8.2 Dosimetry, Dose Limits & Regulations

Personnel Dosimeters

A personnel dosimeter monitors — it does not shield — occupational dose, creating a legal record of accumulated exposure. Four devices are tested:

• The optically stimulated luminescence (OSL) dosimeter is the current standard. Its aluminum oxide detector is read by laser light, is reusable, accurate to about 0.01 mSv (1 mrem), resistant to heat and humidity, and can be worn for a monthly or quarterly badge period.
• The thermoluminescent dosimeter (TLD) uses lithium fluoride (LiF), which stores absorbed energy and releases it as light when heated; LiF is nearly tissue-equivalent and reusable, but the readout destroys the stored signal.
• The film badge is the historical device — inexpensive with a permanent record, but heat/humidity sensitive and single-use.
• The pocket ionization chamber gives an immediate readout of accumulated exposure, useful in high-dose settings, but is less accurate and can falsely discharge.

Dosimeter Placement

The radiographer wears one dosimeter at collar level, outside the lead apron. This estimates dose to the unshielded head, neck, thyroid, and lens — the most exposed region during fluoroscopy — and correlates with effective dose. A declared pregnant worker is issued a second (fetal) dosimeter worn at waist level under the apron to estimate embryo/fetal dose. Badges are never shared, worn at home, or left in the radiation field when not on the body.

Occupational and Public Dose Limits

Dose limits are recommended by the NCRP (National Council on Radiation Protection and Measurements) in Report No. 116 and enforced by state and federal regulators. Effective-dose limits use the tissue-weighted sievert (Sv); note 1 Sv = 100 rem and 1 rem = 10 mSv.

Key points the exam repeats: the annual occupational effective dose limit is 50 mSv (5 rem) for the whole body, excluding personal medical and natural background exposure. The cumulative whole-body limit is 10 mSv multiplied by age in years (a 40-year-old worker: 400 mSv), keeping lifetime dose proportionate to age. The lens of the eye limit is 150 mSv/yr (protecting against the deterministic cataract threshold) and the skin/extremities limit is 500 mSv/yr. The public limit is 1 mSv/yr for continuous exposure (5 mSv for infrequent), and a student under 18 is held to that same 1 mSv/yr.

The Declared-Pregnant Worker

A worker becomes a declared pregnant worker only when she voluntarily declares the pregnancy in writing, stating the estimated conception date. Only then do the special limits apply: 5 mSv (0.5 rem) for the entire gestation and 0.5 mSv per month to smooth exposure and protect the embryo during organogenesis. The monthly cap prevents the whole 5 mSv from being delivered in one month. Declaration is voluntary and may be withdrawn in writing at any time. The employer issues the second waist-level fetal badge and reviews assignments, but cannot force the worker off the job.

ALARA, Units, and Regulatory Structure

ALARA operationalizes the limits: facilities set investigation levels (Level I about 1/10 and Level II about 3/10 of the applicable limit) that trigger review long before any legal limit is reached. For dose quantities, absorbed dose is the gray (Gy) while equivalent and effective dose are the sievert (Sv), which multiplies absorbed dose by a radiation weighting factor (1 for x-rays) and, for effective dose, tissue weighting factors. The negligible individual dose is 0.01 mSv/yr.

Regulatory oversight is layered, and ARRT expects you to separate who recommends from who enforces:

• NCRP — recommends dose limits (advisory, not enforcement).
• ICRP — the international counterpart to the NCRP.
• NRC (Nuclear Regulatory Commission) — regulates reactor-produced (by-product) radioactive materials and delegates authority to Agreement States.
• FDA / CDRH — regulates the manufacture and performance standards of x-ray equipment.
• State health departments — license and inspect diagnostic x-ray facilities and personnel.

Common Exam Traps

Several distinctions are tested repeatedly. First, the dosimeter is a record, not protection — it does nothing to attenuate the beam, so wearing one never substitutes for time, distance, or shielding. Second, the 50 mSv effective-dose limit is the whole-body limit; the higher 150 mSv (lens) and 500 mSv (skin/extremities) values apply to specific tissues, and candidates often reverse them. Third, occupational limits exclude the worker's own diagnostic medical exposure and natural background (about 3 mSv/yr in the U.S.). Fourth, pregnancy limits apply only after a written declaration; an undeclared pregnant worker is legally treated as any other occupational worker. Finally, remember the conversion 1 rem = 10 mSv (1 Sv = 100 rem) when a question mixes SI and traditional units.

The radiographer is personally accountable for wearing the monitor, applying ALARA, and staying within limits every shift.