The foundation of medical imaging, utilizing high-energy electromagnetic radiation to visualize the internal structures of the body. X-rays remain the most frequent radiographic procedure, essential for skeletal, thoracic, and emergency imaging.
X-rays are electromagnetic radiation with wavelengths of 0.01–10 nm, occupying the spectrum between UV and gamma rays. In diagnostic radiology, they are produced artificially using an X-ray tube.
X-rays are generated by two mechanisms inside the tube: Bremsstrahlung radiation (braking radiation — ~80–90% of output) and characteristic radiation (~10–20%).
Tap the tube components, then switch the atom lens between X-ray production in tungsten and photon interactions in tissue.
An incident tube electron passes close to the positive tungsten nucleus, is deflected and slowed, and the lost kinetic energy leaves as an X-ray photon. Most electron energy still becomes heat in the target.
Click on a component in the diagram to learn about it.
Filament heating releases electrons by thermionic emission.
kVp accelerates those electrons across the evacuated glass tube.
Electrons strike the rotating tungsten disc, spreading heat around the focal track while producing X-ray photons.
Useful photons exit through the window, pass through anatomy, and form contrast at the detector.
When an X-ray beam enters the body, photons interact with tissue in five main ways. The dominant interactions in diagnostic radiology are photoelectric absorption and Compton scattering.
Exam Tip: In the diagnostic kVp range (40–150 kVp), photoelectric absorption and Compton scattering are the two interactions you must know in depth. Photoelectric gives you contrast; Compton gives you scatter (noise). Grids and collimation reduce scatter reaching the detector.
Adjust the exposure factors below and see how they affect the simulated radiographic image brightness and contrast.
Inverse Square Law: I₁/I₂ = D₂²/D₁² — intensity is inversely proportional to the square of the distance from the source.
Change the distance and see how intensity (and required mAs) changes. Formula: I₂ = I₁ × (D₁ / D₂)²
When kVp changes, mAs must be adjusted to maintain the same image exposure.
Exact formula (kVp² law): mAs₂ = mAs₁ × (kVp₁ / kVp₂)² — use this for precise calculations.
15% Rule (approximation): ±15% kVp ≈ double/halve the mAs effect — a clinical shortcut. Note: (1.15)² = 1.32, not 2.0, so results will differ slightly from the exact formula.
X-ray remains the most performed imaging study in the world, with billions of exams performed annually.
Radiation Protection (ALARA): Always apply the principle of As Low As Reasonably Achievable. Use appropriate kVp/mAs, collimation, shielding, and correct positioning. Pregnancy must be considered before any ionizing imaging of the abdomen/pelvis.
Explore ultrasound — safe, real-time imaging with no ionizing radiation.
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