3.2 Radiation Safety, Dosimetry & Regulatory Standards

Justify the scan and optimize the technique

For patient imaging, first determine whether the result can address the clinical question. Then use a scan mode that produces adequate quality for that question with exposure as low as reasonably achievable (ALARA). Position the patient accurately, confirm the planned sites and use available previous studies to avoid unnecessary acquisitions. A faster or lower-output setting is appropriate only if it provides the quality the patient needs. For an inadequate image, decide whether reanalysis can address the problem before repeating the exposure.

Manage operator and public exposure

For a point source under otherwise unchanged conditions, intensity scales as $1/d^2$. As an illustrative approximation, a point-source-like field producing 0.16 µSv per scan at 1 m would produce $0.16(1/4)^2=0.01$ µSv per scan at 4 m. This calculation is a model. A DXA scatter field originates over an extended region and nearby surfaces can affect it. A room survey and workload assessment guide actual protection decisions.

If a local assessment supplies an illustrative 0.02 µSv per scan at the console and the worker is there for 5,000 such scans, the estimated contribution is 100 µSv (0.10 mSv). Occupancy, other sources and differences between procedures matter. Use the dose quantity and monitoring rules specified by the facility. A simple workload calculation alone cannot exempt a site from structural shielding or personal monitoring.

Apply the correct regulatory framework

In the United States, the FDA regulates manufacture of X-ray devices, while state and other applicable authorities regulate their clinical use. The U.S. Nuclear Regulatory Commission's 10 CFR Part 20 applies to specified licensees and activities. Its 50 mSv annual adult occupational total effective dose equivalent limit and 1 mSv general annual public limit can also be relevant through state incorporation. For example, California incorporates a specified edition of Part 20 by reference under 17 CCR § 30253. Check the governing rules for registration, personnel qualifications, surveys and monitoring. The IAEA international framework uses an occupational effective dose limit of 20 mSv per year averaged over five years and 50 mSv in any one year for adult workers in planned exposure situations. The applicable jurisdiction determines the legal limit.

Screen for interference and pregnancy

The ACR DXA practice parameter identifies recent oral or intravenous contrast and radionuclides as possible limitations and refers to local institutional guidance. Ask what was administered and when. Check the manufacturer's instructions and local policy before proceeding, rescheduling or choosing an appropriate alternative site. Contrast and dense items can overlap the measurement region. Some radiopharmaceuticals can interfere with detection. Do not apply a universal waiting period to all agents and scanners. Remove external objects in the field and assess whether internal material compromises the specific ROI.

Check pregnancy status under the facility's policy. If pregnancy is confirmed or possible, defer an elective central DXA and refer any question about justified imaging to the responsible clinician. For a patient who discovers pregnancy after a scan, record the scan date, equipment, mode, sites and any repeats. Notify the responsible clinician and involve a qualified medical physicist for an exposure estimate when needed. Use fetal absorbed dose in mGy and clinical context for counseling.

Published phantom measurements show low conceptus doses for the pencil-beam spine and hip protocols studied. Dose depends on the device, acquisition and stage of pregnancy. ACOG notes that fetal anomalies, growth restriction and pregnancy loss have not been reported below 50 mGy. ICRP advises that a conceptus dose below 100 mGy should not be considered a reason to terminate pregnancy. These statements address different questions. Explain that routine DXA is generally low-dose and support case-specific counseling rather than using a fixed fetal-dose estimate or making a clinical decision from the maternal effective dose.

FDA medical X-ray oversight · U.S. 10 CFR Part 20 · California radiation regulations · IAEA international standards · ACR DXA parameter · ACOG diagnostic imaging guidance · ICRP 103 · Original fetal-dose study