Ionizing radiation (see also Radiation Exposure and Contamination Radiation Exposure and Contamination Ionizing radiation injures tissues variably, depending on factors such as radiation dose, rate of exposure, type of radiation, and part of the body exposed. Symptoms may be local (eg, burns)... read more 
) includes
High-energy electromagnetic waves (x-rays, gamma rays)
Particles (alpha particles, beta particles, neutrons)
Ionizing radiation is emitted by radioactive elements and by equipment such as x-ray and radiation therapy machines.
Most diagnostic tests that use ionizing radiation (eg, x-rays, CT, radionuclide scanning) expose patients to relatively low doses of radiation that are generally considered safe. However, all ionizing radiation is potentially harmful, and there is no threshold below which no harmful effect occurs, so every effort is made to minimize radiation exposure.
There are various ways to measure radiation exposure:
The absorbed dose is the amount of radiation absorbed per unit mass. It is expressed in special units of gray (Gy) and milligray (mGy). It was previously expressed as radiation-absorbed dose (rad); 1 mGy = 0.1 rad.
The equivalent dose is the absorbed dose multiplied by a radiation weighting factor that adjusts for tissue effects based on the type of radiation delivered (eg, x-rays, gamma rays, electrons). It is expressed in sieverts (Sv) and millisieverts (mSv). It was previously expressed in roentgen equivalents in man (rem; 1 mSv = 0.1 rem). For x-rays, including CT, the radiation weighting factor is 1.
The effective dose is a measure of cancer risk; it adjusts the equivalent dose based on the susceptibility of the tissue exposed to radiation (eg, gonads are most susceptible). It is expressed in Sv and mSv. The effective dose is higher in young people.
Medical imaging is only one source of exposure to ionizing radiation (see table Typical Radiation Doses Typical Radiation Doses* 
). Another source is environmental background exposure (from cosmic radiation and natural isotopes), which can be significant, particularly at high altitudes; airplane flights result in increased exposure to environmental radiation as follows:
From a single coast-to-coast airplane flight: 0.01 to 0.03 mSv
From average yearly background radiation exposure in the US: About 3 mSv
From yearly exposure at high altitudes (eg, Denver, Colorado): Possibly > 10 mSv
Radiation may be harmful if the total accumulated dose for a person is high, as when multiple CT scans are done, because CT scans require a higher doses than most other imaging studies.
Radiation exposure is also a concern in certain high-risk situations, as during the following:
Pregnancy
Infancy
Early childhood
Young adulthood for women who require mammography
In the US, CT accounts for about 15% of all imaging tests but for up to 70% of total radiation delivered during diagnostic imaging. Multidetector CT scanners, which are the type most commonly used in the US, deliver about 40 to 70% more radiation per scan than do older single detector CT scanners. However, recent advances (eg, automated exposure control, iterative reconstruction algorithms, 3rd-generation CT detectors), are likely to significantly lower radiation doses used for CT scans. The American College of Radiology has initiated programs —Image Gently (for children) and Image Wisely (for adults)— to respond to concerns about the surge in exposure to ionizing radiation used in medical imaging. These programs provide resources and information about minimizing radiation exposure to radiologists, medical physicists, other imaging practitioners, and patients.
Radiation and cancer
Estimated risk of cancer due to radiation exposure in diagnostic imaging has been extrapolated from studies of people exposed to very high radiation doses (eg, survivors of the atomic bomb explosions at Hiroshima and Nagasaki). This analysis suggests a small but real risk of cancer if radiation doses are in the tens of mGy (as used in CT). A CT pulmonary angiogram, routinely done to detect pulmonary embolism, delivers about as much radiation to the breasts as about 10 to 25 two-view mammograms.
Risk is higher in young patients because
They live longer, giving cancers more time to develop.
More cellular growth (and thus susceptibility to DNA damage) occurs in the young.
For a 1-year-old who has a CT scan of the abdomen, estimated lifetime risk of developing cancer is increased by 0.18%. If an older patient has this test, risk is lower.
Risk also depends on the tissue being irradiated. Lymphoid tissue, bone marrow, blood, and the testes, ovaries, and intestines are considered very radiosensitive; in adults the central nervous and musculoskeletal systems are relatively radioresistant.
Radiation during pregnancy
Risks of radiation depend on
Dose
Type of test
Area being examined
The fetus may be exposed to much less radiation than the mother; exposure to the fetus is negligible during x-rays of the following:
Head
Cervical spine
Extremities
Breasts (mammography) when the uterus is shielded
The extent of uterine exposure depends on gestational age and thus uterine size. The effects of radiation depend on the age of the conceptus (the time from conception).
Recommendations
Diagnostic imaging using ionizing radiation, especially CT, should be done only when clearly required. Alternatives should be considered. For example, in young children, minor head injury can often be diagnosed and treated based on clinical findings, and appendicitis can often be diagnosed by ultrasonography. However, necessary tests should not be withheld, even if the radiation dose is high (eg, as with CT scans), as long as the benefit outweighs the potential risk.
Before diagnostic tests are done in women of child-bearing age, pregnancy should be considered, particularly because risks of radiation exposure are highest during early (1st trimester), often unrecognized, pregnancy. The uterus should be shielded in such women when possible. Recent studies have raised a controversy surrounding this standard recommendation, namely, that shielding might increase the radiation dose to the uterus and fetus.
