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Exam category: Safety → Radiation Physics and Radiobiology
Why it matters: Four related-but-distinct quantities get tested constantly, mostly by asking you to tell them apart — what each one measures, in what unit, and in what tissue.
There's a chain: how much radiation is in the air (air kerma) → how much energy a specific tissue actually absorbs (absorbed dose) → how biologically damaging that absorbed dose is for that tissue, accounting for radiation type (equivalent dose) → how much whole-body risk that represents once you account for which organs were exposed and how sensitive they are (effective dose). Each step adds a layer of "how much this actually matters," not just "how much radiation was there."
Air kerma is the kinetic energy photons transfer to electrons during ionization — essentially, radiation intensity measured in air. Unit: gray in air (Gya).
Before air kerma, this same "how much ionization in the air" idea was called exposure: the amount of ionization x-rays or gamma rays produce in a mass of air. You'll still see its units:
Exposure is defined only for air and only for x-rays/gamma rays. Modern practice reports air kerma (Gy) instead, so treat roentgen and C/kg as the legacy names for that same first link in the chain.
Memory cue — the intensity unit grew up: the "ionization in air" quantity went roentgen (R) → coulomb/kg (C/kg, SI) → air kerma (Gy, today) — same idea, three eras of units.
The radiation energy actually absorbed per unit mass of tissue. Unit: gray in tissue (Gyt). For the same air kerma, absorbed dose varies by tissue type — Gya and Gyt are not interchangeable, even though both use the gray.
Absorbed dose alone doesn't account for the fact that some types of radiation cause more biological damage than others for the same absorbed energy. Equivalent dose corrects for this with a radiation weighting factor (Wr):
Equivalent dose = Wr × Absorbed dose
For x-rays, gamma rays, and electrons — everything used in diagnostic radiography — Wr = 1. (Other radiation types, like neutrons or alpha particles, get much higher weighting factors because they're more damaging per unit of absorbed dose — not something diagnostic radiography deals with directly, but it's why the weighting factor exists at all.)
Diagnostic imaging almost always irradiates only part of the body, but radiation risk data (like the atomic bomb survivor studies) is based on whole-body exposure. Effective dose converts a partial-body exposure into an equivalent whole-body risk figure by weighting each exposed tissue's dose by that tissue's relative radiosensitivity — the tissue weighting factor (Wt):
Effective dose (E) = Σ (Equivalent dose × Wt) — summed across every exposed tissue.
More radiosensitive tissues (higher Wt) contribute more to effective dose for the same absorbed dose. This is the number actually used to estimate a patient's overall risk from a partial-body exam — not absorbed dose to any single organ.
Students use "dose" generically and lose track of which specific quantity a question is actually asking about. If a question mentions partial-body exposure and overall patient risk, it wants effective dose. If it's about energy deposited in a specific tissue, it wants absorbed dose. Matching the right term to the right scenario is most of what's actually being tested here.
Source: Bushong, Radiologic Science for Technologists, 12th ed., Ch. 1, Ch. 35, Ch. 40.
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