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 (current module)
Mammography
Fluoroscopy delivers dynamic, real-time imaging but at the cost of increased radiation exposure compared with static radiography.
To manage this safely, fluoroscopic systems continuously measure and display dose metrics that quantify patient exposure, while incorporating multiple dose-saving features designed to maintain diagnostic image quality at the lowest achievable dose.
Why dose metrics are needed
Because fluoroscopy involves prolonged beam-on time and variable geometry, exposure cannot be predicted from technique factors alone.
Instead, real-time measurements of beam output are used to:
- monitor instantaneous and cumulative patient dose;
- alert operators when dose thresholds approach deterministic levels;
- provide records for quality assurance and dose audit.
These measurements are taken using calibrated transmission ionisation chambers positioned near the X-ray tube.
Primary fluoroscopic dose quantities
| Quantity | Definition | Typical Unit | Purpose |
|---|---|---|---|
| Air kerma rate | Rate of energy deposition in air at a defined reference point | mGy/s or mGy/min | Indicates instantaneous dose intensity |
| Cumulative air kerma | Sum of air kerma at that point during the procedure | mGy | Estimates potential skin dose |
| Dose–area product (DAP) | Product of air kerma and beam area ( D × A ) | Gy·cm² | Represents total energy imparted to the patient; correlates with stochastic risk |
| Fluoroscopy time | Total beam-on duration | s or min | General indicator of procedural workload |
Relationship between metrics
DAP = D × A
where D is air kerma (Gy) and A is irradiated area (cm²).
DAP increases with both field size and dose rate and is independent of distance from the source, since beam divergence balances inverse-square fall-off. The fact that DAP doesn’t change with distance is a classic exam question.
Factors influencing fluoroscopic dose
| Parameter | Effect on Dose | Notes |
|---|---|---|
| Fluoroscopy time | Directly proportional | Main determinant of total dose |
| Pulse rate | Higher rate → higher cumulative dose | 7.5–15 fps common |
| Field size | Larger field → higher DAP | Collimation reduces both dose and scatter |
| Magnification mode | Increases tube output | Use only when necessary |
| Patient thickness / projection angle | Greater attenuation → higher tube output | Oblique angles and large patients increase dose |
Dose-saving features
Modern systems integrate several hardware and software measures that collectively reduce dose without compromising image quality.
A. Pulsed fluoroscopy
- X-ray beam emitted in short pulses rather than continuously.
- Typical pulse rates = 3–15 fps.
- Dose reduction roughly proportional to pulse rate (e.g. halving pulse rate halves dose).
- Maintains acceptable temporal resolution for most procedures.
B. Last-image hold (LIH) and fluoro-store
- Retains final frame after beam is off, allowing review or repositioning without further exposure.
- “Fluoro-store” saves selected frames for documentation at no additional dose.
C. Additional filtration
- Copper (0.2–0.9 mm) or aluminium filters remove low-energy photons that contribute to skin dose but not image formation.
- Can reduce entrance surface dose by 30–50 %.
D. Collimation
- Restricts beam to anatomy of interest.
- Decreases scatter and DAP; improves contrast.
E. Automatic Dose Rate Control (ADRC)
- Adjusts kVp and mA to maintain constant detector dose rate.
- Prioritises kVp increases before mA to maximise dose efficiency.
F. Magnification control
- Reduces input field size to increase geometric detail.
- Decreases photon flux → system compensates with higher exposure.
- Use selectively; contributes substantially to cumulative dose.
Dose monitoring and alerts
- Real-time displays show DAP and cumulative air kerma during the procedure.
- Audible/visual warnings activate at predefined thresholds.
- Post-procedure summaries document total fluoroscopy time, DAP, and air kerma for audit and patient record.
- Exceeding 2 Gy skin dose typically triggers patient follow-up for potential deterministic effects.
Practical optimisation
To maintain diagnostic quality while minimising dose:
- Use the lowest pulse rate compatible with procedural requirements.
- Keep collimation tight at all times.
- Employ added filtration and lowest reasonable magnification.
- Use last-image hold for planning rather than continuous beam.
- Monitor displayed dose metrics and reposition the beam if thresholds are approached.
Key points and exam tips:
- Air kerma rate reflects instantaneous intensity; DAP reflects total energy delivered.
- Pulsed fluoroscopy, filtration, and collimation are the most effective dose-saving strategies.
- Magnification and high pulse rates increase dose.
- Cumulative air kerma ≈ skin dose, DAP ≈ whole-body risk.
- Common exam question: “List the dose metrics used in fluoroscopy and describe the main features that reduce patient dose.”
Up next
Next, we will move on to Image Quality and System Optimisation, describing how spatial, temporal, and contrast resolution are balanced in fluoroscopic systems, and how these parameters interact with radiation dose.