Infographics / Fluoroscopy

Quantum mottle in the intensified image

Left to right, the same low-contrast disc is imaged with more detected x-ray quanta N (30, 100, 1000, 10000). With few photons the image is dominated by random grainy mottle and the disc is buried; as N rises the mottle falls and the disc emerges. A plot shows relative noise equal to one over the square root of N, a falling curve, so more photons (higher exposure and dose) give less mottle. Notes state the noise is random Poisson fluctuation, not fixed-pattern phosphor variation, is the dominant noise source at normal signal, worsens with a thicker input screen, and obscures low-contrast detail first.

Quantum mottle: image grain versus x-ray photon count and the 1/√N relation Left to right, the same low-contrast disc is imaged with more detected x-ray quanta N (30, 100, 1000, 10000). With few photons the image is dominated by random grainy mottle and the disc is buried; as N rises the mottle falls and the disc emerges. A plot shows relative noise equal to one over the square root of N, a falling curve, so more photons (higher exposure and dose) give less mottle. Notes state the noise is random Poisson fluctuation, not fixed-pattern phosphor variation, is the dominant noise source at normal signal, worsens with a thicker input screen, and obscures low-contrast detail first. Quantum (mottle) noise: grain versus photon count The same low-contrast disc, imaged with more detected x-ray quanta N N = 30 N = 100 N = 1000 N = 10 000 1/√N ≈ 18% ≈ 10% ≈ 3% ≈ 1% disc emerges disc buried in mottle increasing exposure rate → more photons (N ↑), improved SNR — but higher patient dose Why: relative noise = 1/√N 30 100 1k 10k photon count N (log scale) relative noise N = detected x-ray quanta; σ = √N relative noise = σ / N = 1/√N random, frame-to-frame (Poisson) — not fixed-pattern phosphor variation dominant noise source at normal signal thicker input screen → fewer independent quanta per pixel → more mottle obscures low-contrast detail first; high-contrast edges survive
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