Photons Through Matter
When X-ray photons pass through the body, they interact with atoms via photoelectric absorption, Compton scattering, and (at high energies) pair production. Each interaction removes photons from the primary beam, and the surviving fraction carries the shadow information that forms a radiographic image. The Beer-Lambert law I = I₀e^(−μx) captures this exponential attenuation in its simplest form.
Attenuation Coefficients
The linear attenuation coefficient μ depends on tissue composition, density, and photon energy. At diagnostic energies (20-150 keV), soft tissues cluster near μ = 0.2 cm⁻¹, fat is slightly lower, and bone ranges from 0.5-2.0 cm⁻¹. The Z³/E³ dependence of photoelectric absorption explains why bone (calcium, Z=20) is so visible against soft tissue (mostly carbon/oxygen, Z=6-8) — and why contrast agents with iodine (Z=53) or barium (Z=56) are so effective.
Subject Contrast
Radiographic contrast arises from differential attenuation between adjacent structures. If a lesion has a slightly different μ than surrounding tissue, the transmitted beam intensity differs, creating contrast. This simulation shows how tissue thickness, attenuation coefficient, and photon energy all modulate the contrast between a target structure and its background — the physical foundation for detecting abnormalities on a radiograph.
From Film to Digital
Whether captured on film, computed radiography plates, or flat-panel digital detectors, the fundamental signal is the spatial pattern of transmitted X-ray intensity. Modern digital detectors offer wider dynamic range and post-processing capabilities, but the physics of attenuation remains unchanged. Understanding Beer-Lambert is essential for optimizing technique factors (kVp, mAs) and minimizing patient dose while maintaining diagnostic image quality.