Domain II Study Guide — Indigo Field Guide

1Start here

Most of Domain II is protection.

Four levers do the heavy lifting: collimation, filtration, distance, and time (ALARA). Learn those cold, add a handful of units, dose limits, and biology facts, and you own the 25%. Radiation biology is tested lightly — don't over-invest there.

2The units

Three things measured, two names each.

What it measures

Traditional

SI · metric

Conversion

Exposureoutput, in the air

Roentgen (R)

C/kg

—

Absorbed doseenergy into tissue

rad

Gray (Gy)

1 Gy = 100 rad

Equivalent dosebiological harm

rem

Sievert (Sv)

1 Sv = 100 rem

Hook: Air → Roentgen · Tissue → Gray · Harm → Sievert. Traditional = old US names (R, rad, rem); SI = metric (C/kg, Gray, Sievert).

3The four levers

Collimate · Filter · Distance · ALARA

Collimation restricts the beam — rectangular collimation exposes up to ~70% less tissue; the beam must be ≤ 2.75 in (7 cm) at the skin. Filtration (aluminum) removes low-energy photons — ≥ 2.5 mm Al at 70 kVp+. Distance — the inverse-square law: double the distance, one-quarter the intensity. ALARA (As Low As Reasonably Achievable) is the umbrella over all of it.

4Dose limits (MPD)

The numbers to memorize.

Occupational worker — yearly max→5 rem/year (0.05 Sv)

Pregnant worker — whole gestation→0.5 rem total (0.05 rem/month)

General public→0.1 rem/year (1 mSv)

5Operator rules

Stand back ≥ 6 ft from the tubehead, or behind a barrier.

Position 90–135° to the beam (behind the patient's head).

Never hold the receptor or the tubehead during an exposure.

Your biggest dose is scatter off the patient — not the primary beam.

6Dosimetry

A dosimeter records your cumulative occupational dose — it never shields.

Worn at the torso/collar, read on a schedule (film, TLD, or OSL).

It measures your exposure, not the patient's — a device, not a source.

Elevated reading? Evaluate equipment & technique first.

7Biology (tested lightly)

The handful that shows up.

How radiation mostly damages tissue→Indirect — hits water → free radicals → DNA

Most radiosensitive→rapidly dividing: bone marrow, reproductive cells, lymphocytes

Least radiosensitive→mature, non-dividing: nerve, muscle, mature bone

No threshold, chance-based (cancer, genetic)→stochastic

Threshold, dose-driven severity (burns, cataracts)→deterministic

Affects the exposed person vs. offspring→somatic vs. genetic

8The apron question

Two right answers — know which room you're in.

Exam room — what DANB still tests→lead apron + thyroid collar intraoral; no collar on a panoramic

Real room — 2024 ADA/AAOMR update→no shielding needed — collimation + digital protect better, even in pregnancy

On the exam: shield. Dosimeters are still recommended for pregnant staff either way.

9Most-missed traps

Where Domain II points get lost.

"SI unit for exposure"→C/kg — Roentgen is the old unit

"Raise the kVp — what happens to contrast?"→lower contrast (longer gray scale)

"Operator's greatest exposure source"→scatter off the patient — not the primary beam, not the badge

"Thyroid collar on a panoramic?"→no — it blocks the beam and leaves an artifact

"What does a dosimeter do?"→measures your dose — it doesn't shield or emit

Domain II in 8 lines

✓ Units: Exposure = R / C·kg · Absorbed = rad / Gray (1 Gy = 100 rad) · Equivalent = rem / Sievert (1 Sv = 100 rem).

✓ Protection: collimate (≤2.75 in, rectangular ~70% less), filter (≥2.5 mm Al), distance (inverse-square), ALARA.

✓ Limits: worker 5 rem/yr · pregnant 0.5 rem/gestation · public 0.1 rem/yr.

✓ Operator: ≥6 ft, 90–135°, never hold receptor/tube; scatter is your main dose.

✓ Dosimeter: records occupational dose, never shields; elevated → check equipment first.

✓ Biology: indirect damage dominates; marrow/reproductive/lymphocytes most sensitive; stochastic (no threshold) vs. deterministic.

✓ kVp up → contrast down; mAs up → image darker.

✓ Aprons: on the exam, shield — apron + collar, no collar on panos.