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An experiment in computed tomography
Physics DepartmentPortland State University, Portland, OR
Elliot Mylott ([email protected])Ryan KlepetkaJustin DunlapRalf Widenhorn ([email protected])
Buzug, Thorsten M. Computed Tomography: From Photon Statistics to Modern Cone-Beam CT
Objectives
To design a lab that Introduces students to
CT Utilizes light instead of X-
rays Requires little setup Fast data collection and
analysis Reinforces concepts of
light/matter interaction Can highlight CT specific
issues
An Example of X-rays and Projections for Students
X-ray tube
X-ray film Projecti
ons
CT and Back Projection
X-ray source rotates around an object accumulating projection data
Projection data reflect the attenuation of the X-ray by the object
Projection data is spread back onto a reconstructed image
Multiple scans from different angles result in an image of the original object
Pro
jectio
n D
ata
X-ray Source
Pro
jec
tio
n
Dat
a
1
111
1
1
1
11
1
Projection Data
Back projection image
1
An Example of Back Projection for StudentsThree light sources on a cylinder
Shadows (projections) converge at the cylinder
Scanner Generations
3rd Generation
•Rotation Only•Large Fan Beam•Large Detector Array
1st Generation 2nd Generation 4th Generation
•Translation & Rotation•Pencil Beam•Single Detector
•Translation & Rotation•Small Fan Beam•Small Detector Array
•Rotation Only•Large Fan Beam•Stationary Detector Array
•Rotation Only•Pencil Beam (Simulated Fan Beam)•Single Detector (Simulated Large Detector Array)
Hidden Objects
http://amasci.com/amateur/irgogg2.html
Apparatus
Comparison to X-ray CT Scanners
Rotate X-ray Source
Rotate Scanned Area
Comparison to X-ray CT Scanners
Analog Projection Data
Attenuation0 1
Photogate Scan
X-ra
y tu
be Digital
Projection Data
Focus-Center Distance (FCD)
Rotational Axis
Scanning line
y
x
IR Source
IR Detector
Scanner Geometry
Variables
•FCD – distance between the center of the grid and the rotary motion sensor
•θ - The angle between the rotary motion sensor and the x-axis
• φ – The angle between the focal length line and the scanning line
Output
Equation for the scanning line (y=mx+b)
LabVIEW
•Displays data in real time•No programming experience needed•Can be adjusted for different computer speeds•Will be available as an .exe file
Creating an Image
Creating an Image
Original Positions Final Reconstruction
θ=180
θ=360
CT Concepts - Artifacts
Back projection results in additional, unwanted data.
CT Concepts -Windowing
Change in Gray Scale Value
Conclusion