A count is a single event recorded by radiation detection equipment.

[Whole Body Radiation Counter] Very sensitive device intended to measure even the smallest amount of radionuclides in the entire body. Range usually from Bq to kBq. Heavy shielding is used to keep out naturally existing background radiation.

(GM counter or GM tube) The Geiger-Mueller counter is a radiation measuring instrument. A Geiger-Mueller detector consists of a gas-filled tube that discharges electrically when ionizing radiation passes through it. A counter records the events.

The absorbed dose is the average energy absorbed per unit mass.

The tissue absorbed energy in a small mass volume:
D = (dW/dm) [ Gy ]
D = absorbed dose in Gray (Gy); dW = in the tissue energy absorbed; DM = small volume of the mass.

The SI unit of absorbed dose is the joule per kilogram and its special name is the gray (Gy). In units often used by federal and state agencies, absorbed dose is given in rad; 1 rad = 0.01 Gy.

Absorbed dose is a feature that should increase dose awareness and help users in dose optimization. Absorbed dose in CT is quoted using the CTDI (computed tomography dose index)

CTDIvol (volume-averaged CT dose index) and the dose-length product (DLP) give an indication of the average absorbed dose and relative radiation risk to a standard patient. The user is being warned to scan parameter settings that may lead to high doses, and can adjust the protocol if appropriate. It should be noted that CTDIvol and DLP do not take patient size into account, and will give overestimates and underestimates for large and small patients, respectively.

(ALARA) 'As low as reasonably achievable' is a precautionary principle that should be part of basic radiation safety considerations in protection to the exposure as well as in other technologies of the medical, the nuclear and the industrial fields.
ALARA is based on three principles: Justification means that possible exposure to humans should yield a sufficient benefit to society to justify the risks of the radiation exposure. The ICRP in 1977 states that 'all exposures shall be kept as low as reasonably achievable, economic and social factors being taken into account'. The radiation exposure must be reduced to the lowest level possible, considering the costs of such a limitation in dose.