Computed Tomography Test Computed Tomography Science 4 — Questions and Answers
Question 1: The Nyquist theorem in CT states that to accurately represent a spatial frequency, the sampling rate must be at least:
- Equal to the frequency of interest
- Half the frequency of interest
- Twice the frequency of interest (Correct answer)
- Four times the frequency of interest
Correct answer: Twice the frequency of interest
The Nyquist criterion requires a sampling frequency at least twice the highest spatial frequency to avoid aliasing artifacts in the reconstructed image.
Question 2: What is the primary advantage of dual-energy CT (DECT) over single-energy CT?
- Faster gantry rotation
- Material decomposition and iodine quantification (Correct answer)
- Wider detector coverage per rotation
- Lower tube heat load
Correct answer: Material decomposition and iodine quantification
DECT acquires data at two different kVp levels, enabling material decomposition to identify and quantify iodine, calcium, uric acid, and other substances.
Question 3: Quantum noise in CT images is primarily a function of:
- Detector element size
- The number of photons reaching the detector (Correct answer)
- Reconstruction matrix size
- Table speed
Correct answer: The number of photons reaching the detector
Quantum (photon) noise arises from statistical fluctuations in photon counts; fewer photons (low mAs or high attenuation) result in increased noise.
Question 4: In CT, the term 'z-axis resolution' refers to resolution along which dimension?
- Left-to-right patient axis
- Anterior-to-posterior patient axis
- Superior-to-inferior patient axis (Correct answer)
- Diagonal plane of the gantry
Correct answer: Superior-to-inferior patient axis
The z-axis is the longitudinal axis running head-to-foot along the patient, and z-axis resolution determines the ability to distinguish structures in that direction.
Question 5: Which factor primarily determines the minimum slice thickness achievable in a multi-detector CT system?
- Focal spot size of the X-ray tube
- Individual detector element width in the z-direction (Correct answer)
- Reconstruction matrix size
- Gantry rotation speed
Correct answer: Individual detector element width in the z-direction
The detector element width in the z-axis sets the minimum detector aperture and therefore the thinnest slice that can be acquired.
Question 6: In CT image quality assessment, the modulation transfer function (MTF) is used to evaluate:
- Image noise as a function of dose
- Spatial resolution as a function of spatial frequency (Correct answer)
- Temporal resolution at different heart rates
- Contrast sensitivity at low kVp
Correct answer: Spatial resolution as a function of spatial frequency
The MTF describes how faithfully the CT system reproduces contrast at various spatial frequencies, characterizing the system's spatial resolution capability.
Question 7: The partial volume effect in CT produces inaccurate HU values when:
- The patient moves during acquisition
- Structures of different densities share the same voxel (Correct answer)
- The kVp is set too high for the anatomy
- The reconstruction kernel is too sharp
Correct answer: Structures of different densities share the same voxel
When a voxel contains tissues of different densities, the displayed HU is a weighted average, potentially misrepresenting the true attenuation of each component.
Computed Tomography Test Computed Tomography Science 4
The Nyquist theorem in CT states that to accurately represent a spatial frequency, the sampling rate must be at least: