CRI Digital Radiography Systems 3 — Questions and Answers
Question 1: What is the primary cause of 'quantum mottle' in digital radiographic images?
- Inadequate collimation during exposure
- Insufficient number of X-ray photons reaching the detector (Correct answer)
- Excessive scatter radiation from the patient
- Incorrect window/level settings applied during post-processing
Correct answer: Insufficient number of X-ray photons reaching the detector
Quantum mottle (noise) results from statistical fluctuation in the number of X-ray photons absorbed by the detector; too few photons create a grainy, mottled appearance.
Question 2: In digital radiography, which process corrects for non-uniform detector pixel sensitivity across the imaging plate?
- Dark field correction
- Gain calibration (flat-field correction) (Correct answer)
- Edge enhancement filtering
- Histogram equalization
Correct answer: Gain calibration (flat-field correction)
Flat-field (gain) correction normalizes pixel-to-pixel sensitivity variations by dividing each pixel's value by a reference flat-field image acquired without a patient.
Question 3: Which DICOM standard attribute defines the manufacturer-specific exposure index for a digital radiographic image?
- (0028,1050) Window Center
- (0018,1166) Grid
- (0018,1405) Relative X-Ray Exposure (Correct answer)
- (0028,0301) Burned In Annotation
Correct answer: (0018,1405) Relative X-Ray Exposure
DICOM tag (0018,1405) Relative X-Ray Exposure stores the manufacturer-specific exposure index value that indicates detector dose level.
Question 4: What distinguishes a storage phosphor plate used in CR from a conventional intensifying screen?
- The storage phosphor emits light immediately upon X-ray exposure for direct film exposure
- The storage phosphor traps energy in metastable electron states that are released by laser stimulation during readout (Correct answer)
- The storage phosphor is a direct conversion semiconductor material
- The storage phosphor has a single-use design that must be discarded after each patient
Correct answer: The storage phosphor traps energy in metastable electron states that are released by laser stimulation during readout
CR storage phosphors trap excited electrons in F-centers (metastable states); stimulated luminescence is then triggered by a laser beam during the readout process.
Question 5: For industrial radiographic inspection per ASME or ASTM standards, which image quality indicator (IQI) type is most commonly used in the United States?
- Wire-type IQI (EN 462-1)
- Plaque-type (hole-type) IQI per ASTM E1025 (Correct answer)
- Step-wedge sensitivity indicator
- Duplex wire IQI for unsharpness measurement
Correct answer: Plaque-type (hole-type) IQI per ASTM E1025
The plaque-type (hole-type) IQI per ASTM E1025 is the standard image quality indicator most widely used in US industrial radiographic practice.
Question 6: What is the effect of increasing the X-ray tube voltage (kVp) on the detective quantum efficiency (DQE) of a digital detector?
- DQE always increases because higher energy photons are more efficiently absorbed
- DQE can decrease if higher-energy photons penetrate through the detector without absorption (Correct answer)
- DQE is unaffected by kVp since it is a fixed detector property
- DQE increases proportionally with kVp up to 150 kVp then plateaus
Correct answer: DQE can decrease if higher-energy photons penetrate through the detector without absorption
At higher kVp, photons have greater penetrating power and a larger fraction may pass through the detector without interaction, reducing the DQE.
Question 7: Which type of digital noise has a frequency-dependent structure and is best characterized using the noise power spectrum (NPS)?
- Electronic readout noise only
- Structured (correlated) noise including fixed-pattern noise and quantum noise at all spatial frequencies (Correct answer)
- Salt-and-pepper noise from dead pixels
- Motion unsharpness from patient movement
Correct answer: Structured (correlated) noise including fixed-pattern noise and quantum noise at all spatial frequencies
The noise power spectrum describes how noise energy is distributed across spatial frequencies, capturing both quantum noise and fixed-pattern (structured) noise contributions.
CRI Digital Radiography Systems 3
What is the primary cause of 'quantum mottle' in digital radiographic images?