CAMRT Image Production and Quality Questions and Answers 2 — Questions and Answers
Question 1: What is the primary factor affecting spatial resolution in digital radiography?
- Exposure time
- Detector element (del) size (Correct answer)
- Source-to-image distance
- Tube current (mA)
Correct answer: Detector element (del) size
Del size is the primary determinant. Smaller dels resolve finer details.
Typical del sizes: 100-200 micrometres for general radiography, 25-50 for mammography. A 200-micrometre del yields 2.5 lp/mm maximum.
Question 2: In CR, what is the function of the photostimulable phosphor plate?
- Directly converts X-rays to electrical signals
- Stores X-ray energy as a latent image released by laser stimulation (Correct answer)
- Amplifies the signal before digital conversion
- Filters scatter radiation
Correct answer: Stores X-ray energy as a latent image released by laser stimulation
The PSP plate stores X-ray energy as trapped electrons, read out by laser scanning.
A focused laser scans the plate, releasing trapped electrons as blue-violet photostimulated luminescence, collected by a photomultiplier tube and digitized.
Question 3: What effect does increasing kVp have on image contrast in digital radiography?
- Contrast increases from more X-rays reaching detector
- Contrast decreases from more uniform penetration (Correct answer)
- Contrast unchanged due to post-processing
- Contrast increases from reduced scatter
Correct answer: Contrast decreases from more uniform penetration
Higher kVp reduces subject contrast because higher energy photons penetrate more uniformly.
Compton interaction predominates at higher energies, reducing differential attenuation. Digital post-processing cannot recover subject contrast absent from raw data.
Question 4: What does the exposure indicator (EI) signify when significantly above target?
- Patient dose is excessive
- Detector overexposure, meaning more radiation than necessary (Correct answer)
- Better spatial resolution
- Improved tissue differentiation
Correct answer: Detector overexposure, meaning more radiation than necessary
Above-target EI indicates the detector received more radiation than needed for a diagnostic image.
The deviation index DI = 10 x log10(EI/EI_T). A DI of +3 means approximately double overexposure. Digital detector latitude can mask overexposure (exposure creep).
Question 5: What is the modulation transfer function (MTF) used for in imaging?
- Measuring patient dose
- Quantifying how well a system preserves spatial frequency information (Correct answer)
- Calculating detector dynamic range
- Assessing detector response uniformity
Correct answer: Quantifying how well a system preserves spatial frequency information
MTF measures how effectively contrast is transferred at various spatial frequencies.
At low spatial frequencies MTF is near 1.0. It decreases toward zero at higher frequencies. The frequency where MTF reaches a threshold defines the limiting resolution.
Question 6: What causes Moire artefact in digital radiography?
- Patient motion
- Aliasing between grid frequency and detector pixel frequency (Correct answer)
- Detector malfunction
- Scatter radiation
Correct answer: Aliasing between grid frequency and detector pixel frequency
Moire artefact results from aliasing when grid line frequency approaches the detector Nyquist frequency.
Prevention: use grids with frequencies below Nyquist, oscillate the grid, angle it relative to the pixel matrix, or use software scatter correction.
CAMRT Image Production and Quality Questions and Answers 2
What is the primary factor affecting spatial resolution in digital radiography?