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 procedures represent the highest radiation dose environment in diagnostic imaging. Both patients and staff are exposed to direct and scattered radiation for extended periods, making radiation protection a critical part of fluoroscopic practice.
Protection measures are guided by the principles of justification, optimisation, and dose limitation.
Sources of Radiation Exposure
| Source | Description | Primary Concern |
|---|---|---|
| Primary beam | X-rays directly exiting the tube and passing through the patient | Patient dose |
| Scattered radiation | Deflected photons from patient and table | Staff dose |
| Leakage radiation | X-rays escaping from the tube housing (≤1 mGy/h at 1 m) | Minor; regulated by design |
In interventional and theatre fluoroscopy, the patient becomes the main scatter source. Most staff exposure originates from photons scattered from the patient’s skin surface.
Staff Exposure Pathways
- Scattered radiation: the dominant contributor, especially near the beam entrance side.
- Backscatter from table and walls: adds to operator exposure.
- Reflected scatter from ceiling or floor: relevant during lateral or oblique projections.
Typical occupational dose levels (without protection) can exceed several millisieverts per procedure, highlighting the importance of shielding and correct positioning.
Radiation Protection Principles
Radiation protection centres around three core principles. We’ve covered these previously.
A. Time
- Minimise fluoroscopy time; use intermittent screening.
- Utilise last-image hold instead of continuous exposure.
- Keep record of cumulative fluoroscopy time.
B. Distance
- Staff dose follows the inverse square law: I ∝ 1 / r2
- Doubling the distance from the patient reduces exposure to one-quarter.
- Stand on the image receptor side of the C-arm whenever possible, scatter is significantly lower there.
C. Shielding
- Employ structural and personal shielding to absorb scatter radiation.
- Barriers include fixed protective screens, ceiling-mounted shields, and lead table skirts.
Staff Protection Measures
| Protective Measure | Typical Specification | Purpose / Effect |
|---|---|---|
| Lead apron | 0.5 mm Pb equivalence | Reduces body dose by >90 % |
| Thyroid shield | 0.35–0.5 mm Pb | Protects thyroid gland from scatter |
| Ceiling-suspended screen | 0.5 mm Pb | Shields head and upper body |
| Under-table lead skirt | 0.5 mm Pb | Protects lower body and legs |
| Lead glass viewing window | 1.5–2.0 mm Pb | Fixed barrier for control rooms |
| Lead gloves | 0.25 mm Pb | Limited benefit; avoid beam entry |
| Personal dosimeters | Film or electronic badges | Mandatory dose monitoring |
Best practice:
- Apron and thyroid shield must always be worn during active fluoroscopy.
- Ceiling and table-mounted shields positioned as close to the patient as practicable.
- Operators should avoid leaning over the patient or placing hands in the primary beam.
Patient Protection Measures
| Technique | Effect |
|---|---|
| Collimation | Reduces exposed area and scatter |
| Added filtration (e.g. 0.9 mm Cu) | Removes low-energy photons; lowers skin dose |
| Pulsed fluoroscopy | Reduces exposure duration |
| Low-dose and variable frame-rate modes | Adjust image quality to procedural need |
| Optimal geometry | Keep image receptor close to patient and X-ray tube as far as practical |
| Avoid repeat projections | Plan beam angles to minimise overlap of entrance fields |
| Monitor dose indicators | Track cumulative air kerma and DAP during long procedures |
Occupational Dose Limits (ICRP/IRR)
I know, sorry, lot’s of tables here. It’s just the easiest way to summarise the information.
| Tissue / Category | Dose Limit | Averaging Period |
|---|---|---|
| Effective dose (whole body) | 20 mSv/year (averaged over 5 years; max 50 mSv in any single year) | Annual |
| Lens of eye | 20 mSv/year (ICRP 118) | Annual |
| Skin, hands, feet | 500 mSv/year | Annual |
| Pregnant worker (abdomen surface) | 1 mSv for remainder of pregnancy | — |
Personal dose monitoring is mandatory in all fluoroscopy suites. Real-time dosimeters are recommended during interventional procedures to provide immediate feedback.
Equipment Design and Room Shielding
- Tube housing: limits leakage to <1 mGy/h at 1 m at maximum output.
- Control booth walls and lead glass: typically 1.5 mm Pb equivalence.
- Warning lights and audible indicators: show when beam is active.
- Interlocks: prevent exposure when doors or shields are open.
Key Points and Exam Tips:
- Scatter radiation is the principal source of staff dose in fluoroscopy.
- Apply time, distance, and shielding principles consistently.
- Standing on the detector side of the patient greatly reduces exposure.
- Ceiling-suspended screens and lead skirts are the most effective protective barriers.
- Cumulative air kerma provides an estimate of skin dose; DAP reflects overall risk.
- Occupational dose limits: 20 mSv/year effective dose, 20 mSv/year to the lens, 500 mSv/year to extremities.
- Common exam question: “Describe the main sources of radiation exposure in fluoroscopy and outline protective measures for staff and patients.”