Lecture 10: X-ray CT

Question 1

What it the purpose of x-ray CT?

Answer 1

To generate 2D images, showing the variation for the linear attenuation coefficient, µ, with different tissue types in slices through the body. The images contain depth information.

Question 2

What are the 3 types of slices that can be taken for x-ray CT?

Answer 2

• Axial slice
• Coronal slice
• Sagittal slice

Question 3

Describe briefly how an x-ray CT scanner works

Answer 3

1) A set of projections are taken through the body by rotating the system around the body.
2) For a set of N different angles around the body, N projections are taken (each in 1D).
3) A 2D image is reconstructed of the chosen slice from the set of 1D projections.

Question 4

Describe the shape of an x-ray CT scanner

Answer 4

Question 5

Describe how a translate-rotate x-ray CT system collects projections

Answer 5

1) A series of ‘pencil beams’ are shone through the subject as a single x-ray source.
2) The x-ray source and detector are translated across the subject.
3) The attenuation of the pencil beams allows measurement of a projection from a single angle.
4) The system is rotated to collect the next projection.

Question 6

What is the equation for the projection of a CT scan?

Answer 6

P = projection
I = intensity
I_0 = initial intensity
µ = linear attenuation coefficient
dS = incremental distance

Question 7

What is the projection of a CT scan?

Answer 7

A plot of the intensity as a sum of the attenuation.