In digital radiography, which of the following best describes the 'quantum mottle' (noise) on a radiograph?
- A Random variation in optical density caused by statistical fluctuations in the number of X-ray photons absorbed by the detector ✓
- B Artefact caused by movement of the patient during exposure
- C Geometric unsharpness due to a large focal spot size
- D Overexposure leading to burnt-out areas on the image
Correct answer: A. Random variation in optical density caused by statistical fluctuations in the number of X-ray photons absorbed by the detector
Explanation
Quantum mottle (quantum noise) results from the statistical (Poisson) variation in the number of X-ray photons detected per unit area. When fewer photons are used (low mAs), larger percentage fluctuations produce a grainy, mottled image. Increasing mAs (more photons) reduces quantum mottle but increases patient dose. It is distinct from motion blur (patient movement), geometric unsharpness (focal spot), or overexposure artefact. It is the dominant source of noise in radiography at typical exposures.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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