A matter of exposure
The standard unit of measure for radiation absorbed by an individual is called the "Sievert," or Sv (sometimes identified by a smaller unit called the "millisievert," or mSv). Common medical imaging tests such as X-rays or mammograms generally expose patients to a radiation dose of less than 1 mSv. Other procedures using CT, nuclear stress tests, or fluoroscopy-guided exams often involve radiation int he range of 5-40 mSv. New advances in CT technology, like GE Healthcare's ASiR, have helped reduce the typical range of radiation for CT exams to 1-15 mSv.
Radiation can also be expressed in the unit Gray (Gy), a measurement of the absorbed dose of radiation. This term replaces a former unit of measurement called the rad.
A single exposure at these diagnostic levels may not pose much risk to the patient. But when a patient has numerous tests over a period of time, the cumulative exposure may raise the level of risk. To minimize cumulative exposure, physicians should determine whether a procedure using medical radiation is necessary to achieve the diagnosis or whether an alternative imaging procedure may offer the same diagnostic benefit.
The April 2011 issue of American Journal of Radiology has a series of articles on the assessment of benefits and risks associated with medical radiation in imaging procedures. Healthcare providers can get more details by clicking on the PubMed links below.