Which unit is used to express effective dose in medical imaging risk assessments?

• A. Gy
• B. mSv
• C. Sv
• D. J/kg

Answer: (B)

Effective dose represents overall radiation risk to a person by combining how much energy is deposited in different tissues with how sensitive each tissue is to radiation. The appropriate unit for this risk-related quantity is the sievert. In medical imaging, exposures are usually small, so we report risk in millisieverts to keep the numbers convenient and comparable to typical imaging doses.

Absorbed dose, measured in Gray (or equivalently joules per kilogram), only tells you how much energy is deposited in tissue and does not account for differences in tissue radiosensitivity. That’s why Gray or J/kg aren’t used to express risk. While the sievert is the unit for effective dose (a weighted sum across tissues), reporting in millisieverts (mSv) reflects the common, practical range of imaging exposures.