What is Radiation?

What is Radiation?We are all exposed to radiation every day, mainly from the sun and soil. Other sources of radiation are man-made and include consumer products as well as diagnostic imaging tests like X-rays, CT scans, and nuclear medicine studies.

Radiation is any energy that comes from a source and travels through space, such as light or heat. X-rays are a form of radiant energy, like light or radio waves, but unlike light, X-rays can penetrate the body, which enables X-rays to produce pictures or “images” of internal body structures. At UCSF and most other imaging centers, these images are viewed on a computer monitor and stored electronically.

In the field of Interventional Radiology, the X-ray images are used to assist with the placement of tubes or other devices in the body or with other therapeutic procedures and treatments.

Measuring Radiation Dosage

What is Radiation?The scientific unit of measurement for radiation dose is the Gray (Gy), which is a measure of the amount of X-ray energy absorbed per unit mass. Other radiation units include rad, rem, roentgen, and Sievert.

X-ray imaging procedures normally focus on a specific body part (for example the head or chest) and expose only that region of the body to a radiation dose. Furthermore, different tissues and organs have varying sensitivity to radiation exposure, so the actual radiation risk from an X-ray procedure will vary depending on the body part imaged and the tissues exposed. The term “effective dose” is used to account for the portion of the body exposed during a procedure and averages the risk of radiation over the entire body.  The unit of effective dose is the Sievert.  X-ray procedures result in a small fraction of a Sievert, usually one one-thousandth of a Sievert, indicated by a milliSievert (mSv).

The effective dose accounts for the relative sensitivities of the different tissues exposed. More importantly, it allows for estimation of risk and comparison to more familiar sources of exposure like natural background radiation. More information is available at the radiologyinfo.org website.

Naturally-occurring “Background” Radiation Exposure

We are exposed to radiation from natural sources all the time. According to recent estimates, the average person in the United States receives an effective dose of about 3 milliSieverts (mSv) per year from naturally occurring radioactive materials and cosmic rays, which is radiation that comes from outer space. These natural “background” doses vary throughout the country.

Altitude plays a role in the amount of cosmic radiation, so people living on 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. However, 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.

In simple terms, the radiation exposure from one chest X-ray is equivalent to the amount of radiation exposure one experiences from our natural surroundings over three days.