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X-ray diagnostics and Computed tomography

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15th century head examination

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Projection Imaging with X Rays X-Ray Source Patient Detector I0I0 I

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This frontal chest radiograph demonstrates a dense (radio- opaque) left lung field consistent with a hemothorax in a patient with a gunshot wound to the chest. Chest X-Ray

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Filter Data Acquisition System (DAS) Source Detector Pre-CollimatorPost-Collimator Patient Scattering

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Exponential Attenuation of X-ray xx NoNo NiNi x X-rays Attenuated more NoNo NiNi N i : input intensity of X-ray N o : output intensity of X-ray : linear X-ray attenuation

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Ray-Sum of X-ray Attenuation xx NoNo NiNi Ray-sum Line integral

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First Generation One detector Translation-rotation Parallel-beam

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Second Generation Multiple detectors Translation-rotation Small fan-beam

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Third Generation Multiple detectors Translation-rotation Large fan-beam

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Fourth Generation Detector ring Source-rotation Large fan-beam

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A Little Bit History Nobel prizes Roentgen (1901): Discovery of X-rays Hounsfield & Cormack (1979): Computed tomography

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This frontal chest radiograph demonstrates a dense (radio-opaque) left lung field consistent with a hemothorax in a patient with a gunshot wound to the chest. Chest X-Ray COMPUTED TOMPGRAPHY, CT This CT image demonstrates the large bullae characteristic of patients with Chronic Obstructive Pulmonary Disease (COPD) representing lung destruction

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CT DETECTORS

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Third & Fourth Generations (From Picker) (From Siemens)

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E-Beam CT Scanner l Speed: 50, 100 ms l Thickness: 1.5, 3, 6, 10 mm l ECG trigger cardiac images (From Imatron)

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Spiral/Helical/Volumetric CT Continuous & Simultaneous Source rotation Patient translation Data acquisition

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A rapid development Single-slice 1989 Dual-slice 1992 Quad-slice 1998

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Projection Imaging with X Rays X-Ray Source Patient Detector I0I0 I

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Image Analysis Visualization & analysis 3D, 4D Networked, PC-based Image fusion Computer aided diagnosis Image-based surgery

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FUTURE MONOENERGETIC RADIATION DUAL ENERGY AROUND THE K-EDGE ENERGY SENSITIVE PIXELDETECTORS

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IMAGE RECONSTRUCTION

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Projection & Sinogram Sinogram t Sinogram: All projections P( t) f(x,y) t y x X-rays Projection: All ray-sums in a direction

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SINOGRAM CONSTRUCTION

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Computed Tomography P( t) f(x,y) P( t) f(x,y) t y x X-rays Computed tomography (CT): Image reconstruction from projections

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Reconstruction Idea

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Algebraic Reconstruction Technique (ART) Guess 0 Error Guess 2 Error Update a guess based on data differences Guess 1

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Fourier Transformation Fourier Transform f(x,y) F(u,v) Image Space Fourier Space

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Fourier Slice Theorem v u F(u,v) P( t) f(x,y) t y x X-rays F[P( t)]

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From Projections to Image y x v u F -1 [F(u,v)] f(x,y) P( t) F(u,v)

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Filtered Backprojection f(x,y) P( t)P’( t) 1) Convolve projections with a filter 2) Backproject filtered projections

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Example: Projection Sinogram Ideal Image Projection

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Example: Backprojection Projection

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Example: Backprojection SinogramBackprojected Image

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Example: Filtering Filtered SinogramSinogram

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Example: Filtered Backprojection Filtered Sinogram Reconstructed Image

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Some examples

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Image Display CT Number - Hounsfield unit Air: -1024 Water: 0 Bone: +175 to +3071 Viewing Parameters Window level (L) Window width (W) Zoom factor - 1024 +3071 0255 W L

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ARTIFACTS

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Different numbers of projections (96, 24, 12) Each projection contains 200 rays

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Number of projections (100).The projections have varying numbers of rays (200, 50, 25)

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Comparison 3rd and 4th generation 3rd generation has the X-ray tube in the apex (source fan) 4th generation has a single detector in apex (detector fan) High stability output of X-ray tube High stability of detectors

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Problems with a bad detector is handled differently in 3rd and 4th generation

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One detector that is bad in the 3rd generation will create a ring artifact. A small ring diameter if central and a large diameter for a peripheral detector.

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Beam-Hardening Artifacts Cause Effective energy is shifted to higher value as the X-rays pass through an object Correction Prefilter the X-ray beam near the focus Avoid highly absorbing bony regions Algorithms

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Beam-Hardening Artifacts X-ray path KeV

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Beam-Hardening Artifacts (From J Hsieh at GE) Without correction With correction

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Blurring Artifacts (Volume Averaging) Causes Large CT slice thickness and high contrast structures only partially included Finite source size Finite sampling rates Correction Volume Artifact Reduction (VAR) mode Deblurring

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Blurring Artifacts (Volume Averaging) (Blurred data from GH Esselman at Wash U) BlurredDeblurred Volume averaging

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Stair-Step Artifacts (Helix) Associated with inclined surfaces in reformatted slices Causes Large reconstruction interval Asymmetric helical interpolation Correction Collimation and feed less than feature sizes, and small reconstruction interval Adaptive interpolation

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Stair-step Artifacts (From JA Brink at Yale U)

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Metal Artifacts Cause Metal blocks parts of projection data Correction Avoid metal parts Algorithms

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Metal Artifacts (From DD Robertson at Wash U)

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Metal Artifacts Dental phantom Filtered backprojection FB with linear interpolation EM-like Iterative reconstruction

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Motion Artifacts Causes Patient motion Organ motion heart beating breathing swallowing Correction Fast scanning Algorithms

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Motion Artifacts =0 o =90 o Filtered backprojection EM-like Iterative reconstruction Time varying phantom

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Radiation Dose Dose - radiation energy transferred to an anatomic structure during X-ray scanning The unit of dose is Gray (Gy) sometimes Rad (0.01 Gy) Typical values for a CT transaxial scan are in the range of 30 to 50 mGy

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Radiation Profile: Single Scan Radiation spreads outside the designated slice due to scattering Ideal profile Real profile

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CT Dose Index CTDI: CT dose index T: slice thickness D(z): local dose z: longitudinal coordinate T

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Radiation Profile: Multiple Scans Radiation dose from multiple scans are accumulated in the central slice Real profile

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Multiple Scan Average Dose MSAD: multiple scan average dose I: inter-slice distance N: Number of scans I

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Dose Measurement Cylindrical phantoms of 16 cm & 32 cm Pencil ionization chamber Dosimeter 16 or 32 cm

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MSAD Estimation MSAD is directly proportional to mA directly proportional to scan time increases with kVp as compared to dose at 120 kVp 0.2-0.4 times less at 80 kVp 1.2-1.4 times more at 140 kVp increases slightly with decreasing slice thickness similar at the iso-center and near surface for head significantly less at the iso-center than near surface for body

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LEC ( 2 ) RAD 323. Reconstruction techniques dates back to (1917), when scientist (Radon) developed mathematical solutions to the problem of reconstructing.

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