The Photoelectric Effect
1. Definition
The photoelectric effect occurs when an incident X-ray photon interacts with an inner-shell (bound) electron. The photon is totally absorbed, and its energy is used to eject the electron (now called a photoelectron). The vacancy in the inner shell is then filled by an electron from an outer shell, resulting in the emission of characteristic X-rays or Auger electrons.
2. High-Yield Characteristics
- Diagnostic Importance: It provides the diagnostic information (image contrast) by allowing differential absorption between different tissues (e.g., bone vs. soft tissue).
- Energy Requirement: The incident photon energy must be equal to or slightly greater than the binding energy of the inner-shell electron.
- Absorption: It is a process of total photon absorption, meaning zero scatter is produced from this specific interaction.
3. Key Physics Relationships
| Variable | Relationship to Photoelectric Effect |
|---|---|
| Atomic Number (Z) | Highly proportional to  . This is why bone (high Z) appears white. |
| Photon Energy (E) | Inversely proportional to $Latec E^3$. As photon energy increases, interaction probability drops rapidly. |
4. Exam Pearls
- Contrast Media: Agents like Iodine (Z=53) and Barium (Z=56) are used specifically because their high atomic numbers maximize the Photoelectric Effect, creating high contrast.
- K-Edge Effect: The probability of the photoelectric effect increases dramatically when the incident photon energy is just above the binding energy of the K-shell electron (the “K-edge”).
- Board Exam Summary: If you need to differentiate tissues, you rely on the Photoelectric effect. If you need to reduce patient dose or increase penetration, you rely on kVp, which shifts the interaction balance toward Compton scattering.