An ionization chamber has radiation absorption characteristics most similar to which body medium?

• A. Air
• B. Bone
• C. Fat
• D. Blood

Answer: (A)

Ionization chambers are gas-filled detectors; their sensitive medium is air. The photons interact and deposit energy in that gas just as they would in air, and the chamber is calibrated in air so its reading corresponds to dose in air (air kerma/exposure). Because air has a very low density and a specific set of interaction characteristics, the detector’s absorption behavior mirrors that of air much more than it does of denser body tissues. Dense tissues like bone or blood, and even fat, have different densities and atomic compositions, which change how photons are absorbed and how energy is deposited. So the chamber’s response best matches air, making air the closest match for its absorption characteristics.