Which one of the following imaging modalities imparts the highest dose of ionizing radiation to the patient?
CT head and neck
The correct answer is CT head and neck. CT acquires many thin-section X-ray projections around the patient, so cumulative dose far exceeds any plain film. CT head and neck delivers roughly 2-4 millisieverts, about one to two years of natural background radiation or 100-200 chest radiographs. This is why CT contributes a disproportionate share of population medical radiation and why the ALARA principle and justified indications matter, especially in children and pregnancy.
Ultrasound uses sound waves and delivers no ionizing radiation, making it the modality of choice in pregnancy and for FAST in trauma. A chest X-ray is among the lowest-dose studies at about 0.02 mSv. Screening mammography irradiates only the breast, about 0.4 mSv for a two-view bilateral study, higher than a chest film but an order of magnitude below CT.
Key point: Dose ranking is ultrasound and MRI (zero) < chest X-ray < mammography < CT. Ultrasound and MRI are preferred in pregnancy whenever they can answer the question.
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