Ionizing radiation is measured in units commonly referred to as effective dose in millisievert (mSv) or rad, rem, roentgen, sievert, or gray. Because different tissues and organs have varying sensitivity to radiation exposure, the actual radiation risk to different parts of the body from an x-ray procedure varies. For example, the thyroid is extremely radiation sensitive, because of this lead lined shields are placed over the thyroid when X-Rays are performed near them.
The term effective dose is used when referring to the radiation risk averaged over the entire body. The effective dose accounts for the relative sensitivities of the different tissues exposed. More importantly, it allows for quantification of risk and comparison to more familiar sources of exposure that range from natural background radiation to radiographic medical procedures. Naturally-occurring "background" radiation exposure for the average person in the U.S. is responsible for an effective dose of about 3 mSv per year from naturally occurring radioactive materials and cosmic radiation from outer space.
These natural "background" doses vary throughout the country. People living in the plateaus of Colorado or New Mexico receive about 1.5 mSv more per year than those living near sea level. The added dose from cosmic rays during a coast-to-coast round trip flight in a commercial airplane is about 0.03 mSv. Altitude plays a big role, but the largest source of background radiation comes from radon gas in our homes (about 2 mSv per year). Like other sources of background radiation, exposure to radon varies widely from one part of the country to another.
A simple way of looking at radiation exposure from diagnostic imaging, could be to compare the radiation dose from the test to the amount of radiation exposure one experiences from one's natural areas.
Here are some radiation exposure approximations from common sources:
Procedure Effective Radiation Exposure
Chest X-Ray 10 days 0.1 mSv
Abdomen and Pelvis CT scan 3 years 10 mSv
Head CT scan 8 months 2 mSv
Mammography 7 weeks 0.4 mSv
Coronary ct Angiography 4 years 12 mSv
Bone Density 3 hours 0.001 mSv
The risk of a procedure is usually low relative to the benefit. However, radiation is cumulative. This table could help guide you to discussing the risk with your physician.
