Milligrays (mGy) to Millirads (mRad) conversion

What is the Milligray?

The milligray is a submultiple of the gray, the SI unit of absorbed radiation dose, equal to one-thousandth of a gray. It is the practical unit for the low doses encountered in medical imaging and everyday radiation exposure.

Definition

One milligray is 10⁻³ gray, i.e. one millijoule of radiation energy absorbed per kilogram of matter:

$$1\ \text{mGy} = 1.00000\times10^{-3}\ \text{J/kg}$$

Since 1 Gy = 1 J/kg, the milligray equals 0.001 J/kg. In the older CGS system, 1 mGy = 0.1 rad, and 1000 mGy = 1 Gy = 100 rad.

Origin and History

The milligray takes its name from the British radiobiologist Louis Harold Gray (1905–1965) combined with the metric "milli-" prefix. The gray was adopted into the SI in 1975, and its milli-submultiple quickly became standard for reporting diagnostic-imaging doses.

Law and Notable Facts

As an SI-prefixed unit, the milligray is fully official. It dominates diagnostic radiology reporting because typical imaging doses fall in the single-to-tens-of-milligray range, keeping figures conveniently sized compared with fractions of a gray.

Real-World Examples and Conversions

• 1 mGy = 0.001 Gy = 0.1 rad.
• A chest X-ray delivers roughly 0.1 mGy; a CT scan delivers tens of mGy locally.
• Average annual natural background dose is around 2–3 mGy.
• 1000 mGy = 1 gray.

What is the Millirad?

The millirad is one-thousandth of a rad, a non-SI submultiple used to express the small absorbed radiation doses encountered in diagnostic imaging and environmental monitoring. It quantifies the energy that ionizing radiation deposits per unit mass of material.

Definition

One millirad equals one-thousandth of a rad, or 10 micrograys of absorbed dose:

$$1\ \text{mRad} = 1.00000 \times 10⁻⁵\ \text{Gy}$$

Because $1\ \text{rad} = 0.01\ \text{Gy}$ and the gray is defined as $1\ \text{Gy} = 1\ \text{J/kg}$, one millirad corresponds to $10⁻⁵\ \text{J/kg}$, equivalent to $0.1\ \text{erg/g}$.

Origin and History

The millirad arose naturally from the rad, which the International Commission on Radiological Units and Measurements defined in 1953. As radiation-protection measurements increasingly dealt with the very low doses of diagnostic radiology and background exposure, the millirad became a convenient working unit.

Law and Notable Facts

Like the rad, the millirad has officially been replaced by SI submultiples of the gray (typically the milligray or microgray), but it persists in United States medical physics practice. One milligray equals 100 millirad, keeping the same factor-of-100 relationship as the parent units.

Real-World Examples and Conversions

A single dental X-ray delivers on the order of a few millirad. A cross-country airline flight exposes passengers to roughly 2 to 5 millirad from cosmic radiation. Natural background radiation in many regions amounts to several hundred millirad per year, of which 100 millirad equals 1 milligray.