CRI Digital Radiography Systems 2 — Questions and Answers
Question 1: What is the primary function of a scintillator layer in an indirect flat-panel detector?
- Convert X-rays directly into electrical charge
- Convert X-rays into visible light for photodiode capture (Correct answer)
- Amplify the electrical signal before analog-to-digital conversion
- Filter scattered radiation before image capture
Correct answer: Convert X-rays into visible light for photodiode capture
In indirect flat-panel detectors, the scintillator (commonly cesium iodide) converts X-ray photons into visible light, which is then captured by a photodiode array.
Question 2: Which artifact in computed radiography (CR) is caused by a partially erased image remaining on the phosphor plate?
- Moiré pattern
- Ghost image (residual signal) (Correct answer)
- Quantum mottle
- Dead pixel artifact
Correct answer: Ghost image (residual signal)
A ghost image (residual signal or ghosting) occurs when the CR phosphor plate was not fully erased before the next exposure, leaving remnants of a prior image.
Question 3: In digital radiography, what does the term 'exposure index' (EI) primarily indicate?
- The peak kilovoltage used during exposure
- The relative amount of radiation reaching the detector (Correct answer)
- The degree of image sharpness achieved
- The scatter-to-primary ratio of the exposure
Correct answer: The relative amount of radiation reaching the detector
The exposure index is a numeric value that reflects the radiation exposure level received by the digital detector, used to assess technique adequacy.
Question 4: What is the typical advantage of a cesium iodide (CsI) scintillator over a gadolinium oxysulfide (GOS) screen in flat-panel detectors?
- Lower cost of manufacturing
- Needle-shaped crystal structure that reduces light diffusion and improves resolution (Correct answer)
- Higher frame rate during fluoroscopy
- Greater resistance to radiation damage over time
Correct answer: Needle-shaped crystal structure that reduces light diffusion and improves resolution
CsI grows in needle-shaped columns that channel light toward the photodiodes, significantly reducing lateral light spread and improving spatial resolution compared to granular GOS.
Question 5: Which factor most directly affects the spatial resolution of a computed radiography system?
- The kVp setting used during exposure
- The phosphor plate thickness and laser spot size during readout (Correct answer)
- The grid ratio used during the exposure
- The ambient room temperature during scanning
Correct answer: The phosphor plate thickness and laser spot size during readout
Spatial resolution in CR is primarily limited by phosphor layer thickness (affecting light spread) and the size of the laser beam used to stimulate the plate during readout.
Question 6: In a direct conversion flat-panel detector, which material is most commonly used as the photoconductor?
- Gadolinium oxysulfide
- Cesium iodide
- Amorphous selenium (a-Se) (Correct answer)
- Silicon dioxide
Correct answer: Amorphous selenium (a-Se)
Amorphous selenium is the dominant photoconductor in direct conversion flat-panel detectors because it directly converts X-ray photons into electrical charge without an intermediate light stage.
Question 7: What is the purpose of applying a 'look-up table' (LUT) in digital radiography image processing?
- To correct for detector pixel dead zones
- To map raw detector values to displayed brightness levels for optimal viewing (Correct answer)
- To remove scatter from the raw image data
- To calibrate the X-ray tube output before acquisition
Correct answer: To map raw detector values to displayed brightness levels for optimal viewing
A look-up table (LUT) maps input pixel values to specific output display values, controlling image contrast and brightness to produce a diagnostically useful presentation.
CRI Digital Radiography Systems 2
What is the primary function of a scintillator layer in an indirect flat-panel detector?