Basics of X-ray Physics
X-ray safety

Key points

  • X-rays are potentially harmful to both patients and hospital staff
  • Requests for radiological examinations should be clinically justified

It's the law!

Referrers for radiological investigations are required to provide sound clinical reasons to justify exposing patients to radiation. Local rules of the X-ray department must be adhered to, as ignoring them may result in breaking the law.

All X-rays may cause alteration of cellular division and other intracellular processes and are therefore potentially harmful to the human body. For this reason all medical exposure to radiation should be justified in terms of risk to benefit ratio.

Biological effects of X-ray exposure

Some biological effects such as skin damage are dose dependent, whereas others, such as development of cancer, are random.

Dose dependent biological effects become measurable above 50mSv (millisieverts) and a whole body dose of greater than 10Sv (sieverts) is universally lethal. The average radiation dose per person in the UK is 2.6mSv per year, 2.2mSv of which is background and 0.4mSv relates to medical exposure. Diagnostic investigations typically involve doses between 0.02mSV (chest X-ray) to 10mSv (CT abdomen). This means that a chest X-ray amounts to 3 days of background radiation and a CT abdomen is equivalent to 4.5 years!

Some body parts are more susceptible to the random damaging effects of radiation. These are generally tissues with rapidly dividing cells, for example, radiation dose to the stomach is over 20 times more likely to result in a fatal cancer than the same dose to bone.

Radiation exposure to reproductive organs carries further potential risk to future generations. Children are more radiosensitive than adults and irradiation of a fetus should be avoided wherever possible.

Safety principles

Several principles should be adhered to in order to reduce risk to patients.

JUSTIFICATION - Potential benefit of radiation exposure should outweigh risk.

OPTIMIZATION - Measures should be in place to reduce dose to patients and staff.

LOCAL RULES - Measures to ensure wider regulations are enforced, for example that X-ray machines are correctly installed and used, and that referrals are justified.

Risks to staff

Staff are also at potential risk from radiation exposure. The doses that radiologists and radiographers are exposed to are generally small. However, local rules are enforced to ensure that dosimetry badges and protective clothing such as lead aprons are worn, in order to monitor and reduce staff radiation dose.

Knowledge of the 'inverse square law' helps in reducing dose. This states that the dose to a given area is quadrupled be halving the distance from the radiation source. Simply put, standing back from a source of radiation reduces dose to staff. This is particularly important during interventional radiology cases when radiologists or radiographers are working close to the X-ray beam.

The inverse square law

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The inverse square law

  • The strength of the X-ray beam is inversely proportional to the square of distance from the source (X).
  • Standing back by double the distance from a source of radiation (d to 2d) will quarter the dose to the radiologist or radiographer.

Radiation safety should be taken seriously by all referring doctors. If you are in doubt about justifying a procedure or investigation you should discuss it with a radiologist. For more information please use the 'iRefer' guidelines available online from the Royal College of Radiologists.