CT: Image Artefacts, Radiation dose

Question 1

What may the imaging system produce?

Answer 1

Artificial structures which deviate from reality

= Artefacts

Question 2

What are important causes of artefacts?

Answer 2

1. Patient movement
2. Beam hardenng
   3, Scattered radiation
3. Partial volume effects
4. Metallic implants
5. Exceeding limits of field of measurement

Question 3

In CT, what does patient movement lead to?

Answer 3

Local blurring of contours

Disturbances throughout the entire image

Question 4

What are two main ways of minimising patient movement artefact?

Answer 4

1. Short scan times

2. Immobilization aids

Question 5

Why can part of individual projections be noisy?

Answer 5

Insufficient photons passing through the widest part of patient

Question 6

What happens when the photon starvation views are reconstructed?

Answer 6

The noise is magnified, resulting in streaks

Question 7

What does streak distort?

Answer 7

Information about soft tissue

Question 8

Why does beam hardening artefact result?

Answer 8

The broad polychromatic spectrum of X-day radiation is attenuated differently, depending on the energy of radiation, the object type and projection direction

Question 9

What does the beam hardening cause?

Answer 9

Variable Increases in the mean energy of the spectrum when encountering thicker objects and especial bony structures