As X-ray photons pass through the body they interact with tissues through several physical processes. These interactions determine how much radiation is absorbed, scattered, or transmitted, and therefore play a central role in image formation and radiation dose.

The two dominant interactions in diagnostic radiology are the photoelectric effect and Compton scattering. The photoelectric effect results in complete absorption of the photon and contributes strongly to image contrast, particularly in high atomic number materials such as bone. Compton scattering results in partial energy transfer and produces scattered photons that degrade image contrast.

The probability of these interactions depends on photon energy, electron density, and the atomic number of the absorbing material. At lower photon energies the photoelectric effect predominates, while at higher energies Compton scattering becomes more likely.

Understanding these interaction mechanisms is fundamental to diagnostic imaging physics and is heavily examined in FRCR Part 1 Physics, ABR Core, and RANZCR examinations.

The questions in this section explore the relative probabilities of different interaction processes, their dependence on photon energy and tissue composition, and their influence on image contrast and radiation dose.

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