Radiographic Imaging Principles

Interaction of X-Radiation With Matter

• Fundamental physics concepts related to radiation interaction and scatter are crucial for radiographers in selecting technical factors like kVp (peak kilovoltage) and mAs (milliampere-seconds)
• kVp controls the quality (penetrating power) and to some extent the quantity of x-ray photons in the beam.
• mAs controls the quantity of photons in the x-ray beam.
• X-rays can interact with biological tissue in three ways: absorption, scattering, and passing through without interaction.
• Absorption is the transfer of x-ray energy to the biological tissue atoms.
• The amount of energy absorbed per unit mass is the absorbed dose (D).
• Scatter occurs when x-rays interact with tissue atoms and change direction.
• Direct transmission refers to photons that pass through the patient without interaction, while indirect transmission happens when photons interact, are scattered, but still hit the image receptor.
• Attenuation encompasses both absorption and scatter, preventing photons from reaching the image receptor. Attenuation is reduction in the intensity of the primary photon beam either from absorption or scatter.
• Primary radiation refers to the x-ray photons produced in the x-ray tube.
• Exit radiation or image-formation photons strike the image receptor.
• Attenuated photons are those that do not reach the image receptor because they were absorbed or scattered.
• All scatter does not result in indirect transmission.

X-Ray Beam Production and Energy

• Diagnostic x-ray beams are produced by a stream of high-energy electrons bombarding a positively charged target (anode).
• Tungsten or tungsten alloys are commonly used for the anode target due to their high melting points and high atomic numbers.
• X-ray photons are produced as electrons interact with the target atoms.
• X-ray photons have a spectrum of energies.
• The maximum energy of x-ray photons equals the energy of the electrons bombarding the target, measured in kilovolts peak (kVp).
• The filtered x-ray photon beam is referred to as primary radiation.
• Inherent filtration is the combination of the x-ray tube glass wall and additional aluminum placed within the collimator assembly.
• X-ray photons can have no more energy than the electrons striking the anode/target.

Direct and Indirect Transmission X-Ray Photons

• X-rays pass through the patient and interact to create an image.
• Direct transmission photons pass through without interaction.
• Indirect transmission photons interact with tissue but still strike the image receptor.
• Indirect transmission is always a result or scatter.

Attenuation

• Attenuation is the reduction in the intensity of the primary photon beam.
• Attenuation can either be from absorption or scattering.
• Absorption happens when photons lose all their energy when interacting with a tissue atom.
• Scatter happens when photons lose part of their energy and are redirected.

Probability of Photon Interaction With Matter

• Photon interactions with matter are random.
• The probability of interaction can be predicted on average for a large number of photons
• Interaction characteristics are described in tables.
• The probability of interactions varies greatly depending on atomic number and energy of x-ray photons.

Processes of Interaction

• Five types of x-ray interactions are possible:
  • Coherent scattering
  • Photoelectric absorption
  • Compton scattering
  • Pair production
  • Photodisintegration

Coherent Scattering

• Involves no energy loss, only a change in direction.
• Important at low energies.

Photoelectric Absorption

• Most important mode of x-ray interaction for radiographic imaging.
• X-ray photon gives all its energy to an inner-shell electron, causing its ejection (photoelectron).
• This creates a vacancy, which is filled by an outer-shell electron, emitting characteristic x-ray.
• More likely in materials with higher atomic numbers and lower photon energies.
• Probability is proportional to Z3/E3 ( where Z is atomic number and E is energy.

Compton Scattering

• Incoming photons interact with loosely bound outer shell electrons.
• Photon energy is transferred, resulting in a scattered photon and Compton scattered electron, with a changed trajectory.
• Important source of scattered radiation in diagnostic radiology.

Pair Production

• Occurs at high photon energies (greater than 1.022 MeV).
• Photon energy is converted into matter in the form of electron-positron pair.
• Important at high energies.

Photodisintegration

• Occurs at very high energies (greater than 10 MeV)..
• Photon energy is completely absorbed by the nucleus causing its instability.
• This can lead to emission of particles (neutrons, protons, or alpha particles.)
• Not important in ordinary diagnostic radiology.

Mass Density and Effective Atomic Number

• Different body structures have different densities and atomic numbers.
• Density and atomic number influence attenuation.
• High density/atomic number structures attenuate x-rays more than low density/atomic number structures.
• X-ray absorption depends on both thickness and characteristics of the tissue.
• Tissue with greater density and atomic number appear darker in radiographs.

Use of Contrast Media

• Contrast media have higher atomic numbers than surrounding tissue
• They enhance visualization of specific structures.
• Positive contrast media (e.g., barium, iodine) appear brighter on radiographs because they absorb x-rays more readily.
• X-ray absorption is greater due to higher Z-number.