• Radiation absorbed dose (D)

• Equivalent dose (H_T)

• Effective dose (E)

• Collective effective dose or collective dose

• Dose limits

• Dose rate

Radiation-absorbed dose (D)

This is a measure of the amount of energy absorbed from the radiation beam per unit mass of tissue and can be measured using a dosimeter.

Equivalent dose (H_T)

This is a measure that allows the different radio-biological effectiveness (RBE) of different types of radiation to be taken into account.

For example, alpha particles penetrate only a few millimetres in tissue, lose all their energy and are totally absorbed, whereas X-rays penetrate much further, lose some of their energy and are only partially absorbed. The biological effect of a particular radiation-absorbed dose of alpha particles would be considerably more severe than a similar radiation-absorbed dose of X-rays.

By introducing a numerical value known as the radiation weighting factor (W_R), which represents the biological effects of different radiations on different tissues, the unit of equivalent dose (H_T) in a particular tissue provides a common unit allowing comparisons to be made between one type of radiation and another, for example:

Equivalent dose (H_T) = radiation-absorbed dose (D) × radiation weighting factor (W_R) in a particular tissue

(For X-rays, the radiation weighting factor (W_R) = 1, therefore the equivalent dose (H_T) in a particular tissue, measured in Sieverts, is equal to the radiation-absorbed dose (D), measured in Grays.)

Effective dose (E)

This measure allows doses from different investigations of different parts of the body to be compared, by converting all doses to an equivalent whole body dose. This is necessary because some parts of the body are more sensitive to radiation than others. The International Commission on Radiological Protection (ICRP) has allocated each tissue a numerical value, known as the tissue weighting factor (W_T), based on its radiosensitivity, i.e. the risk of the tissue being damaged by radiation – the greater the risk, the higher the tissue weighting factor. The sum of the individual tissue weighting factors represents the weighting factor for the whole body.

The effective dose (E) from an individual clinical radiograph, where the x-ray beam is absorbed by different tissues, is calculated as follows:

Effective dose (E) = Σ Equivalent dose (H_T) in each tissue × relevant tissue weighting factor (W_T)

The tissue weighting factors recommended by the ICRP in 1990 and revised in 2007 are shown in Table 4.1.

The 2007 revised tissue weighting factors include the salivary glands as an individual weighted tissue and the oral mucosa in the remainder tissues. Therefore the effective dose for dental radiographic examinations may be considerably higher when calculated using these revised tissue weighting factors.

One way of calculating the effective dose is by using a tissue equivalent anthropomorphic phantom with dosimeters placed in the most radiosensitive regions, as shown in Fig. 4.1.

As individual doses are very small from any one examination, a number of radiographic exposures are repeatedly performed (e.g. ten times) using typical exposure factors (time, kV and mA), for that particular examination. The dosemeters are then ‘read’ to give a measurement of the radiation-absorbed dose (D) in the individual tissues. Using this data both the equivalent dose (H_T) and the effective dose (E) may be calculated as shown in the simplified flow diagram shown in Fig. 4.2.