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An X-ray image depends on what happens to photons inside matter. Distinguish the interaction mechanisms, then use attenuation measurements to explain a reconstructed CAT image.
What you'll be able to do
Simple scattering changes a photon’s direction without a substantial energy change in the elementary model. In photoelectric absorption, the photon is absorbed and transfers energy to a bound electron, with part used to overcome its binding energy. In Compton scattering, a photon transfers energy to an electron and emerges with lower energy and a longer wavelength.
Pair production converts photon energy into an electron–positron pair near a nucleus, which helps conserve momentum. It needs at least , about 1.02 MeV. That threshold is above typical diagnostic X-ray photon energies, so recognising the process does not mean it dominates an ordinary radiograph.
For a narrow monochromatic beam and uniform material, intensity decreases exponentially with thickness. Equal thicknesses remove equal fractions of the remaining beam. The attenuation coefficient depends on material and photon energy.
Barium or iodine contrast agents increase attenuation in selected structures that otherwise show poor contrast with surrounding tissue. Scattered photons reaching the detector can obscure spatial information, so beam geometry and scatter control matter. Keep incident, transmitted and absorbed quantities separate when interpreting a detector reading.
A CAT scanner rotates an X-ray tube producing a thin fan-shaped beam around the patient. A ring of detectors records transmitted intensities through many paths. Computer software reconstructs the distribution of attenuation in a section and displays it as an image.
A plain radiograph superimposes structures along the beam direction; a CAT section can distinguish overlapping internal features. CAT can give useful detail and rapid acquisition, but uses ionising radiation and generally requires more exposure than one plain image. Explain the benefit for the stated diagnostic task rather than calling every extra projection an independent photograph of a slice.
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.