An experiment in computed tomography

Physics DepartmentPortland State University, Portland, OR

Elliot Mylott (emylott@pdx.edu)Ryan KlepetkaJustin DunlapRalf Widenhorn (ralfw@pdx.edu)

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