Transcript CT 成像原理介紹_1
CT 成像原理介紹 Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical 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 to view the anatomy structure of human body without operation But it's superimposed And we couldn't view soft tissue History of Computed Tomography 1963 - Alan Cormack developed a mathematical method of reconstructing images from x-ray projections 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 1971 Early 1970s CT Broke the Barrier For the first time we could view: - Tomographic or “Slice” anatomy - Density difference But it's time consuming And resolution needs to be improved Concept of X-ray Attenuation SCATTERED XRAYS An X-ray beam passing through the body is attenuated (loses its energy) by : Absorption Scattering Incident X-ray BODY TISSUE Transmitted ray Absorption by the tissue is proportional to the density More dense tissue MORE ATTENUATION Less dense tissue LESS ATTENUATION How does CT Work? X-ray generation Data acquisition Recon. & postpro. How does CT Work? X-ray goes through collimator therefore penetrate only an axial layer of the object, called "slice" How does CT Work? Patient is placed in the center of the measurement field X-ray is passed through the patient’s 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 image How is CT Image generated? 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 generated? The attenuation values are transferred to the computer where they are coded & used to create a slice image CT Generations & Design “Generation” is used to label CT tube-detector designs 3rd Generation Design Rotating tube & detector 4th Generation Design Fixed ring detector 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 Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications What is Spiral Scan? -- just 4“C” Continuously rotating tube/detector system Continuously generating X-ray Continuously table feed Continuously data acquisition Continuous data acquisition A Volume Data Reconstruction of arbitrary slices (either contiguous or overlapping) within the scanned volume Distance between the slices is called Increment B Contiguous Image Reconstruction Slice Thickness Increment = Slice Thickness No Overlap No Gaps Increment Overlapping Image Reconstruction SliceThickness Overlap Increment < Slice Thickness Overlap of slices Closer image interval More images created Increment Image Reconstruction with Gaps Slice Thickness Increment > Slice Thickness Gaps between slices Images are further apart Less images created Increment Deep Inspiration Shallow Inspiration Standard CT / Slice Imaging Misregistration due to different respiratory levels between slices Unable to resconstruct images at arbitrary position Partial Volume Effect Slice imaging is slow Spiral CT / Volume Imaging Scan the whole region of interest in one breath hold No gaps since radiation always transmits the whole volume Reconstruction of overlapping images without additional dose Retrospective reconstruction of slices in arbitrary position within the scanned volume Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications Scan Parameters X-ray Tube Voltage Table Speed (mm/rot) (kVp) X-ray Tube Current (mA) Scan Time (s) Slice thickness or Collimation (mm) or Feed per 360 rotation Pitch Interpolation Process Increment (mm) Table Speed & Pitch Table Speed is defined as distance traveled in mm per 360º rotation Feed per rotation Pitch => Table Collimation Table Feed Collimation 10 mm/rot 15 mm/rot 20 mm/rot 10 mm 10 mm 10 mm Pitch 1.0 1.5 2.0 30 s Pitch 2 covers 2x distance as Pitch 1 10mm P1 30s More Coverage in the same time with extended Pitch!! 10mm P2 Scan Range = 300mm 30s 10mm P1 10 mm/s 15s 10mm P2 20 mm/s Cover the same volume in shorter time with extended Pitch Interpolation Algorithm Converts volume data into slice images Interpolation To reduce artifacts due to table motion during spiral scanning, we use a special reconstruction process called INTERPOLATION Slim Algorithm Wide 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 scanning produces a broader image thickness Pitch 2 scanning does not increase image noise PITCH 1 PITCH 2 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 signal Image RESOLUTIO N SS AllPpoints within the slice contribute equally & points outside of the slice do not contribute to the image at all . signal Ideal SSP Collimation = width of x-ray beam =slice profile Z-axis (mm) Slice Profile (SP) Effective slice thickness of an image Slice Profile Resolution Factors influencing Slice Profile • Collimation • Pitch • Interpolation algorithm (360° or 180°) Factors influencing SSP •Collimator width collimation => SSP Spiral CT •Table speed or Pitch •Interpolation Algorithm => mathematical process required to reconstruct axial images from the spiral volume data set Pitch & Slice Profile Slim vs Wide – SSP Comparison Slice Profile Slim Pitch One 5.0 mm Pitch Two 6.5 mm %Broaden 0 30 Wide 6.3 m 10.8 WIDE 720 degree More photons SSP Spatial resolution Smoother SLIM 464 degree Less photons SSP Spatial resolution Noisier Slim - Advantages •Improved 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 C o l l i m a ti o n 5 .0 m m W id e S lim I n te r p o l a t io n In te r p o l a t i o n P itc h 1 .0 6 .3 m m % B roa den ed 26 P itc h 2 .0 1 0 .8 m m % B roa den ed 116 5 .0 m m 0 6 .5 m m 30 Optimizing the Scanning Parameters SCAN RANGE = 150mm 10/10/10 (15s) Lesion smaller than 1cm 5/10/5 (15s) Slice Profile = 10mm Slice Profile = 6.5mm Smallest Possible Effective Slice Thickness Scan Length (mm) Table Speed (mm/s) Smallest Collimation (mm) Scan Duration (s) on the scan length & patient’s Scan Duration Depends breath-hold compliance Table Speed Pitch Factor 1 < Pitch < 2 to cover the whole volume in one breath-hold Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications Injection Protocols Site Volume Peripheral vein eg. antecubital vein 19-20 gauge needle or IV catheter 80 - 150 ml patients’ weight & region of interest Flow Rate 2 - 5 ml/s cardiac output Scan Delay Delay between injection initiation & the start of the scan sequence Concentration 300 mg I/ml non-ionic contrast Tailoring Scan & Injection Protocols Injection Duration must be equal to or greater than Scan Time Enhancement Curve of the Target Region HU HU 250 250 200 200 150 150 100 100 50 50 CONTRAST Time[s] Bolus Duration < scan time Insufficient, inhomogeneous opacification CONTRAST NaCl Time[s] Bolus Duration = scan time Uniform, maximum opacification Contrast Bolus Timing Determines optimal scan delay for spiral CTA sequence HU Time-density curve of the target region 250 200 150 Early Optimal Window Late 100 50 CONTRAST NaCl Time[s] Test Bolus Procedure 10-20 ml of contrast is injected at the chosen rate for spiral After a delay of 8-10s, low-dose, single-level axial images are acquired every 2s at the starting point of the imaging volume Dynamic Evaluation to generate a Time-density curve Imaging Volume for spiral CTA Dynamic scans at this position Dynamic Scans Dynamic Evaluation ROI placed in the Aorta Time-density curve Scan Delay Peak Enhancement Time Computed Tomography CT Basics Principle of Spiral CT Scan Parameter & Image Quality Optimizing Injection Protocols Clinical Applications Dual Phase Liver Exam Liver Metastases Arterial Phase Venous Phase Single Plane Imaging with Multiplanar Results Oblique recon. of Aorta 2D reconstruction based on a serial of axial images along a certain axis Sagittal Coronal CT Angiography Spine 3D image: AVM Max. Intensity Projection Surface Shaded Display (3D) Femoral Arteries CT Angiogram 3D Post-processing 3D Bronchoscopy Colour Segmentation 3D Lesion in the right upper lobe branch Volume Rendering Technique Transparent & color image Solid Image Transparent image Virtual Endoscopy Bronchoscopy • Real Time Fly through • Reverse Perspective • Axial Image reference • High Resolution