ARRT CT Physics 2 — Questions and Answers
Question 1: What is the primary purpose of the bow-tie filter in a CT scanner?
- Increase tube current
- Compensate for the cylindrical shape of patients to reduce peripheral dose (Correct answer)
- Remove scatter radiation from the detector
- Calibrate the detector array
Correct answer: Compensate for the cylindrical shape of patients to reduce peripheral dose
The bow-tie (wedge) filter shapes the beam so less radiation hits the thinner periphery of the patient, reducing dose and preventing detector saturation.
Question 2: In CT, the term 'pitch' for single-slice CT is calculated as:
- Table speed / gantry rotation time
- Table feed per rotation / slice thickness (Correct answer)
- kVp / mAs
- Detector width / field of view
Correct answer: Table feed per rotation / slice thickness
Pitch = table feed per 360° rotation divided by the nominal slice thickness; a pitch >1 means gaps exist between consecutive slices.
Question 3: Which artifact appears as bright or dark bands radiating from high-density structures like metal implants?
- Ring artifact
- Beam hardening artifact
- Streak artifact (Correct answer)
- Partial volume artifact
Correct answer: Streak artifact
Streak artifacts arise from highly attenuating objects (metal, dense bone) causing inconsistent projections that reconstruct as linear streaks.
Question 4: What does the term 'half-scan reconstruction' refer to in CT?
- Reconstructing only half the detector channels
- Using 180° + fan angle of data instead of a full 360° to reduce motion artifact (Correct answer)
- Reducing matrix size by half to speed processing
- Acquiring data with only half normal mAs
Correct answer: Using 180° + fan angle of data instead of a full 360° to reduce motion artifact
Half-scan reconstruction uses approximately 180° + fan beam angle of data, which can reduce cardiac or other motion artifacts by shortening effective scan time.
Question 5: The spatial resolution of a CT image is primarily limited by which factor?
- Patient age
- Focal spot size and detector aperture (Correct answer)
- Room temperature
- Type of reconstruction filter used alone
Correct answer: Focal spot size and detector aperture
Focal spot size and detector cell aperture are the primary hardware factors limiting spatial resolution in CT.
Question 6: Which of the following best describes 'overranging' (z-overscanning) in helical CT?
- Scanning beyond the prescribed FOV diameter
- Extra rotations acquired beyond the requested scan range to complete helical reconstruction (Correct answer)
- Using a higher kVp than needed
- Applying too wide a display window
Correct answer: Extra rotations acquired beyond the requested scan range to complete helical reconstruction
Overranging refers to the additional tube rotations at the start and end of a helical scan needed to reconstruct the first and last requested slices, increasing dose.
Question 7: When noise in a CT image is described as 'quantum mottle,' it most directly reflects:
- Electronic noise in the detector
- Statistical variation in the number of photons detected (Correct answer)
- Software processing errors
- Mechanical vibration of the gantry
Correct answer: Statistical variation in the number of photons detected
Quantum mottle (quantum noise) results from statistical variations in photon fluence reaching the detector, following Poisson statistics.
ARRT CT Physics 2
What is the primary purpose of the bow-tie filter in a CT scanner?