Computed Tomography Test Computed Tomography Science 5 — Questions and Answers
Question 1: Which type of CT detector converts X-ray photons directly into electrical charge using amorphous selenium?
- Indirect-conversion flat-panel detector
- Direct-conversion flat-panel detector (Correct answer)
- Xenon ionization detector
- Scintillator-photodiode detector
Correct answer: Direct-conversion flat-panel detector
Direct-conversion detectors (e.g., amorphous selenium) convert X-rays directly to electrical charge without an intermediate light step, potentially improving spatial resolution.
Question 2: Spectral CT using photon-counting detectors differs from energy-integrating detectors mainly because it:
- Measures only the total energy deposited per acquisition
- Counts individual photons and records their energy levels (Correct answer)
- Requires higher tube current for equivalent image quality
- Uses a scintillator crystal coupled to a PMT
Correct answer: Counts individual photons and records their energy levels
Photon-counting detectors register each photon individually and bin them by energy, enabling true spectral decomposition without energy overlap between bins.
Question 3: In CT, the concept of 'noise power spectrum' (NPS) provides information about:
- The frequency-dependent distribution of image noise (Correct answer)
- The maximum achievable mA for a given kVp
- Patient dose as a function of scan length
- The dynamic range of the display monitor
Correct answer: The frequency-dependent distribution of image noise
The NPS (or Wiener spectrum) describes how noise energy is distributed across spatial frequencies, capturing the texture and magnitude of noise in CT images.
Question 4: Which reconstruction algorithm uses a statistical model of the imaging process to minimize a cost function iteratively, achieving better noise properties than FBP?
- Filtered back-projection (FBP)
- Maximum-likelihood expectation maximization (MLEM) (Correct answer)
- Simple back-projection (SBP)
- Algebraic reconstruction without penalty
Correct answer: Maximum-likelihood expectation maximization (MLEM)
MLEM iteratively updates the image estimate to maximize the likelihood that the projection data would be observed given the current image, reducing noise and artifacts.
Question 5: In helical CT, the 'pitch factor' is defined as table advancement per rotation divided by:
- Total detector width (beam collimation) (Correct answer)
- Tube rotation time
- Reconstruction field of view
- Number of detector rows
Correct answer: Total detector width (beam collimation)
Pitch = table feed per 360° rotation / total beam collimation width; a pitch of 1 means table advance equals the beam width with no overlap or gap.
Question 6: The 'effective dose' in CT differs from absorbed dose (Gy) in that it accounts for:
- Tube output variation with kVp
- Tissue weighting factors and radiation type (Correct answer)
- Patient positioning relative to isocenter
- Detector efficiency across the array
Correct answer: Tissue weighting factors and radiation type
Effective dose (Sv) applies tissue weighting factors to equivalent doses in each organ, reflecting the overall stochastic risk to a standard individual from non-uniform irradiation.
Question 7: Which CT image artifact results from inconsistent attenuation readings caused by rapid patient breathing during thoracic imaging?
- Ring artifact
- Beam hardening streak
- Respiratory motion artifact (Correct answer)
- Windmill artifact
Correct answer: Respiratory motion artifact
Respiratory motion during thoracic CT causes misregistration of lung boundaries and blurring of structures, classified as a motion artifact.
Computed Tomography Test Computed Tomography Science 5
Which type of CT detector converts X-ray photons directly into electrical charge using amorphous selenium?