One of the most significant areas of advance in clinical medicine over the last 20 years has been in the imaging technologies. It is difficult to point to the single method or application that has benefited most from those advances because nearly all specialities in medicine have been involved. Perhaps the most significant trend, however, has been the development and introduction of imaging methods that do not use ionizing radiation, such as ultrasonography (US) and magnetic resonance imaging (MRI). Imaging methods using x-rays or nuclear medicine products are the largest single man-made source of radiation burden to the population. Research and development into other methods that can replace techniques that use ionizing radiation is a justifiable goal in itself, for the benefit of both patients and hospital staff involved in imaging. It is difficult/impossible to provide accurate estimates of risk of imaging with ionizing radiation on an individual basis, but suffice it to say, the risks are small but present and must be taken into account when devising imaging policies.

The effects of ionizing radiation used in medical imaging in adults are divided into two types depending on the mechanism of risk accumulation with increasing dose. Non-stochastic effects occur in body areas such as the cornea where radiation will definitely have a deleterious effect that is dose-dependent. This is not usually a concern for patients but it is for staff involved in high-exposure procedures such as interventional angiography. Most of the concern for patient risk is the induction of malignant processes, which are described as being stochastic events, i.e., they are chance occurrences with the risk increasing with exposure. The risk of tumor develop- ment is not equal all over the body, some parts being quite resistant to radiation and others being very sensitive (ova, bone marrow, breast, thyroid, etc.). Irradiation of the ovaries and testicles presents concerns other than tumor generation in the individual, as there is risk of DNA damage that will be passed to a future offspring and could manifest as malformation or tumor.

When experts in radiation biology produce dose-related risks for radiation exposure in a medical environment they take account of the relative exposures of different parts of the body with different imaging methods because of the variation of radiation sensitivity, as described above. They take account of the patient’s age as well, as it is highly likely that cells that are dividing rapidly are more susceptible to the damaging effects of radiation. Because an induced malignancy may take many years to develop, a lower age is an increased risk because there is more time for a problem to develop. Hence clinical imagers are very keen to limit radiation exposure in children wherever possible. It is imperative that the developing fetus should not be exposed to unnecessary radiation because of the known risks of teratogenicity and tumor formation. Although x-ray based methods of accessing the fetus have been used in the past, this is not the case now and even irradiating the mother during pregnancy should be performed only for well-considered reasons. Fetal assessment by imaging, therefore, is the realm of methods that do not use ionizing radiation.