1、CT 成像原理介紹,Computed Tomography,CT BasicsPrinciple of Spiral CTScan Parameter & Image QualityOptimizing Injection ProtocolsClinical Applications,X-Ray Discovery,X-ray was discovered by a German scientist Roentgen 100 years ago. This made people for the first time be able toview the anatomy structure o
2、f human body without operation,But its superimposed And we couldnt view soft tissue,My name is Godfrey HounsfieldI work for the Central Research Labs. of EMI, Ltd in EnglandI developed the the first clinically useful CT scanner in 1971,History of Computed Tomography,Early 1970s,1963 - Alan Cormack d
3、eveloped 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 consumingAnd resolution needs to be improved,Concept of X-ray Attenuation,An X-ray beam passing thr
4、ough the body is attenuated (loses its energy) by : Absorption Scattering,Incident X-ray,Transmitted ray,SCATTERED X-RAYS,BODY TISSUE,Absorption by the tissue is proportional to the density,Less dense tissue,More dense tissue,MORE ATTENUATION,LESS ATTENUATION,How does CT Work?,X-ray goes through col
5、limator therefore penetrate only an axial layer of the object, called “slice“,How does CT Work?,Patient is placed in the center of the 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 t
6、hese signalsThese digitized signals called raw data are sent to a computer which create the CT image,How 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 attenuation,How is CT Image generate
7、d?,The attenuation values are transferred to the computer where they are coded & used to create a slice image,How is CT Image generated?,CT Generations & Design,“Generation” is used to label CT tube-detector designs,3rd Generation Design Rotating tube & detector,4th Generation Design Fixed ring dete
8、ctor,Slip-ring Technology,Power is transmitted through parallel sets of conductive rings instead of electrical cables Continuous Gantry Rotation Prerequisite for Spiral CT,Non Slip-ring Scanner,Slip-ring Scanner,Computed Tomography,CT BasicsPrinciple of Spiral CTScan Parameter & Image QualityOptimiz
9、ing Injection ProtocolsClinical Applications,What is Spiral Scan? - just 4“C”,Continuously rotating tube/detector system Continuously generating X-ray Continuously table feed Continuously data acquisition,Reconstruction of arbitrary slices (either contiguous or overlapping) within the scanned volume
10、 Distance between the slices is called Increment,A,B,Volume Data,Continuous data acquisition,Increment,Slice Thickness,Increment = Slice ThicknessNo OverlapNo Gaps,Contiguous Image Reconstruction,Increment,Overlap,Slice,Thickness,Overlapping Image Reconstruction,Increment Slice ThicknessOverlap of s
11、licesCloser image intervalMore images created,Increment Slice ThicknessGaps between slicesImages are further apartLess images created,Image Reconstruction with Gaps,Increment,Slice Thickness,Standard CT / Slice Imaging,Deep Inspiration,Shallow Inspiration,Misregistration due to different respiratory
12、 levels between slices,Partial Volume Effect,Unable to resconstruct images atarbitrary position,Slice imaging is slow,Scan the whole region of interest in one breath hold,Reconstruction of overlapping images without additional dose,Retrospective reconstruction of slices in arbitrary position within
13、the scanned volume,No gaps since radiation alwaystransmits the whole volume,Spiral CT / Volume Imaging,Computed Tomography,CT BasicsPrinciple of Spiral CTScan Parameter & Image QualityOptimizing Injection ProtocolsClinical Applications,Scan Parameters,X-ray Tube Voltage (kVp) X-ray Tube Current (mA)
14、 Scan Time (s) Slice thickness or Collimation (mm),Table Speed (mm/rot) or Feed per 360 rotationPitchInterpolation ProcessIncrement (mm),Table Speed & Pitch,Table Speed is defined as distance traveled in mm per 360 rotation,Pitch =,Table Feed per rotationCollimation,10mm P1,10mm P2,30s,30s,More Cove
15、rage in the same time with extended Pitch!,Pitch 2 covers 2x distance as Pitch 1,Scan Range = 300mm,10mm P1 10 mm/s,10mm P2 20 mm/s,30s,15s,Cover the same volume in shorter time with extended Pitch,To reduce artifacts due to table motion during spiral scanning, we use a special reconstruction proces
16、s called INTERPOLATION,Interpolation,Converts volume data into slice images,Interpolation Algorithm,Wide Algorithm,Slim Algorithm,2 x 360 = 720 raw data,2 (180+52) = 464 raw data,Wide algorithm produces a broader image thickness Wide algorithm uses more raw data = less image noise,PITCH 2,PITCH 1,Pi
17、tch 2 scanning produces a broader image thickness Pitch 2 scanning does not increase image noise,30% increase in image thickness with Pitch 2,Slice Sensitivity Profile ( SSP ),SSP describes the effective slice thickness of an image and to what extent anatomy within that slice contribute to the signa
18、l,SSP,RESOLUTION,All points within the slice contribute equally & points outside of the slice do not contribute to the image at all .,Image signal,Ideal SSP,Z-axis (mm),Collimation = width of x-ray beam =slice profile,Slice Profile (SP),Effective slice thickness of an image,Slice Profile Resolution,
19、Factors influencing Slice ProfileCollimationPitchInterpolation algorithm (360 or 180),Factors influencing SSP,Collimator widthcollimation = SSP Spiral CT Table speed or Pitch Interpolation Algorithm = mathematical process required to reconstruct axial images from the spiral volume data set,Pitch & S
20、lice Profile,Slim vs Wide SSP Comparison,Slice Profile Slim %Broaden Wide %Braden Pitch One 5.0 mm 0 6.3 mm 26 Pitch Two 6.5 mm 30 10.8 mm 116,WIDE 720 degree More photons,SLIM 464 degree Less photons,SSPSpatialresolution,SSP Spatial resolution,Smoother image,Noisier image,Slim - Advantages,Improved
21、 Z Resolution Reduced partial volume artifacts Slim + extended Pitch Longer coverage Same coverage with shorter scan time or thinner slices Less radiation dose,Wide - Advantages,Noise Reduction Smoother image Useful for scanning huge patient Only for scanning at Pitch One,Slice Profile Comparison,Le
22、sion smaller than 1cm,SCAN RANGE = 150mm,10/10/10 (15s),5/10/5 (15s),Slice Profile = 10mm,Slice Profile = 6.5mm,Optimizing the Scanning Parameters,Smallest Possible Effective Slice Thickness,Scan Length (mm),Scan Duration (s),Table Speed (mm/s),Smallest Collimation (mm),Scan Duration,Depends on the
23、scan length & patients breath-hold compliance,Table Speed Pitch Factor,1 Pitch 2 to cover the whole volume in one breath-hold,Computed Tomography,CT BasicsPrinciple of Spiral CTScan Parameter & Image QualityOptimizing Injection ProtocolsClinical Applications,Peripheral vein eg. antecubital vein 19-2
24、0 gauge needle or IV catheter,Volume,Flow Rate,Concentration,Injection Protocols,300 mg I/ml non-ionic contrast,2 - 5 ml/s cardiac output,80 - 150 ml patients weight & region of interest,Site,Scan Delay,Delay between injection initiation & the start of the scan sequence,Tailoring Scan & Injection Pr
25、otocols,Injection Duration must be equal to or greater than Scan Time,50,100,150,200,250,Times,HU,CONTRAST,50,100,150,200,250,Times,HU,CONTRAST,NaCl,Bolus Duration scan time Insufficient, inhomogeneous opacification,Bolus Duration = scan time Uniform, maximum opacification,Enhancement Curve of the T
26、arget Region,50,100,150,200,250,Times,HU,Optimal Window,Early,Time-density curve of the target region,Late,Contrast Bolus Timing,Determines optimal scan delay for spiral CTA sequence,CONTRAST,NaCl,Test Bolus Procedure,Imaging Volume for spiral CTA,10-20 ml of contrast is injectedat the chosen rate f
27、or spiral,After a delay of 8-10s, low-dose,single-level axial images are acquired every 2s at the startingpoint of the imaging volume,Dynamic scans at this position,Dynamic Evaluation to generate a Time-density curve,Dynamic Evaluation,Time-density curveScan Delay Peak Enhancement Time,Dynamic Scans
28、,ROI placed in the Aorta,Computed Tomography,CT BasicsPrinciple of Spiral CTScan Parameter & Image QualityOptimizing Injection ProtocolsClinical Applications,Arterial Phase,Venous Phase,Dual Phase Liver Exam Liver Metastases,Single Plane Imaging with Multiplanar Results,2D reconstruction based on a
29、serial of axial images along a certain axis,Sagittal,Coronal,Oblique recon. of Aorta,CT Angiography,Spine 3D image: AVM,Max. Intensity Projection,Surface Shaded Display (3D),Femoral Arteries CT Angiogram,3D Post-processing,3D Bronchoscopy Lesion in the right upper lobe branch,Colour Segmentation 3D,Volume Rendering Technique,Transparent image,Transparent & color image,Solid Image,Virtual Endoscopy,Real Time Fly throughReverse PerspectiveAxial Image referenceHigh Resolution,Bronchoscopy,The END,
