BASIC PRINCIPLES OF RADIATION DOSIMETRY
The field of medical radiation dosimetry arose from the practical need to predict the biological effects of ionizing radiation before patient exposure. The link between increasing the amount of energy imparted by a certain type of radiation and increasing biological effects was demonstrated early in the 20th century.
A convenient way to quantify radiation is by using the concept of absorbed dose. Absorbed dose (also known as physical dose) is defined as the amount of energy deposited by radiation divided by the mass of the object in which the energy was deposited:
where ARE is the absorbed radiation energy and m the mass of the object in which the energy was absorbed. Dose rate, a related concept, is the increase of absorbed dose per unit time (ΔD/Δt).
Thus, absorbed dose is the density of the energy deposited by radiation in an object. The SI unit for dose measurement is the gray (Gy). One gray is the absorbed dose equal to 1 J of radiation energy absorbed by 1 kg of matter (1 Gy = 1 J/kg). An earlier unit of measurement is the rad, defined as the dose of radiation that imparts 100 erg to 1 g of matter. One gray is equivalent to 100 rad.
Methods of Measuring Absorbed Dose
The most common method of measuring dose is air dosimetry. The method uses a calibrated gas radiation detector that measures the charge induced by radiation. Technically, the quantity measured is exposure, not absorbed dose. Exposure, defined as the amount of electric charge produced by radiation in a unit mass of air, is measured in roentgen (R). The roentgen is almost the exposure equivalent of the rad. More exactly, an absorbed dose of 1 rad corresponds to an exposure of 1.07 R. Similar to dose rate, the exposure rate is the increase of exposure in the unit time. The exposure rates common in nuclear medicine practice range from 0.1 to 10 mR/h.
Gas detectors can provide instantaneous readings of the exposure rate. Survey meters, which are a sensitive form of gas radiation detectors, are routinely used in nuclear cardiology laboratories. At the end of each shift, the staff involved in handling radiation is thoroughly surveyed with a survey meter (including hands and shoe soles) for radioactive contamination. The high sensitivity of survey meters makes them suitable for evaluation of contamination of victims of accidents and criminal acts resulting in release of radioactivity. Under these circumstances, portable survey meters from the nuclear cardiology laboratory and the trained personnel can become ad hoc multipliers of the civil defense assets, and can help limit the damage related to radiological incidents.
Other methods of measuring radiation dose are film dosimeters and thermoluminescence detectors (TLDs). Radiation interacts with photographic film, and the resulting film exposure can ...