Lecture 20 – Photon Interactions with Matter Final

Attenuation

The progressive decrease in the number of photons in the beam as it passes through matter due to absorption and scatter.

Differential attenuation (subject contrast)

Differences in attenuation between various tissues; gives rise to subject contrast. Dependent on tissue density, tissue thickness, and atomic number (all direct relationships).

Three possible outcomes for a photon interacting with matter

Absorption (photoelectric), scatter (change in direction), or transmission (no interaction). In diagnostic imaging, most photons that reach the IR were transmitted.

Scatter radiation (definition in this course)

Any photon that has undergone a change in direction after interacting with matter; usually carries enough energy to reach the IR or the staff and degrades contrast and adds dose.

Secondary radiation

Characteristic radiation emitted by atoms after absorbing x-ray photons. In practice, secondary radiation is often grouped with scatter, including leakage radiation when designing shielding.

Coherent (classical/Thomson) scatter

Low-energy (<10 keV) interaction where the photon interacts with the whole atom, is deflected with no energy loss, and causes no ionization. It contributes very little to image formation or dose in diagnostic energies.

Photoelectric effect (PE) – key features

Incident photon interacts with an inner-shell electron, ejecting it (photoelectron) and being completely absorbed; the vacancy is filled by outer-shell electrons, releasing characteristic x-rays or Auger electrons. Major contributor to image contrast and patient dose.

Energy range where PE is most likely

When photon energy is just slightly greater than the K-shell binding energy of the absorber material; probability is high at low photon energies and in high-Z materials.

Atomic number dependence of PE

Probability of photoelectric interaction is directly related to approximately Z³ (atomic number cubed), so higher-Z materials have a much higher PE probability.

Compton scatter – key features

Incident photon interacts with a loosely bound outer-shell electron, ejecting it (Compton or recoil electron) and being deflected with reduced energy. The atom is ionized; the scattered photon may undergo more Compton events or exit the patient.

Effect of Compton scatter on imaging and dose

Compton scatter decreases contrast resolution (adds unwanted exposure to the IR) and is the major source of occupational dose to radiographers.

Effect of increasing kVp on PE and Compton

As kVp increases, the probability of both PE and Compton interactions per photon decreases (more photons transmit), but the fraction of interactions that are Compton becomes larger relative to PE.

Pair production

High-energy interaction (>1 MeV) where the incident photon is absorbed in the nuclear electromagnetic field, and its energy is converted into an electron–positron pair. The positron later annihilates with an electron, producing two 0.511 MeV photons emitted 180° apart. Not used in diagnostic radiography.

Photodisintegration

Very high-energy interaction (>10 MeV) where the incident photon is absorbed by the nucleus, which then emits a nuclear fragment. Has no role in diagnostic radiography but changes the target atom’s atomic number.

kVp increase: effect summary (from review table)

As kVp increases: more photons are transmitted; photoelectric and Compton probabilities per photon decrease, but a greater proportion of interactions are Compton; patient dose ↓, IR exposure ↑, occupational dose ↑, and contrast resolution ↓ (more gray, less black-and-white).

Atomic number (Z) increase: effect summary (from review table)

As tissue atomic number increases: more photons are attenuated; more interactions are photoelectric; patient dose ↑, IR exposure ↓, occupational dose ↓, and contrast resolution ↑ (more subject contrast between tissues).

Reasons the x-ray beam is polyenergetic

Because of fluctuating kilovoltage (voltage ripple), Bremsstrahlung production over a range of energies, multiple electron interactions with the target, characteristic x-ray energies, and filtration that selectively removes low-energy photons.