Ionizing radiation (see also Radiation Exposure and Contamination) 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 quantify 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 used to estimate tissue reactions (or stochastic effects) of exposure to ionizing radiation; 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. The effective dose helps clinicians assess and compare health risks associated with various medical radiation procedures and can also be compared to an assigned occupational dose in radiation protection standards.
Medical imaging is only one source of exposure to ionizing radiation (see table 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 United States: About 3 mSv
From yearly exposure at high altitudes (eg, Colorado, New Mexico): About 1.5 mSv additional over background
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
The National Council on Radiation Protection and Measurements (NCRP) in United States (Report No. 184) shows that CT scans accounted for 63% of the collective dose from all medical imaging procedures in 2016, compared to 50% in 2006. While the number of CT scans increased by 20% over that decade, the overall dose per person for CT procedures was essentially unchanged. Between 2006 and 2016,the estimated nontherapeutic medical radiation dose declined by 15 to 20%. The estimated average individual effective dose per person in the United States was 2.92 mSv in 2006 and 2.16 mSv in 2016.
Multidetector CT scanners, which are the type most commonly used in the United States, 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 professionals, 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). Direct epidemiological evidence from human populations demonstrates that exposure to ionizing radiation increases the risk of some cancers when doses exceed approximately 50 to 100 mSv for protracted exposure (eg, in occupational settings) or 10 to 50 mSv for acute exposure (eg, from atomic bomb exposure) (1).
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% (2). 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 potential benefit outweighs the potential harm.
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 (first 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.
References
1. Brenner DJ, Doll R, Goodhead DT, et al: Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 100(24):13761-13766, 2003. doi:10.1073/pnas.2235592100
2. Brenner D, Elliston C, Hall E, Berdon W: Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR Am J Roentgenol 176(2):289-296, 2001. doi:10.2214/ajr.176.2.1760289
