Oral and Maxillofacial Radiology and Physics
Radiology and physics: what is tested
This domain covers x-ray production, image formation, radiation dose and protection, and radiographic interpretation, including panoramic and CBCT principles.
Core rules
- X-rays are produced by thermionic emission at the filament; mA controls tube current and quantity; kVp controls tube voltage and penetrating quality.
- Bremsstrahlung produces a continuous spectrum; characteristic radiation produces discrete peaks.
- Photoelectric absorption predominates at low energy and high atomic number; Compton scattering predominates at higher energy and causes scatter.
- Attenuation follows exponential decay; half-value layer is the thickness that reduces intensity by half.
- Intensity follows the inverse square law: doubling distance quarters the intensity.
- Dose units: absorbed dose in gray, equivalent and effective dose in sievert.
- Deterministic effects have thresholds; stochastic effects are assumed linear with no threshold.
- Panoramic radiography uses tomography and a focal trough; ghost images appear on the opposite side.
- CBCT provides three-dimensional imaging with higher dose than a single periapical but lower than medical CT.
Worked example
A radiograph is repeated at twice the original distance. Reasoning: by the inverse square law, intensity is proportional to 1/d squared. Doubling distance reduces intensity to one quarter. To keep image density, exposure time must increase fourfold, but patient dose and technique must be optimized rather than simply increased.
Traps
- Claiming kVp controls the number of x-rays; mA does.
- Mixing gray and sievert; effective dose is in sievert.
- Forgetting that CBCT is not a low-dose substitute for all periapical radiographs.
- Missing ghost images on panoramic films.