One milligray is exactly 0.001 gray. The prefix carries the entire distinction between diagnostic imaging and radiotherapy — the two differ by four or five orders of magnitude, and the unit is the same.
About the units
The gray is one joule per kilogram of absorbed ionising radiation. Because the useful range spans from the microgray of background exposure to the tens of gray of cancer treatment, the prefixed forms do most of the work in practice. Diagnostic radiology operates in milligray, radiotherapy in gray, and environmental monitoring in microgray or nanogray per hour. Getting the prefix wrong is not a rounding error but a change of clinical universe: 20 mGy is a routine CT scan, 20 Gy is a fraction of a cancer treatment.
The exact factor
The factor is exactly 1,000. Reference values in milligray: a dental X-ray delivers about 0.005 mGy to the whole body; a chest radiograph 0.1 mGy; a mammogram 3 mGy to the breast tissue; an abdominal CT 10 to 20 mGy per organ; and an interventional cardiology procedure can reach several hundred mGy to the skin. Radiotherapy fractions are typically 1.8 to 2 Gy, which is 1,800 to 2,000 mGy — a thousand times a chest X-ray in a single session.
Where you meet this conversion
Diagnostic radiology and dose recording. Imaging equipment reports dose in milligray or in derived quantities such as dose-length product for CT, and cumulative patient dose is tracked across examinations. Regulatory dose reference levels for common examinations are published in milligray. Paediatric imaging protocols work hard to reduce these figures, because a child's tissue is more radiosensitive and has more years for a stochastic effect to appear. Interventional procedures monitor skin dose in milligray in real time to avoid deterministic injury.