Table 4

Synopsis of Radiation Exposure and Dose Metrics

MetricUnitUtility
Absorbed Dose-Related Parameters:
Characterize Dose to Organ/Tissue or Whole Body
Absorbed doseGyAmount of ionizing radiation energy deposited per unit mass of tissue. 1 Gy = 1 Joule of energy deposited per kg of tissue. This metric is a concentration of energy deposition—not the total quantity of energy deposited.
Equivalent doseSvAbsorbed dose adjusted by a radiation weighting factor that adjusts for the specific tissue-injuring potential of the particular radiation type. Photons (x-rays and gamma rays) have a weighting factor of 1. Electrons also have a weighting factor of 1. Neutrons have larger weighting factors that vary with their energy level. For medical imaging, because only photons and positrons are used, absorbed dose and equivalent dose take the same value.
Effective dosemSvCalculated whole-body quantity used to roughly compare potential stochastic risks from different partial-body exposures. It is expressed as the uniform whole-body dose that would confer the stochastic risk equivalent to that caused by a regional exposure.
Modality-Specific Parameters
X-ray fluoroscopic air kerma (free-in-air)GyUsed to assess level of radiation present at a location. In x-ray fluoroscopy, cumulative air kerma at the interventional reference point can be used to approximate beam entrance port skin dose. (For isocentric C-arms, the reference point is located 15 cm from isocenter in the direction toward the x-ray source. This point in space approximates the location of beam entry into the patient, but due to variation in table height and tube angulation, is only an estimate of beam entrance port skin dose).
X-ray fluoroscopic Air-KAP, also referred to as dose-area product (DAP)Gy·cm2Used to assess the total quantity of radiation delivered by an external beam. It is the product of the cumulated amount of air kerma and the area of a radiographic or fluoroscopic field. KAP is often used as the basis for estimating effective dose from a fluoroscopic procedure.
Computed tomographic dose index (CTDIFDA, CTDI100, CTDIw, and CTDIvol)mGyUsed to assess relative level of radiation applied during a CT imaging sequence. This metric is a concentration of energy deposition in the exposed volume. It is not a total deposited energy quantity, as it does not incorporate the actual exposed volume (See DLP below). Different versions are used for varied purposes.
Computed tomographic dose-length product (DLP)mGy·cmUsed to assess integrated amount of radiation applied along an axial length of a patient during a CT examination. Can be used to estimate effective dose from the procedure.
Radionuclide injected dosemCiA measure of the quantity of radioactivity injected for a nuclear scintigraphy study. The relationship of injected dose to other dose parameters is complex and includes the nature of the nuclide’s radiation, the nuclide’s half-life, the distribution in the body, and the elimination kinetics.

CT = computed tomography; CTDI = computed tomographic dose index; KAP = Kerma-Area Product.