3.7 DXA Dose Quantities & Comparisons
Key Takeaways
Absorbed dose uses Gy. Equivalent dose to a tissue and reference effective dose use Sv.
An X-ray radiation weighting factor of one does not turn an entrance dose into effective dose.
Choose the dose quantity, scanner, mode and coverage before comparing protocols.
Background comparisons give a sense of scale. They do not establish a lifetime limit for justified medical imaging.
Identify the radiation quantity before converting it
Radiation measurements can describe energy absorbed in tissue, radiation-weighted dose to an organ, or an estimated effective dose for comparison. The quantity determines which number answers the question.
| Quantity | Unit and meaning |
|---|---|
| Absorbed dose, | Gray (Gy), equivalent to joules deposited per kilogram. One Gy equals 100 rad. Embryo or fetal dose estimates use absorbed dose, commonly mGy. |
| Equivalent dose to tissue , | Sievert (Sv). Weight the absorbed dose in the tissue by radiation type. For diagnostic X-rays the radiation weighting factor is one, so a tissue dose of 0.20 mGy from X-rays corresponds numerically to 0.20 mSv of equivalent dose to that same tissue. |
| Effective dose, | Sievert (Sv). Sum tissue-weighted equivalent doses for the reference person. One Sv equals 100 rem. The quantity helps compare exposure protocols and is an approximate indicator of population-averaged detriment. |
| Entrance air kerma or estimated skin entrance dose | Commonly reported in Gy or a submultiple. Read the measurement definition. An entrance measurement alone does not establish an organ dose or effective dose. |
In the radiological protection framework,
Here is mean absorbed dose to tissue from radiation type . The factors account for radiation type, and factors account for the tissue contribution to population detriment. In the effective dose calculation, organ doses are based on reference models. An effective dose cannot serve as an individual risk prediction. A weighting factor of one for X-rays does not make Gy and Sv interchangeable for different quantities. For example, the ICRP thyroid weighting factor of 0.04 would give a contribution of 0.008 mSv for a reference thyroid equivalent dose of 0.20 mSv. Contributions from other irradiated tissues are needed to estimate the total effective dose.
Work through the conversions
The prefix matters. Since 1 Gy is 1,000 mGy, 1 rad is 10 mGy. Since 1 Sv is 1,000 mSv, 1 rem is 10 mSv. An absorbed dose of 0.05 mGy is 50 µGy. An effective dose of 0.05 mSv is 50 µSv. Converting the prefix does not convert the quantity.
A GE Lunar iDXA manual lists an estimated skin entrance dose of 146 µGy for an AP spine acquisition in standard mode in its specified table. It also lists 37 µGy for its thin mode. These entries demonstrate the effect of mode and patient thickness assumptions. They do not give the whole-body effective dose for either mode. A reading in air or at the entrance surface does not include the tissue dose distribution.
Compare effective doses for a named protocol
A Stanford research protocol for adult Hologic Discovery A or Horizon A scans gives estimated effective doses of 6.7 µSv for the spine and 4.7 µSv for one hip in Fast Array mode, totaling 11.4 µSv. Its adult Array estimates are 13.3 and 9.3 µSv, totaling 22.6 µSv. These estimates illustrate the dependence on the scanner and mode. The table is not a dose promise for another clinic or patient.
Using an illustrative background effective dose of 8 µSv/day, the Fast Array total corresponds to days and the Array total to days. Background exposure varies by location. This comparison gives scale and does not compare the exact organ doses, dose rate or individual risk of the two exposures.
Central DXA is generally low-dose. A dedicated axial QCT protocol usually exposes the patient to more radiation than central DXA. Comparing the two requires the actual scan coverage and mode. Review a local or manufacturer dose estimate when communicating about a particular exam. An effective dose for the patient also differs from dose to staff near the device, which depends on the room and workload.
Explain the dose to a patient
A patient asks whether previous imaging has exhausted a lifetime radiation quota. Explain that clinical imaging decisions use expected benefit and dose optimization. Check why the examination is needed and explain the local technique. If comparing with background exposure, name the quantity and assumptions. A dose comparison can support an informed decision without imposing a lifetime limit for clinically justified medical examinations.
ICRP dose quantities · GE operator manual · Stanford protocol examples · IAEA Basic Safety Standards · ISCD Adult Official Positions
A physicist estimates 0.20 mGy of absorbed dose to the thyroid from diagnostic X-rays. Which equivalent dose is implied for that tissue?
20 mSv.
0.20 mSv.
0.008 mSv.
0.002 Sv.
A scanner protocol specifies estimated skin entrance dose of 146 µGy. What can this number be reported as directly?
Whole-body effective dose of 146 µSv.
A fetal absorbed dose of 0.146 mGy.
A personal operator dose of 0.146 mSv.
The estimated skin entrance dose for the specified acquisition.
An adult DXA protocol estimates effective doses of 6.7 µSv for spine and 4.7 µSv for one hip. Using 8 µSv/day as an illustrative background rate, what scale comparison is supported?
11.4 µSv total, about 1.4 days of the stated background rate.
6.7 µSv total, exactly one day for all facilities.
22.6 µSv total, about 2.8 days for the stated settings.
11.4 mSv total, establishing a lifetime radiation quota.
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