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 (current module)
Fluoroscopy
Mammography
While patient exposures are justified for diagnostic benefit, staff exposures must be kept as low as reasonably achievable (ALARA) without compromising workflow or image quality.
Radiation workers are subject to annual dose limits and protected through a combination of time, distance, and shielding.
The Three Fundamental Principles of Staff Protection
| Principle | Description | Effect |
|---|---|---|
| Time | Minimise the time spent near a radiation source | Halves exposure if time is halved |
| Distance | Increase distance from the source | Follows the inverse square law: ( I ∝ 1/d2) |
| Shielding | Place protective material between source and staff | Attenuates both primary and scattered X-rays |
Time
- Exposure is directly proportional to time spent in the radiation field.
- In fluoroscopy and interventional work:
- Use pulsed fluoroscopy and last-image hold.
- Record fluoroscopy time as part of dose documentation.
- Radiographers should leave the room or stand behind barriers during expos
Distance (Inverse Square Law)
I2 = I1(d1/d2)2
I ∝ 1/d2
- Doubling the distance from the source reduces exposure by a factor of four.
- Staff should stand at least 2 m from the X-ray tube and patient whenever possible.
- For mobile radiography, stand 90–135° to the primary beam direction — the angle of lowest scatter intensity.
Shielding
A. Structural Shielding
- Primary barriers (e.g. lead-lined walls, doors) absorb the direct beam.
- Secondary barriers (e.g. control room walls, lead glass) absorb scattered and leakage radiation.
- Designed using workload, use factor, occupancy, and distance parameters.
B. Personal Protective Equipment (PPE)
| Protection | Typical Lead Equivalence | Notes |
|---|---|---|
| Lead apron | 0.35–0.5 mm Pb | Worn by staff in fluoroscopy or mobile exams |
| Thyroid collar | 0.5 mm Pb | Protects radiosensitive thyroid gland |
| Lead glasses | 0.35 mm Pb | Reduces lens dose (important after ICRP limit reduction) |
| Gloves | 0.25 mm Pb | Protect hands if within scatter field |
| Mobile lead screens | 1–2 mm Pb equivalent | Used in wards and theatres for added protection |
Correct storage: aprons should be hung, not folded, to avoid cracking.
Controlled and Supervised Areas
Defined under radiation protection regulations (e.g. IRR/IRMER in the UK).
| Area Type | Definition | Typical Location / Control |
|---|---|---|
| Controlled area | Area where specific protection measures are required due to potential for significant exposure | X-ray rooms, fluoroscopy suites; access restricted to trained personnel |
| Supervised area | Area where exposure is unlikely to exceed a small fraction of limits | Adjoining corridors, staff rooms; routine monitoring only |
Requirements for controlled areas:
- Clearly marked and signposted.
- Warning lights (“X-rays on”).
- Door interlocks to prevent exposure when open.
- Access limited to essential personnel.
Staff Dose Monitoring
In most centres, staff likely to receive >1 mSv/year must have personal dosimetry.
| Type | Principle | Usage |
|---|---|---|
| TLD (Thermoluminescent dosimeter) | Stores energy from ionisation; emits light on heating | Worn on torso, exchanged monthly/quarterly |
| OSL (Optically stimulated luminescence) | Emits light when stimulated by laser; high sensitivity | Modern replacement for TLDs |
| Film badge | Photographic darkening; now largely obsolete | Historical use only |
| Electronic personal dosimeter (EPD) | Real-time dose readout | Interventional procedures |
Staff Dose Limits (ICRP 103)
| Quantity | Limit | Notes |
|---|---|---|
| Effective dose (whole body) | 20 mSv/year averaged over 5 years (≤50 mSv in any one year) | Applies to classified radiation workers |
| Public effective dose | 1 mSv/year | Excludes medical exposure |
| Lens of eye | 20 mSv/year (lowered from 150 mSv) | Occupational limit |
| Skin / extremities | 500 mSv/year | Localised exposure |
Staff exposures in diagnostic radiology are usually <1 mSv/year which is well below these limits.
Key Takeaways and Exam Tips:
- Time, distance, shielding are the three pillars of occupational protection.
- Inverse square law: doubling distance → one-quarter dose.
- Lead apron (0.35–0.5 mm Pb) essential for fluoroscopy and mobile work.
- Controlled areas: access restricted, interlocked doors, warning lights.
- Personal dosimetry: TLD/OSL badges routinely used.
- Dose limits: 20 mSv/year occupational, 1 mSv/year public.
- Common exam question: “Describe the methods used to minimise occupational radiation exposure in diagnostic radiology.”
Up next
This brings us to the end of the radiation safety in X-ray imaging module. Up next, fluoroscopy! See you there!