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The unique radiation exposure conditions that exist in computed tomography (CT), during which thin slices of the patient are irradiated by a narrow, fan-shaped beam of x rays emitted from the x-ray tube during its rotation around the patient, have required the use of special dosimetry techniques to characterize the radiation doses to patients and to monitor CT system performance. This section describes the basic dosimetry quantities used to indicate patient doses during CT.

Absorbed dose - The fundamental quantity for describing the effects of radiation in a tissue or organ is the absorbed dose. Absorbed dose is the energy deposited in a small volume of matter (tissue) by the radiation beam passing through the matter divided by the mass of the matter. Absorbed dose is thus measured in terms of energy deposited per unit mass of material. Absorbed dose is measured in joules/kilogram, and a quantity of 1 joule/kilogram has the special unit of gray (Gy) in the International System of quantities and units. (In terms of the older system of radiation quantities and units previously used, 1 Gy equals 100 rad, or 1mGy equals 0.1 rad.)

Equivalent dose - The biological effects of an absorbed dose of a given magnitude are dependent on the type of radiation delivering the energy (i.e., whether the radiation is from x rays, gamma rays, electrons (beta rays), alpha particles, neutrons, or other particulate radiation) and the amount of radiation absorbed. This variation in effect is due to the differences in the manner in which the different types of radiation interact with tissue.

The variation in the magnitude of the biological effects due to different types of radiation is described by the "radiation weighting factor" for the specific radiation type. The radiation weighting factor is a dimensionless constant, the value of which depends on the type of radiation. Thus the absorbed dose (in Gy) averaged over an entire organ and multiplied by a dimensionless factor, the radiation weighting factor, gives the equivalent dose. The unit for the quantity equivalent dose is the sievert (Sv). Thus, the relation is

equivalent dose (in Sv) = absorbed dose (in Gy) x radiation weighting factor

In the older system of units, equivalent dose was described by the unit rem and 1 Sv equals 100 rem or 1 mSv equals 0.1 rem.

For x rays of the energy encountered in CT, the radiation weighting factor is equal to 1.0. Thus, for CT, the absorbed dose in a tissue, in Gy, is equal to the equivalent dose in Sv.

Effective dose - The risk of cancer induction from an equivalent dose depends on the organ receiving the dose. A method is required to permit comparison of the risks when different organs are irradiated. The quantity "effective dose" is used for this purpose. The effective dose is calculated by determining the equivalent dose to each organ irradiated and then multiplying this equivalent dose by a tissue-specific weighting factor for each organ or tissue type. This tissue- or organ-specific weighting factor accounts for the variations in the risk of cancer induction or other adverse effects for the specific organ. These products of equivalent dose and tissue weighting factor are then summed over all the irradiated organs to calculate the "effective dose." (Note that effective dose is a calculated, not measured quantity.) The effective dose is, by definition, an estimate of the uniform, whole-body equivalent dose that would produce the same level of risk for adverse effects that results from the non-uniform partial body irradiation. The unit for the effective dose is also the sievert (Sv).

Quantities specific to CT - A number of special dose quantities have been developed to characterize the doses associated with CT. It is beyond the scope of this discussion to describe these unique dose descriptors. They include the Computed Tomography Dose Index, referred to as the CTDI, the "weighted" CTDI (CTDIW), the "volume" CTDI (CTDIVOL), the "multiple scan average dose" (MSAD), and the "dose-length product" (DLP). See "Other Resources Page" for links to detailed descriptions of dose quantities and indices used for CT.