Digital Radiography: Dose, Noise & Contrast Windowing

Exposure Technique

Tube current-time (mAs) 100 mAs

Tube voltage (kVp) 90 kVp

Display Processing (Window/Level)

Window width (contrast) 60%

Window level (brightness) 100%

Image Quality

Photon flux N₀

800

Subject contrast

10.5%

CNR

2.97

Relative dose

1.27

How it works

Every exposure fires a finite number of X-ray photons at the detector. That number is a random (Poisson) process — the source of quantum mottle, the grainy noise visible on every real digital radiograph:

N ≈ k · mAs            (mean photons per detector cell)
σ(N) ≈ √N              (Poisson noise, "quantum mottle")
S_lesion = N·(1 − C)   (C = subject contrast, set by kVp)
CNR = C·N / σ(N) = C·√N

Raising mAs linearly increases photon count N, so noise σ grows only as √N — dose buys you CNR at a shrinking return (double the mAs, gain only √2 in CNR). Raising kVp makes the beam more penetrating and more uniform between tissues, which lowers subject contrast C even as it raises N — this is the classic dose-vs-contrast trade-off radiographers balance every exposure.

The acquired image is not what you see directly — a window/level transform remaps raw counts to the 0–255 display range before you read it:

display = clamp( (raw − (L − W/2)) / W × 255,  0, 255 )

Window width (W) — narrower = higher displayed contrast but a smaller range of tissue stays visible before clipping to pure black/white.
Window level (L) — the raw value mapped to mid-gray; shifts which tissue density sits at the center of the visible range.
New Exposure — resamples the Poisson noise at the current N and C, showing that no two exposures at identical technique factors are pixel-identical.

Real-world relevance: this trade-off between dose, quantum noise and window/level processing is exactly what a radiographer and a PACS workstation negotiate on every digital X-ray, CT slice and fluoroscopy frame taken in a hospital.

X-ray photon stream Flat-panel detector image

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