Radiation in medical imaging: risk, ALARA and pregnancy
- Modality
X-rays and CT carry a small radiation risk, so each study should be needed and done with the least radiation that still answers the question.
What it is
Radiographs, CT, fluoroscopy and nuclear medicine use ionizing radiation, which can damage DNA. Two kinds of effect matter. Deterministic (tissue) effects, such as skin burns, hair loss or cataract, appear only above a threshold dose and get worse as the dose rises; in diagnostic imaging they are seen only with long fluoroscopic procedures. Stochastic effects, mainly cancer, have no known safe threshold: the chance, not the severity, rises with dose, and the risk from one diagnostic study is small. Children are more sensitive and have more years ahead for a cancer to appear. Two principles guide practice: justification (the study must be expected to help the patient more than it harms) and optimization, known as ALARA, «as low as reasonably achievable». Radiation in pregnancy worries patients and doctors alike, but most diagnostic studies give the fetus far less than the level linked to harm, and a needed study should not be withheld.
How the image is made
- Effective dose (millisievert, mSv) compares studies; natural background radiation is roughly 3 mSv a year.
- A chest radiograph is very low, about the same as a few days of natural background radiation.
- A CT of the chest or abdomen is roughly the same as one to three years of background radiation, many times a chest film.
- Ultrasound and MRI use no ionizing radiation.
- Fetal dose depends on whether the fetus is in the beam: studies of the head, chest or limbs give it very little.
What it is good for
- Justification: order a study only when its result will change management.
- Optimization (ALARA): the lowest dose that still gives a diagnostic image, with child-sized protocols.
- Protection of staff: time (shorter exposure), distance (dose falls quickly with distance) and shielding (lead aprons, screens).
Limitations and risks
- Deterministic effects have a threshold (skin injury after long fluoroscopy); stochastic effects (cancer) have no known threshold.
- Children and young adults carry a higher lifetime risk per study than older adults.
- Repeated CT adds up; check whether a recent study already answers the question.
- Fetal risk depends on gestational age: early, an all-or-none effect; during organogenesis and early fetal life, malformation and brain effects occur only at doses far above a usual diagnostic study.
- Anxiety about radiation can cause harm too: delaying a CT in suspected pulmonary embolism or trauma in pregnancy is more dangerous than the study.
Next step
- Before abdominal or pelvic imaging in a person who could be pregnant, ask and, if unsure, do a pregnancy test.
- In pregnancy, prefer ultrasound or MRI when they answer the question; if CT is needed, do it.
- If a pregnant patient has already had a study with the fetus in the beam, ask the radiologist or medical physicist to estimate the fetal dose before counselling.
- In children, ask for paediatric protocols and consider ultrasound first.
Test yourself
Think of the answer, then tap the question to see it.
What is the difference between deterministic and stochastic effects of radiation?
Deterministic effects (skin burns, cataract) appear only above a threshold and worsen with dose; stochastic effects (cancer) have no known threshold, and dose raises their probability, not their severity.
What does ALARA stand for and what does it mean in practice?
«As low as reasonably achievable»: once a study is justified, use the lowest radiation that still gives a diagnostic image, such as child-sized protocols.
What are the three ways staff reduce their own radiation exposure?
Time (shorter exposure), distance (dose falls quickly as one moves away) and shielding (lead aprons, thyroid collars, screens).
A pregnant woman has suspected pulmonary embolism and needs a chest CT. Should the study be withheld because of the fetus?
No. The fetus is outside the direct beam and receives very little; the risk of missing an embolism is far greater. The study is done when it is the right test.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Radiation Safety in Medical Imaging (StatPearls (NCBI Bookshelf), 2023); Radiation Exposure In Pregnancy (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.