X-ray physics notes curriculum
Fundamentals of radiation
The X-ray machine
Production of X-rays
Interaction of radiation with matter
X-ray detection and image formation
Image quality
Radiation safety in X-ray imaging
Fluoroscopy
Mammography (current module)
Breast tissue is one of the most radiosensitive organs in diagnostic imaging, and screening mammography involves repeated exposures over a patient’s lifetime. For this reason, radiation dose optimisation is fundamental to mammography physics and quality assurance.
Dose measurement in mammography focuses not on skin dose but on the mean glandular dose (MGD) which is the dose to the radiosensitive glandular component of the breast.
Radiation dose metrics
Unlike general radiography, where entrance surface dose (ESD) or dose–area product (DAP) are used, mammography expresses dose as mean glandular dose (MGD).
This provides a more accurate estimate of stochastic risk, as glandular tissue is the primary site for carcinogenic effects.
| Metric | Definition | Clinical relevance |
|---|---|---|
| Entrance surface air kerma (ESAK) | Dose in air at the breast surface, excluding backscatter | Directly measurable |
| Entrance surface dose (ESD) | ESAK × backscatter factor (≈1.1) | Represents skin dose |
| Mean glandular dose (MGD) | Average absorbed dose to glandular tissue, derived from ESAK | Used for optimisation and DRLs |
Mean glandular dose (MGD)
Definition
MGD represents the average energy deposited per unit mass of glandular tissue in the breast and is the standard dose descriptor for mammography.
MGD = K × g × c × s
Where:
- K = incident air kerma at the breast surface (mGy)
- g = conversion factor for a standard 50% glandular / 50% adipose breast
- c = correction for different glandularity
- s = correction for spectrum (target/filter combination)
These conversion factors are derived from Monte Carlo simulations of energy absorption within model breasts of different thicknesses and compositions.
Typical dose values
| View / Procedure | Mean glandular dose (MGD) | Comments |
|---|---|---|
| Standard 2D view (CC or MLO) | 1.0–2.0 mGy | Within reference limits |
| Tomosynthesis (single acquisition) | 1.0–2.5 mGy | Comparable or slightly higher than 2D |
| Screening examination (2 views/breast) | 2–4 mGy total | Bilateral exposure, four images total |
Factors influencing dose
| Factor | Effect on Dose | Explanation |
|---|---|---|
| Breast thickness | ↑ Dose | More attenuation → higher tube output required |
| Breast composition | ↑ Glandularity → ↑ Dose | Glandular tissue absorbs more energy/is more radiosensitive |
| kVp | ↑ kVp → ↓ Dose | Higher energy photons penetrate more efficiently |
| Target/filter combination | W/Rh or Rh/Rh → ↓ Dose | Optimises beam quality for tissue type |
| Compression | ↓ Dose | Reduces tissue thickness and scatter |
| Anti-scatter grid | ↑ Dose | Grid absorbs some primary photons (Bucky factor ≈2) |
| Detector DQE | ↑ DQE → ↓ Dose | More efficient photon utilisation |
Dose optimisation techniques
At risk of repeating exactly what’s tabulated above…
A. Compression
- Reduces breast thickness and scatter, lowering both dose and noise.
- Ensures uniform exposure across tissue depth.
- The single most effective method for dose reduction.
B. Target/filter selection
- Mo/Mo: thin or fatty breasts (lower kVp, higher contrast).
- Mo/Rh or Rh/Rh: thicker or denser breasts (slightly higher energy, lower dose).
- W/Rh or W/Ag: modern digital systems; optimal balance of penetration and contrast.
C. Tube potential and AEC control
- Automatic Exposure Control (AEC) adjusts exposure based on pre-exposure measurement of breast thickness and density.
- Ensures consistent image brightness and avoids overexposure.
D. Detector efficiency
- High-DQE digital detectors achieve required image SNR at lower doses compared with film–screen systems.
E. Beam filtration
- K-edge filters remove non-diagnostic low-energy photons and unnecessary high-energy photons, maintaining dose efficiency.
Radiation protection for staff and patients
Patient protection
- Justification: mammography performed only when clinically indicated or as part of screening.
- Optimisation: ALARA principle applied through AEC, compression, and regular QA.
- Limitation: typical cumulative dose over lifetime screening remains far below deterministic thresholds.
Staff protection
- Staff exposure in mammography is minimal due to system design (fully shielded housing, automatic exposure control).
- Lead aprons are generally unnecessary during standard operation.
- Dose to staff is primarily from scatter during interventional or biopsy procedures; barriers and shielding are used when required.
Key takeaways and exam tips:
- Mean glandular dose (MGD) is the standard dose metric in mammography.
- Calculated from incident air kerma multiplied by conversion factors for breast thickness, glandularity, and beam quality.
- Typical MGD: 1–2 mGy per view, with DRL = 2 mGy (45 mm breast).
- Compression, target–filter optimisation, and high-DQE detectors are the most effective dose-reduction strategies.
- MGD provides the most relevant measure of patient risk, as glandular tissue is radiosensitive.
- Common exam question: “Define mean glandular dose (MGD) and describe the factors affecting radiation dose in mammography.”
Up next
That brings us to the end of the mammography module! Only one more module to go! Get ready for one last module, digital subtraction angiography. See you there!