CT-成像原理介绍课件

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 structur

2、e of human body without operation But its superimposed And we couldnt view soft tissue My name is Godfrey Hounsfield I work for the Central Research Labs. of EMI, Ltd in England I developed the the first clinically useful CT scanner in 1971History of Computed Tomography Early 1970s1963 - Alan Cormac

3、k 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 difference But its time consuming And resolution needs to be improvedHow does CT Work?Recon. & postpro.Data acquisitionX-ra

4、y generationX-ray goes through collimator thereforepenetrate only an axial layer of the object, called sliceHow does CT Work?The 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 gen

5、erated?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 Generation DesignRotating tube & detector4th Generation DesignFixed ring detecto

6、rComputed 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 systemContinuously generating X-rayContinuously table feedContinuously data acquisitionIncremen

7、tSlice ThicknessIncrement = Slice Thickness No Overlap No GapsContiguous Image ReconstructionIncrementOverlapSliceThicknessOverlapping Image ReconstructionIncrement Slice Thickness Gaps between slices Images are further apart Less images createdImage Reconstruction with GapsIncrementSlice ThicknessS

8、tandard CT / Slice ImagingDeep InspirationShallow Inspiration Misregistration due to different respiratory levels between slicesPartial Volume Effect Unable to resconstruct images at arbitrary position Slice imaging is slow Scan the whole region of interest in one breath hold Reconstruction of overl

9、apping images without additional dose Retrospective reconstruction of slices in arbitrary position within the scanned volume No gaps since radiation always transmits the whole volumeSpiral CT / Volume ImagingComputed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizi

10、ng Injection Protocols Clinical ApplicationsTable Speed & Pitch Table Speed is defined as distance traveled in mm per 360 rotationPitch = Table Feed per rotation Collimation10mm P110mm P230s30sMore Coverage inthe same time with extended Pitch!Pitch 2 covers 2xdistance as Pitch 1To reduce artifacts d

11、ue to table motion during spiral scanning,we use a special reconstruction process called INTERPOLATION Interpolation Converts volume data into slice imagesInterpolation AlgorithmWide AlgorithmSlim Algorithm2 x 360 = 720raw data2 (180+52)= 464raw dataWide algorithm produces a broader image thicknessW

12、ide algorithm uses more raw data = less image noisePITCH 2PITCH 1Pitch 2 scanning produces a broader image thicknessPitch 2 scanning does not increase image noise30% increase in image thicknesswith Pitch 2 Slice Sensitivity Profile ( SSP )SSP describes the effective slice thickness of an image and t

13、o 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 x-ray beam =slice profileSlice Profile (SP)Effective

14、 slice thickness of an image Slice ProfileResolutionFactors influencing Slice Profile Collimation Pitch Interpolation algorithm (360 or 180)Factors influencing SSPCollimator width collimation = SSPSpiral CTTable speed or PitchInterpolation Algorithm= mathematical process required to reconstruct axia

15、l 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 imageNois

16、ier 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 OneSlic

17、e Profile Comparison Lesion smaller than 1cm SCAN RANGE = 150mm 10/10/10 (15s) 5/10/5 (15s) Slice Profile = 10mm Slice Profile = 6.5mmOptimizing the Scanning ParametersSmallest Possible Effective Slice ThicknessScan Length (mm)Scan Duration (s)Table Speed (mm/s)SmallestCollimation (mm)Scan DurationD

18、epends on the scan length & patients breath-hold complianceTable SpeedPitch Factor1 Pitch 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. antecubi

19、tal vein 19-20 gauge needle or IV catheterVolumeFlow RateConcentrationInjection Protocols300 mg I/mlnon-ionic contrast2 - 5 ml/s cardiac output80 - 150 ml patients weight & region of interestSiteScan DelayDelay between injection initiation & the start of the scan sequenceTailoring Scan & Injection P

20、rotocolsInjection Duration must be equal to or greater than Scan Time50100150200250TimesHUCONTRAST50100150200250TimesHUCONTRASTNaClBolus Duration scan timeInsufficient, inhomogeneous opacificationBolus Duration = scan timeUniform, maximum opacificationEnhancement Curve of the Target Region5010015020

21、0250TimesHUOptimal WindowEarlyTime-density curveof the target regionLateContrast Bolus TimingDetermines optimal scan delay for spiral CTA sequenceCONTRASTNaClTest Bolus ProcedureImaging Volumefor spiral CTA 10-20 ml of contrast is injected at the chosen rate for spiral After a delay of 8-10s, low-do

22、se, single-level axial images are acquired every 2s at the starting point of the imaging volume Dynamic scans at this position Dynamic Evaluation to generate a Time-density curveDynamic EvaluationTime-density curve Scan Delay Peak Enhancement Time Dynamic ScansROI placed in the AortaComputed Tomography CT