Continuing improvements in radiologic imaging make it increasingly useful in diagnostic evaluation. Primary care and referring physicians work with radiologists who specialize in diagnostic imaging to choose the best imaging test for each evaluation. Many imaging tests use ionizing radiation (x-rays and radionuclides) and radiographic contrast agents; the associated risks to patients are usually small but should be considered.
Risks of Ionizing Radiation
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 harmful, and there is no threshold below which no harmful effect occurs, so every effort is made to minimize radiation exposure. Doses vary by type of imaging test (see Table 1: Principles of Radiologic Imaging: Typical Radiation Doses*).
There are various ways to measure radiation exposure:
Radiation may be harmful if the total accumulated dose for a person is high, as when multiple scans are done, because most scans require a high dose. Radiation dose is also a concern in certain high-risk situations (eg, during early pregnancy, infancy, or early childhood; in young women who require mammography).
In the US, CT accounts for > 15% of all imaging tests but for about 70% of total radiation delivered during diagnostic imaging. Multidetector CT scanners, which are usually used now, deliver about 40 to 70% more radiation per scan than do older single detector CT scanners.
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, and because more cellular growth (and thus susceptibility to DNA damage) occurs in the young. For a 1-yr-old who has a CT scan of the abdomen, estimated lifetime risk of developing cancer is increased by 0.18%. If an elderly patient has this test, risk is lower.
Risk also depends on the tissue being irradiated; for example, risk is higher for breast and abdominal tissue than for brain tissue.
Radiation during pregnancy
Risks of radiation depend on dose, type of test, and area being examined. The fetus may be exposed to much less radiation than the mother; exposure to the fetus is negligible during head, cervical spine, and extremity x-rays and during 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).
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 risk is high, 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, often unrecognized pregnancy. The uterus should be shielded in such women when possible.
Radiographic Contrast Agents and Contrast Reactions
Radiopaque contrast agents are often used in radiography and fluoroscopy to help delineate borders between tissues with similar radiodensity. Most contrast agents are iodine-based. Iodinated contrast agents may be ionic or nonionic. Ionic contrast agents, which are salts, are hyperosmolar to blood. These agents should not be used for myelography or in injections that may enter the spinal canal (because neurotoxicity is a risk) or the bronchial tree (because pulmonary edema is a risk). Nonionic contrast agents may be low-osmolar (which is still hyperosmolar relative to blood) or iso-osmolar (with the same osmolarity as blood). Newer nonionic contrast agents are now routinely used at many institutions because they have fewer adverse effects.
The most serious contrast reactions are allergic-type reactions and contrast nephropathy (renal damage after intravascular injection of a contrast agent).
Allergic-type contrast reactions
Reactions vary in severity:
The mechanism is anaphylactoid (see Allergic, Autoimmune, and Other Hypersensitivity Disorders: Anaphylaxis); risk factors include a previous reaction to injected contrast agents, asthma, and allergies.
Treatment begins by stopping contrast infusion. For mild or moderate reactions, diphenhydramine 25 to 50 mg IV is usually effective. Treatment of severe reactions depends on the type of reaction and may include oxygen, epinephrine, IV fluids, and possibly atropine (for bradycardia).
In patients at high risk of contrast reactions, imaging tests that do not require iodinated contrast should be used. If contrast is necessary, a nonionic agent should be used, and patients should be premedicated with prednisone (50 mg po 13 h, 7 h, and 1 h before injection of contrast) and diphenhydramine (50 mg po or IM 1 h before the injection). If patients require imaging immediately, they can be given diphenhydramine 50 mg po or IM 1 h before injection of contrast and hydrocortisone 200 mg IV q 4 h until imaging is completed.
In some patients, intravascular injection of an iodinated contrast agent causes serum creatinine to increase transiently. Most of these patients have no symptoms, and nearly all recover normal function within 1 wk. However, < 1% of patients require dialysis or develop chronic kidney disease, indicating contrast nephropathy. Common risk factors include the following:
In patients at risk of developing acute renal failure after receiving iodinated intravascular contrast, reduced dose of contrast, use of iso-osmolality agent, and hydration should be considered. Many hydration regimens exist; one example is IV administration of 0.9% normal saline at 1 mL/kg for 24 h beginning a few hours before the procedure. Acetylcysteine may be given as premedication for patients at risk of developing nephrotoxicity, but its efficacy is uncertain. Oral antihyperglycemic drugs, such as metformin, should be withheld for 48 h after IV contrast administration to avoid drug accumulation if contrast-induced nephrotoxicity occurs. Because many protocols dealing with contrast agents and reactions are specific and continually updated, it is important to discuss such details with the imaging department.
Last full review/revision July 2008 by Jon A. Jacobson, MD