Core Principles - Dose Dodge Dance
- Justification:
- Benefit of procedure must outweigh radiation risk.
- Ensure net positive benefit before any exposure.
- Optimisation: 📌 ALARA (As Low As Reasonably Achievable)
- Keep all exposures minimal, considering socio-economic factors.
- Methods: ↓Time, ↑Distance, appropriate Shielding.
- Dose Limitation:
- Strict adherence to dose limits for occupational & public exposure.
- These limits do not apply to patient's medical exposure.
⭐ Dose limits are for occupational/public protection, not for patients undergoing medical exposure; here, justification and optimisation (ALARA) are paramount.
Dose Metrics & Limits - Number Game
- Absorbed Dose (D): Energy deposited per unit mass (Gray, Gy).
- Equivalent Dose ($H_T$): Absorbed dose adjusted for radiation type. $H_T = \sum_R W_R \cdot D_{T,R}$ (Sievert, Sv). $W_R$ = Radiation Weighting Factor.
- Effective Dose (E): Equivalent dose adjusted for tissue sensitivity. $E = \sum_T W_T \cdot H_T$ (Sv). $W_T$ = Tissue Weighting Factor.
- Diagnostic Reference Levels (DRLs): Standardized dose levels for common procedures; investigate if consistently exceeded.
Annual Dose Limits (ICRP):
| Category | Effective Dose (Annual) | Lens of Eye (Annual) | Skin (1 cm²) (Annual) | Hands/Feet (Annual) |
|---|---|---|---|---|
| Occupational | 20 mSv (avg over 5 yrs, max 50 mSv/yr) | 20 mSv | 500 mSv | 500 mSv |
| Public | 1 mSv | 15 mSv | 50 mSv | N/A |
Patient Dose Factors - Dialing It Down
- Exposure Parameters:
- kVp: Optimal selection; higher kVp can ↓dose with mAs reduction.
- mAs (mA × time): Directly impacts dose. Use lowest diagnostic mAs; ↓time.
- Distance (Source-to-Skin): Maximize. Dose $I \propto 1/d^2$.
⭐ Doubling distance from X-ray source reduces exposure to 1/4 (Inverse Square Law).
- Filtration: Added filters (Al) harden beam, ↓skin dose.
- Collimation: Restrict beam to interest area. ↓Irradiated volume, ↓scatter, ↓dose.
- Shielding: Lead shields (gonadal, thyroid) for organs outside primary beam.
- Patient Size: Adjust technique; larger patients need ↑exposure.
- Grids: For parts >10-12 cm. ↓Scatter, ↑contrast, but ↑dose (2-5x).
- Detector Efficiency (DQE): Higher DQE = ↓radiation needed.
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Modality-Specific Optimisation - Smart Scan Tactics
Tailor strategies per imaging technique to minimize dose, ensuring diagnostic quality.
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| Modality | Key Optimisation Tactics |
|---|---|
| General Radiography | ↑kVp/↓mAs techniques, Automatic Exposure Control (AEC), appropriate Source-to-Image Distance (SID), collimation. |
| Fluoroscopy | Pulsed fluoroscopy, Last Image Hold (LIH), virtual collimation, ↓frame rate, minimize magnification mode use. |
| CT Scan | Automated Exposure Control/Tube Current Modulation (AEC/TCM), iterative reconstruction algorithms, optimize scan range & pitch, pediatric-specific protocols. Bismuth shields (controversial). |
| Mammography | Appropriate breast compression, optimal beam quality (target/filter combination, e.g., Mo/Mo, Mo/Rh, W/Rh). |
| Nuclear Medicine | Justify administered activity (ALARA), pediatric dose reduction (e.g., EANM formula or $D_{ped} = D_{adult} \times \frac{Weight_{ped}}{70}$), select radiopharmaceutical with optimal $T_{eff}$. |
High‑Yield Points - ⚡ Biggest Takeaways
- ALARA (As Low As Reasonably Achievable) is the guiding principle.
- Justification: Ensure medical benefit outweighs radiation risk.
- Optimization: Lowest dose for diagnostic images, using Dose Reference Levels (DRLs).
- Time, Distance, Shielding: Cardinal principles for staff and patient protection.
- Collimation: Restrict beam to area of interest to ↓ dose and scatter.
- High kVp, low mAs techniques generally ↓ patient dose.
- Iterative reconstruction in CT significantly ↓ dose compared to filtered back projection_._
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