The main measurements used in radiology
In depth - July 22, 2019

The main measurements used in radiology

To treat, analyze and manipulate radioactive materials, it is necessary to define the intensity with which these materials disintegrate. This is done by measuring its activity, which expresses the number of disintegrations that take place in the material during one unit of time.

Measuring the intensity of disintegration

In the International System of Units (SI), the activity unit is the becquerel, symbolized by Bq. The becquerel is the activity of a material that experiences a disintegration per second.

Since the disintegration of a radioactive body decreases exponentially, its activity does not remain constant as time passes; it follows the same evolution, slower or faster depending on whether it is an isotope with a long or short life.

Measuring the actions in radiation

To study the actions of radiation on an object, we have defined the measurement of the absorbed dose (usually just “dose”).

A dose is the energy that the radiation transfers to the mass unit in the irradiated material

In the SI system, the dose unit is called gray, symbolized by Gy. A gray is a dose of radiation that transfers 1 joule of energy to 1 kilogram of mass of irradiated material. Studies of the biological effects of radiations show that these effects not only depend on the absorbed dose but also on the type of radiation used. That is, equal doses from two different radiations produce different biological effects.

To take this into account a new measurement is defined: the corresponding dose, which is the product of the absorbed dose by a weighting factor for each type of radiation. This is done to standardize the different types of radiation from the point of view of their biological effects. This factor is 1 for X, gamma and beta radiations; 5 to 20 for neutrons, 5 for protons and 20 for alpha radiations and other particles with different charges. The corresponding dose unit in the SI system is the sievert, symbolized by Sv. A sievert is the absorbed dose of any radiation that produces the same biological effects as 1 Gy of gamma radiation.

The International System of Units and the Radiological System

The International System of Units began to be used not long ago. Previously, radiological magnitudes were measured within a special unit system known as the Radiological System. It is still possible to find publications that use the units in this system.

Here are the correspondences between the units of the international and radiological systems:

Units for radiological measurements in the SI and radiological systems

MeasurementUnit and symbol (SI)Unit and symbol (Radiological system)Correspondence
Activitybecquerel - Bqcurio - Ci1 Ci = 3.7 x 1010 Bq
Absorbed dosegray - Gyrad - rad or rd1 rad = 102 Gy
Corresponding dosesievert - Svrem - rem1 rem = 10-2 Sv

The Radiological System also defined a measurement known as exposure, used to measure the radiation’s ability to produce ions in the air; its unit was the roentgen. This measurement became obsolete, and thus its unit in the Si system (coulomb/kilogram) does not have a name. The correspondence is 1R 0 2.54 x 10-4 C/kg.

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