6.3 Units, Measurement & Dose Quantities

The Four Radiation Quantities

ARRT items expect fluency in four related quantities, each with an SI unit and an older traditional unit. A common memory aid is the phrase R-A-D and R-E-M, but the reliable approach is to learn what each quantity measures and its conversion factor. The four are exposure, absorbed dose, equivalent dose, and effective dose, plus the separate quantity radioactivity.

Notice that absorbed dose, equivalent dose, and effective dose all have the same numerical relationship to their traditional units (a factor of 100), but they answer different questions.

Exposure and Air Kerma

Exposure quantifies the amount of ionization x-rays produce in a known mass of air. Its traditional unit is the roentgen (R) and its SI unit is the coulomb per kilogram (C/kg). Modern practice increasingly reports air kerma (kinetic energy released per unit mass) in gray, written Gy(a) to signify "gray in air." Air kerma is displayed by fluoroscopic units as a real-time dose-tracking value and is replacing the roentgen for describing beam output.

Absorbed Dose

Absorbed dose is the energy actually deposited in a mass of tissue. Its SI unit is the gray (Gy), defined as 1 joule of energy absorbed per kilogram (1 J/kg); 1 Gy equals 100 rad. Because absorbed dose depends on the absorbing medium, tissue dose is written Gy(t). Absorbed dose alone, however, does not describe biologic risk, because equal energy from different radiations does not do equal damage.

Equivalent Dose and the Radiation Weighting Factor

Equivalent dose (H-T) corrects absorbed dose for the type of radiation using the radiation weighting factor (W-R), formerly the quality factor:

Equivalent dose (Sv) = absorbed dose (Gy) x W-R

The unit is the sievert (Sv) (traditional: rem). Low-LET radiations, x-rays, gamma rays, and beta particles, all have W-R = 1, so for diagnostic radiography 1 mGy of absorbed dose equals 1 mSv of equivalent dose. High-LET radiations weigh much more: alpha particles carry W-R = 20, and neutrons range from about 2 to 20 depending on energy. Equivalent dose lets us compare the biologic impact of different radiation types on a single organ.

Effective Dose and Tissue Weighting

Effective dose (E) goes one step further and accounts for which tissues were irradiated and their differing radiosensitivity, so a partial-body exposure can be expressed as the equivalent whole-body risk. It sums each organ's equivalent dose multiplied by that organ's tissue weighting factor (W-T):

Effective dose (Sv) = sum of (W-T x H-T) for all irradiated tissues

The tissue weighting factors are set so they add up to 1.0. Under ICRP 103, the highest weights (0.12 each) go to the red bone marrow, colon, lung, stomach, and breast; the gonads carry 0.08; and thyroid, esophagus, bladder, and others carry 0.04 or less. Effective dose, expressed in sieverts, is the quantity used to set occupational and public dose limits and to compare the risk of, say, a chest radiograph (~0.1 mSv) with a CT of the abdomen (~8-10 mSv). Detailed dose-limit values are covered in the personnel-protection chapter.

Radioactivity and Background Dose

Radioactivity measures the rate at which a radioactive source decays. Its SI unit is the becquerel (Bq), defined as one disintegration per second; the traditional curie (Ci) equals 3.7 x 10^10 Bq. Radiography rarely uses these, but nuclear-medicine cross-over items appear.

Finally, know typical background radiation. In the United States the average person receives about 3.1 mSv per year from natural sources, of which radon gas is by far the largest contributor (~2 mSv), followed by internal, terrestrial, and cosmic sources. Adding medical exposure roughly doubles the total to about 6.2 mSv per year. These benchmarks let you place a patient's imaging dose in everyday context, a skill both the exam and real patient communication reward.

Worked Example: Building an Effective Dose

Work the quantities in sequence. Imagine a chest radiograph delivers 0.1 mGy absorbed dose to the lungs. Because x-rays are low-LET, W-R = 1, so the lung equivalent dose is 0.1 mSv. To find the effective dose contribution, multiply by the lung tissue weighting factor (W-T = 0.12): 0.1 mSv x 0.12 = 0.012 mSv from the lung. Summing every partially irradiated organ's contribution yields the total effective dose of roughly 0.1 mSv for the exam. This chain, absorbed dose (Gy) to equivalent dose (Sv, via W-R) to effective dose (Sv, via W-T), is the single most important conceptual thread in radiation dosimetry, and items frequently test whether you know which weighting factor applies at each step.

Comparing Doses in Everyday Terms

Relating a study to background not only appears on the exam but is also how you reassure an anxious patient without minimizing genuine dose stewardship.