Radiation Units & Measurements – Core Vocabulary

Vocabulary flashcards covering fundamental terms used to quantify, calculate, and interpret radiation exposure, dose, and biological risk in diagnostic imaging.

Exposure

Measure of ionizations produced in air; expressed in coulombs per kilogram (C kg⁻¹).

Air Kerma

Kinetic energy released per unit mass of air; measures ionization energy in air in grays (Gy).

Absorbed Dose

Radiation energy deposited in tissue per unit mass; unit = gray (Gy = J kg⁻¹).

Equivalent Dose (EQD)

Absorbed dose adjusted for radiation type using the radiation weighting factor (WR); unit = sievert (Sv).

Effective Dose (EFD)

Equivalent dose further adjusted for tissue sensitivity using the tissue weighting factor (WT); gauges long-term risk, unit = sievert (Sv).

Radiation in Air

Category of measurements (Exposure, Air Kerma) that quantify beam intensity before interaction with tissue.

Radiation in Tissue

Category of measurements (Absorbed, Equivalent, Effective dose) that quantify energy or risk inside the body.

Linear Energy Transfer (LET)

Rate at which radiation deposits energy along its path in matter; higher LET means denser ionization and more biological harm.

Radiation Weighting Factor (WR)

Numerical value that scales dose for radiation type; low for X-/γ-rays (1), high for α-particles (20).

Tissue Weighting Factor (WT)

Value representing a tissue’s sensitivity to stochastic effects; used in effective-dose calculations.

Relative Biological Effectiveness (RBE)

Comparison of biological damage produced by different radiations delivering the same absorbed dose.

Inverse Square Law

Beam intensity is inversely proportional to the square of the distance from the source (I ∝ 1 ⁄ d²).

Primary Radiation

X-ray photons that exit the tube window before interacting with the patient.

Scatter Radiation

Radiation deflected from its original path after interacting within the patient or other matter.

Leakage Radiation

X-ray photons that escape the protective tube housing other than through the window.

Ionization

Process in which radiation ejects electrons from atoms, creating charged particles (ions).

Coulomb per Kilogram (C kg⁻¹)

SI unit for exposure; 1 C kg⁻¹ = 6.242 × 10¹⁸ ionizations per kilogram of air.

Gray (Gy)

SI unit for absorbed dose and air kerma; equals one joule of energy absorbed per kilogram of matter.

Sievert (Sv)

SI unit for equivalent and effective dose; expresses biological risk rather than physical energy.

milliampere-seconds (mAs)

Product of tube current and exposure time; directly proportional to beam photon quantity and dose.

Kilovolt Peak (kVp)

Maximum voltage applied to the X-ray tube; influences photon energy and quantity, impacting beam intensity and dose.

Photoelectric Effect

Interaction where an X-ray photon is completely absorbed, ejecting an inner-shell electron and increasing patient dose.

Compton Scattering

Interaction where an X-ray photon transfers part of its energy to an outer electron and is deflected, producing scatter.

Transmission

X-ray photons passing through the patient without interaction.

Alpha Particle

High-LET particulate radiation (helium nucleus) with WR = 20, causing dense ionization.

Free Electron

Electron liberated from an atom by ionizing radiation, counted in exposure measurements.

Joule per Kilogram

Base SI expression of a gray; 1 Gy = 1 J kg⁻¹.

Microgray (µGy) / Milligray (mGy)

Sub-units of the gray commonly used in diagnostic imaging to express small doses.

Microsievert (µSv) / Millisievert (mSv)

Sub-units of the sievert used to express small equivalent or effective doses in medical imaging.

Short-Term (Deterministic) Effect

Tissue reaction, e.g., skin erythema, hair loss, predicted by absorbed dose thresholds rather than effective dose.