreeLess photons SSP Spatial resolutionSSPSpatialresolutionSmoother imageNoisier image2021/5/2135Slim - AdvantagesImproved Z
27、ResolutionReduced partial volume artifactsSlim + extended PitchLonger coverageSame coverage with shorter scan time or thinner slicesLess radiation dose2021/5/2136Wide - AdvantagesNoise ReductionSmoother imageUseful for scanning huge patientOnly for scanning at Pitch One2021/5/2137Slice Profile Compa
28、rison2021/5/2138 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 Parameters2021/5/2139Smallest Possible Effective Slice ThicknessScan Length Scan Length (mm)(mm)Scan Duration Scan Durat
29、ion (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 2 to cover the whole volume in one breath-hold2021/5/2140Computed
30、Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications2021/5/2141Peripheral vein eg. antecubital vein 19-20 gauge needle or IV catheterVolumeFlow RateConcentrationInjection Protocols300 mg I/ml300 mg I/mlnon-ionic contrastnon-i
31、onic contrast2 - 5 ml/s 2 - 5 ml/s cardiac output cardiac output80 - 150 ml80 - 150 ml patients weight & region of interestpatients weight & region of interestSiteScan DelayDelay between injection initiation & the Delay between injection initiation & the start of the scan sequencestart of the scan s
32、equence2021/5/2142Tailoring Scan & Injection ProtocolsInjection Duration must be Injection Duration must be equal to or greater thanequal to or greater than Scan Time Scan Time5050100100150150200200250250TimesTimesHUHUCONTRASTCONTRAST5050100100150150200200250250TimesTimesHUHUCONTRASTCONTRASTNaClNaCl
33、Bolus 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 of
34、the Target Region2021/5/21435050100100150150200200250250TimesTimesHUHUOptimal WindowOptimal WindowEarlyEarlyTime-density curveTime-density curveof the target regionof the target regionLateLateContrast Bolus TimingDetermines optimal scan delay for spiral CTA sequenceCONTRASTCONTRASTNaClNaCl2021/5/214
35、4Test Bolus ProcedureImaging 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 ar
36、e single-level axial images 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
37、a Time-density curve2021/5/2145Dynamic EvaluationTime-density curve Scan Delay Peak Enhancement Time Dynamic ScansROI placed in the Aorta2021/5/2146Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications2021/5/2147 Arte
38、rial Phase Arterial Phase Venous Phase Venous PhaseDual Phase Liver ExamLiver Metastases2021/5/2148Single 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 axisSagittalSagi
39、ttalCoronalCoronalOblique recon. of AortaOblique recon. of Aorta2021/5/2149CT AngiographySpine 3D image: AVMSpine 3D image: AVMMax. IntensityMax. IntensityProjectionProjectionSurface ShadedSurface ShadedDisplay (3D)Display (3D)Femoral Arteries CT AngiogramFemoral Arteries CT Angiogram2021/5/21503D P
40、ost-processing3D BronchoscopyLesion in the right upper lobe branchLesion in the right upper lobe branchColour Segmentation 3D Colour Segmentation 3D 2021/5/2151Volume Rendering TechniqueTransparent imageTransparent imageTransparent & color imageTransparent & color imageSolid ImageSolid Image2021/5/2152Virtual Endoscopy Real Time Fly through Real Time Fly through Reverse Perspective Reverse Perspective Axial Image reference Axial Image reference High Resolution High ResolutionBronchoscopyBronchoscopy2021/5/21532021/5/2154
