Understanding X-rays (cont.)

Radiation Versus Radioactive

It is natural that we might confuse X-rays with radiation from radioactivity. You may think that man-made radiation is more dangerous than an equal amount of natural radiation, but this is not necessarily the case.

Most background radiation comes from radioactivity in a person's body. We are all radioactive. A typical adult has over 9,000 radioactive disintegrations in his or her body each second. That's over one-half million per minute. The resulting radiation strikes billions of our cells each hour. There are two scientific quantities used in the discussion of radiation protection: equivalent dose and effective dose. Neither of these quantities can be directly measured.

Effective dose

Effective dose, E, is defined by the International Commission for Radiological Protection (ICRP) and was adopted by the US National Council for Radiation Protection and Measurement (NCRP). The concept of effective dose is appealing but unattainable. E is intended to equate the relative risk of inducing a fatal cancer from a partial body dose (such as radon progeny in the lungs) to the whole body dose that would have the same risk of inducing a fatal cancer.

The effective dose cannot be measured, and it is difficult to calculate. Physicists use computer simulation programs to estimate the organ doses in a standard patient from typical exposure conditions for various X-ray examinations. The results of these simulations can be used to estimate E for various patient exposures. Once a table of effective doses is constructed for a particular X-ray unit, it is a simple matter to calculate the BERT-the time to get the same effective dose from background radiation. Typical effective doses and BERT values for some common X-ray projections are listed here.

Typical effective doses and BERT values for some common X-ray studies in an adult (adapted from IPSM Report 53)

Effective dose should not be confused with the entrance skin dose (ESD), which was commonly used for describing patient radiation up until about 20 years ago. The ESD is easy to measure, but it is not a good measure for the amount of radiation a patient receives. For example, the ESD for a dental intra-oral X-ray (for example, a bitewing) is about 50 times greater than the ESD for a chest X-ray, yet the effective dose from the dental exposure is usually lower than the dose from a chest X-ray.

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