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An X-ray tube converts electron kinetic energy into a spectrum of photons and substantial heating. Beam controls and detector choices influence contrast, sharpness and dose. This lesson is part of the Medical Physics option; study it if this is your centre’s chosen Paper 3 Section B topic.
What you'll be able to do
A heated cathode emits electrons by thermionic emission. A large potential difference accelerates them towards a metal anode. Deceleration in the target produces a continuous bremsstrahlung spectrum: electrons can lose different amounts of energy. Characteristic lines occur when inner-shell vacancies are filled and photons of specific transition energies are emitted.
At most, one electron gives all of its kinetic energy to one photon. Most electron energy becomes heat, so a rotating anode spreads the heating over a larger target area. Cathode heater current changes the electron emission rate; accelerating voltage changes the maximum photon energy and spectral distribution.
A smaller effective focal spot reduces geometrical unsharpness, but concentrates target heating. Collimation restricts the beam to the region of interest, reducing unnecessary exposure and scattered radiation. Filtration removes very low-energy photons that would be absorbed superficially without contributing much to the image.
Different tissues attenuate X-rays differently. A barium contrast agent can make an otherwise poorly distinguished digestive structure more opaque. Higher photon energy increases penetration but may reduce differential absorption and contrast. Exposure time and tube current affect photon number, noise and patient dose.
In a scintillator-based flat-panel detector, absorbed X-rays produce light. Photodiode pixels convert that light into electrical signals, which are scanned electronically to create a digital image. Digital readout is convenient for display, storage and processing and avoids photographic chemical development.
Photographic systems can use intensifying screens: X-rays create visible photons that expose the film more efficiently, reducing required X-ray exposure. Fluoroscopic image intensification converts and amplifies a weak image for viewing. State the energy-conversion stages rather than suggesting an intensifier creates new detail absent from the input.
Try each problem before opening the worked solution. State the physical model, show substitutions with consistent units, and explain what your answer means.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.