Roentgen measures what?

• A. The energy deposited in tissue
• B. Exposure to gamma or x rays in air
• C. The total radioactivity of a source
• D. Dose equivalent in rem

Answer: (B)

Roentgen measures exposure in air caused by X-ray or gamma radiation. It quantifies the electric charge produced per unit mass of air as ionization results from radiation. This makes it an exposure measure, not a dose delivered to tissue, not the total radioactivity of a source, and not a biological effect quantity. Absorbed dose (energy deposited in tissue) is measured in rad or Gray; radioactivity of a source is measured in curies or becquerels; dose equivalent (biological effect) is measured in rem or sievert. The roentgen is an older unit tied specifically to ionization in air, with modern practice often using coulombs per kilogram for exposure and gray for absorbed dose.