Radiation Units – Key Vocabulary

30 vocabulary flashcards covering radiation measurement units, dose quantities, CT dose indices, radioactivity units, and key conversion factors.

Radiation Units

Standardized quantities used to express and compare physical properties of ionizing radiation, reported in either conventional or SI forms.

Conventional Units

Older, non-metric radiation units such as roentgen, rad, rem, and curie.

SI Units

International System units for radiation measurement: coulomb per kilogram, gray, sievert, and becquerel.

Exposure (X)

The amount of ionizing radiation that may strike an object in air; used mainly for beam output calibration.

Roentgen (R)

Conventional unit of exposure equal to 2.58 × 10⁻⁴ C/kg of air.

Coulomb per kilogram (C/kg)

SI unit of exposure measuring the charge of ion pairs produced per kilogram of air.

Absorbed Dose (D)

Energy deposited per unit mass of tissue by ionizing radiation; basis for all dose calculations.

Rad

Conventional unit of absorbed dose; 1 rad = 0.01 Gy.

Gray (Gy)

SI unit of absorbed dose equal to 1 joule per kilogram; 1 Gy = 100 rad.

Quality Factor (Q)

Multiplier that converts absorbed dose to dose equivalent by accounting for radiation type’s biological damage.

Kerma (K)

‘Kinetic Energy Released per unit MAss’; sum of initial kinetic energies of charged particles liberated by uncharged radiation, expressed in gray.

Equivalent Dose (H)

Absorbed dose adjusted by radiation weighting factor to reflect biological effect on a specific tissue; H = D × W_R.

Sievert (Sv)

SI unit for equivalent or effective dose; 1 Sv = 100 rem.

Radiation Weighting Factor (W_R)

Dimensionless number describing the relative biological effectiveness of a radiation type (e.g., 1 for x-rays, 20 for alpha).

Effective Dose (E)

Sum of tissue-weighted equivalent doses reflecting overall stochastic risk to the whole body.

CT Dose Index (CTDI)

Standardized measure of radiation output from a CT scanner obtained in an acrylic phantom.

CTDI100

CTDI measured over a 100-mm pencil ion chamber; linear average not tailored to patient anatomy.

CTDIw (Weighted CTDI)

Dose index combining center and peripheral CTDI100 values: CTDIw = ⅓ (center) + ⅔ (periphery).

CTDIvol

Volume CTDI that adjusts CTDIw for pitch (table movement); CTDIvol = CTDIw / pitch.

Dose-Length Product (DLP)

CT dose metric incorporating scan length: DLP = CTDIvol × scan length (mGy · cm).

Pitch (CT Scanning)

Table travel per 360° rotation divided by collimated beam width; higher pitch generally lowers patient dose.

Monte Carlo Simulation (Radiology)

Computerized random sampling technique used to model radiation transport and estimate patient dose accurately.

Radioactivity

Process of spontaneous nuclear decay accompanied by emission of particles or photons.

Activity (A)

Rate at which a radionuclide decays, expressed in disintegrations per second.

Curie (Ci)

Conventional unit of activity; 1 Ci = 3.7 × 10¹⁰ decays per second.

Becquerel (Bq)

SI unit of activity equal to one decay per second; 1 Bq = 2.7 × 10⁻¹¹ Ci.

Conversion: Gy ↔ rad

1 Gy = 100 rad ; 1 rad = 0.01 Gy.

Conversion: Sv ↔ rem

1 Sv = 100 rem ; 1 rem = 0.01 Sv.

Conversion: Ci ↔ Bq

1 Ci = 3.7 × 10¹⁰ Bq ; 1 Bq ≈ 2.7 × 10⁻¹¹ Ci.

Memory Tip (Roentgen–Gray–Sievert)

Roentgen measures exposure, Gray (or rad) measures absorbed energy, Sievert (or rem) measures biological damage.