Which statement correctly describes the Gray unit?

• A. Gray is the SI unit of absorbed dose.
• B. Gray is the unit of dose equivalent.
• C. Gray is the unit of exposure in air.
• D. Gray is not used in radiology.

Answer: (A)

Absorbed dose is the amount of energy deposited by radiation in a mass, per unit of that mass. The Gray is the SI unit for this quantity, defined as 1 joule of energy absorbed per kilogram of matter. This directly measures how much energy tissue receives from radiation, which is the foundational way we quantify exposure effects in radiology.

Dose equivalent, which reflects the biological impact of that energy, is expressed in Sieverts, not Grays. Exposure in air measures ionization and is expressed in units like coulombs per kilogram (the roentgen is historical). Since Gray characterizes energy deposited in matter, it correctly describes absorbed dose, and it is indeed used in radiology.