CT

Question 1

The X-ray tube in a CT scanner is modified compared to an X-ray tube used in projectional radiography. Identify 4 characteristics of an efficient CT X-ray tube. (4 marks)

Answer 1

• larger anode to absorband dissipate heat
• modiefids cathode assembly to produce smaller focal point for sharper images
• metal envelope and ceraic insulater to provide stability in the heat and prevent overheating
• flying focal spot for precise control over focal spot lacation

1 mark: Longer continuous exposure times at higher kV and mA.
1 mark: Large, thick anode to absorb and dissipate heat.
1 mark: Modified cathode assembly for smaller focal spot (0.6mm).
1 mark: Metal envelope and ceramic insulators to improve efficiency.

Question 2

List and explain 4 desirable characteristics required of a good quality CT image. (8 marks)

Answer 2

arteact free

good spatial resolution

good snr

less noise

Question 3

What do you understand by the term ‘noise’ in CT and why does it occur? (4 marks)

Answer 3

1 mark: Noise refers to variations in CT numbers that are not related to true attenuation coefficients.

1 mark: It results in a grainy or mottled image appearance.

1 mark: Noise occurs due to statistical fluctuations in X-ray photon detection (stochastic noise).

1 mark: Other contributors include insufficient photon detection, electronic noise, and structural noise from reconstruction algorithms.

Question 4

List 3 examples of image reconstruction techniques used in CT. (3 marks)

Answer 4

• back projection
• filtered back projection
• iterative reconstruction

Question 5

List 3 examples of image post-processing techniques used in CT. (3 marks)

Answer 5

1 mark: Multiplanar Reformatting (MPR).

1 mark: Maximum Intensity Projection (MIP).

1 mark: Volume Rendering / 3D Reconstruction.

Question 6

Describe the characteristics of a CT detector. (4 marks)

Answer 6

1 mark: Small detector size for improved spatial resolution.

1 mark: Ability to form an array by combining multiple detectors.

1 mark: High detection efficiency and sensitivity to capture a wide dynamic range of X-ray intensities.

1 mark: Fast response time with negligible afterglow, ensuring accurate image capture.