Abstract X-ray attenuation is fundamental to radiographic image formation. The intensity of the beam emerging from the body varies with material thickness, composition, density, and photon energy. This article explains the Beer–Lambert law, the main interactions of X-rays with matter, factors affecting attenuation, and their relationship to contrast and the appearance of bone, soft tissue, and air in diagnostic images. Keywords X-ray attenuation; Beer–Lambert law; photoelectric effect; Compton scattering; contrast; image formation. Introduction When X-rays pass through the body, not all photons reach the detector. Some are absorbed or scattered, reducing the intensity of the primary beam. Differences in X-ray intensity after passage through tissues become differences in detector signal, forming the image. The Beer–Lambert Law Attenuation in an approximately homogeneous material can be described by the equation: \[ I = I_0 e^{-\mu x} \] Here, \(I_0\) represents the incident beam intensity, \(I\) represents the transmitted beam intensity, \(\mu\) is the linear attenuation coefficient, and \(x\) is the material thickness. The equation shows that increasing thickness or the attenuation coefficient decreases beam intensity exponentially. The Photoelectric Effect In photoelectric absorption, a photon transfers its energy to a bound electron and is completely absorbed. This interaction is relatively more important at lower photon energies and in materials with higher atomic numbers, contributing to contrast between different tissues. Compton Scattering Compton scattering occurs when a photon interacts with a loosely bound electron, loses part of its energy, and changes direction. Scattered photons may reach the detector and add unwanted signal, reducing image contrast. Factors Affecting Attenuation Attenuation depends on photon energy, material composition, density, thickness, and beam geometry. Therefore, the selection of X-ray tube voltage, filtration, and appropriate imaging geometry affects both image quality and radiation dose. Tissue Attenuation and Image Appearance Bone usually appears relatively bright because it attenuates X-rays more strongly than soft tissue. Air-filled regions allow a larger proportion of the beam to pass through and therefore appear darker. Soft tissues generally appear in intermediate shades of gray because their attenuation properties are more similar to one another. Importance of Attenuation in Computed Tomography In computed tomography (CT), attenuation differences are measured from multiple angles and converted into numerical values related to relative attenuation coefficients. These measurements are then used to reconstruct cross-sectional images of the body. Attenuation is therefore a fundamental physical basis for understanding CT image formation. Conclusion Understanding attenuation and the interactions between X-rays and matter is essential for understanding image formation and improving image quality. Controlling beam energy, filtration, imaging geometry, and digital processing helps achieve an appropriate balance among contrast, noise, and radiation dose. Almustaqbal University – The First University in Iraq