mGy

mGy, or milligray, is a unit of absorbed radiation dose in the International System of Units. The gray (Gy) is defined as one joule of energy deposited per kilogram of matter. Therefore, one milligray is one thousandth of a gray: 1 mGy = 0.001 Gy = 0.001 J/kg. The gray is used to quantify energy deposition in a specific mass of tissue or material exposed to ionizing radiation.

Absorbed dose describes the amount of energy delivered to matter, but it does not directly reflect biological effect. In radiation protection and medical decision making, two related quantities are commonly used: equivalent dose and effective dose, measured in sieverts (Sv) or millisieverts (mSv). These account for radiation type and tissue sensitivity and are derived from the absorbed dose by applying weighting factors.

Applications: In medical imaging and radiotherapy, doses are often reported in mGy when describing the energy deposited in tissues. Diagnostic CT scans may involve doses of several milligrays to the imaged region, while other procedures may yield smaller or larger values depending on technique and settings. In radiotherapy, doses are higher and commonly expressed in grays per fraction and total dose, reflecting planned energy delivery to a target.

Measurement and context: Doses in mGy are measured with dosimeters or calculated from imaging parameters, and can be expressed as a dose rate in mGy per unit time. The actual biological effect depends on the dose, dose rate, radiation quality, and tissue characteristics.

Relation to historical units: The gray replaces the older rad unit, with 1 Gy = 100 rad. The milligray is among the commonly used subunits in clinical contexts, alongside mSv for protective dose assessment.

See also: Absorbed dose, Equivalent dose, Effective dose, Dosimetry, Radiation safety.