How X-rays Interact with Living Tissues

X-rays are ionizing photons. When they pass through the body, they may be absorbed, scattered, or transmitted. The relative amount depends on photon energy, tissue composition and thickness.

1. Photoelectric Effect

An X-ray photon transfers most or all of its energy to a bound electron and ejects it from the atom. The resulting vacancy can produce characteristic X-rays or Auger electrons.

The photoelectric effect is more important at lower diagnostic photon energies and in materials with higher atomic number. It contributes strongly to bone contrast.

2. Compton Scattering

The incident photon collides with a loosely bound electron, transfers part of its energy to the electron and changes direction with lower energy.

Compton scattering:

• Produces scattered radiation.
• Reduces image contrast.
• Contributes to radiation dose outside the primary beam.

3. Coherent Scattering

The photon changes direction without significant energy transfer or ionization. Its contribution in diagnostic imaging is comparatively small.

4. Transmission

Some photons pass through the body without interaction and reach the detector. These transmitted photons form a major part of the useful image signal.

Tissue Attenuation

As X-rays pass through tissue, intensity decreases approximately according to:

$$I=I_0e^{-\mu x}$$

where $I_0$ is incident intensity, $\mu$ is the linear attenuation coefficient and $x$ is tissue thickness.

Different tissues have different attenuation coefficients, producing image contrast.

Biological Effects

Ionization can damage cellular molecules, including DNA. Effects may be deterministic/tissue reactions at sufficiently high doses or stochastic risks, such as increased cancer probability, associated with radiation exposure.

Conclusion

X-ray imaging depends on differential attenuation caused mainly by photoelectric absorption and Compton scattering in the diagnostic-energy range. The differences in attenuation between tissues allow internal structures to be visualized.