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
X-ray imaging uses ionising radiation, which carries a potential risk to both patients and staff.
Radiation safety in medical imaging aims to maximise diagnostic benefit while minimising risk through three guiding principles established by the International Commission on Radiological Protection (ICRP):
- Justification – every exposure must result in a net benefit.
- Optimisation – exposure should be as low as reasonably achievable (ALARA).
- Dose limitation – applies to staff and the public, not to patients undergoing justified medical exposure.
Why Radiation Safety Matters
Although diagnostic doses are relatively low, the biological effects of ionising radiation are cumulative and probabilistic.
Repeated small exposures can increase long-term risk of malignancy, while higher doses may cause direct tissue injury.
Understanding these effects enables safe imaging practice, dose monitoring, and appropriate use of protection measures.
Two Types of Biological Effects of Ionising Radiation
Radiation effects are classified into deterministic (tissue reactions) and stochastic (probabilistic) effects, depending on their dose-response behaviour.
I can’t tell you how often this comes up in exams. You need to understand the difference between these risks.
Deterministic (Tissue Reaction) Effects
Definition
Deterministic effects occur when a threshold dose is exceeded, causing cell death or functional impairment in a significant fraction of a tissue or organ.
Characteristics
- Threshold: no effect below a certain dose.
- Severity increases with dose (once threshold is passed).
- Affected individuals are certain to show the effect if threshold exceeded.
- Result from massive cell loss, not mutation.
Examples and approximate thresholds
| Effect | Typical Threshold Dose | Latency / Notes |
|---|---|---|
| Transient skin erythema | ~2 Gy | Hours to days |
| Epilation (temporary hair loss) | 3–5 Gy | 2–3 weeks |
| Permanent epilation | >7 Gy | 3–4 weeks |
| Skin desquamation / ulceration | >10 Gy | Weeks |
| Cataract formation | 0.5 Gy (lens) | Months to years |
| Temporary sterility (testes) | ~0.15 Gy | 1–2 months |
| Permanent sterility | >3.5 Gy | Months |
| Foetal developmental defects | 0.1–0.2 Gy (organogenesis) | Depends on gestation stage |
These thresholds are far above those used in diagnostic imaging, but can be reached during prolonged interventional fluoroscopy or accidental over-exposure.
Stochastic Effects
Definition
Stochastic effects arise from random radiation-induced damage to DNA in individual cells.
If the damage is mis-repaired, it may lead to malignant transformation or heritable genetic mutations.
Characteristics
- No threshold: any dose, however small, carries some probability of effect.
- Probability increases with dose; severity does not.
- Mechanism: single-cell mutation rather than tissue necrosis.
- Main concern in diagnostic radiology due to low but non-zero risk per exposure.
Examples
- Carcinogenesis (solid tumours, leukaemia).
- Heritable effects (theoretical, not observed/no direct evidence in humans at diagnostic levels).
Dose–response relationship
- Follows the linear-no-threshold (LNT) model: Risk ∝ Dose
- Risk is additive and cumulative over a lifetime.
Deterministic vs Stochastic Effects
| Feature | Deterministic (Tissue Reaction) | Stochastic (Probabilistic) |
|---|---|---|
| Threshold | Yes | No |
| Severity | Increases with dose | Independent of dose |
| Probability | 0 below threshold, 100% above | Increases linearly with dose |
| Mechanism | Cell death in tissues | DNA mutation in individual cells |
| Examples | Erythema, cataract, sterility | Cancer, genetic mutation |
| Relevant to | High-dose fluoroscopy / therapy | Diagnostic imaging |
| ICRP concern | Prevent tissue damage | Limit long-term risk |
Key Takeaways and Exam Tips:
- Deterministic effects have thresholds and severity increases with dose; mainly relevant in high-dose procedures.
- Stochastic effects have no threshold; risk increases linearly with dose. Main concern in diagnostic imaging.
- ALARA and justification form the foundation of radiation safety.
- Typical diagnostic risks are low but cumulative over a lifetime.
- Common exam question: “Compare stochastic and deterministic radiation effects and explain their relevance in diagnostic imaging.”
Up Next
Next, we’ll move on to Dose Quantities and Units, where we’ll briefly review how radiation dose is quantified in practice, including absorbed, equivalent, and effective doses and how clinical dose indicators like DAP and ESD are used in X-ray imaging.