ALARA (As Low As Reasonably Achievable)

Recommendations to minimize one's exposure to radiation to the lowest level possible, such as choosing alternate imaging methods (if appropriate), increasing the distance from the source of radiation, decreasing exposure time and using the right shielding, if appropriate.

Background Radiation

Radiation that is present in very low levels on earth and in the atmosphere. This radiation is emitted from many sources, including granite and other stones, cosmic rays, and certain elements. The average dose a person receives from background radiation is approximately 3 mSv per year.

Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine testing: a catalog. Radiology. 2008;248:254-263.

Computed Tomography (CT) scan

A CT scan can detect many conditions that may not show up on a standard X-ray. Depending on the technology utilized, the typical radiation exposure from a CT scan is 1.5-20 mSv

Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine testing: a catalog. Radiology. 2008;248:254-263.


A commonly cited index for modern CT equipment is CTDIvol. It is an expression of radiation dose in a CT dosimetry phantom, and is derived from the CT dose index, or CTDI. CTDI represents the radiation dose of a single CT slice from the primary beam, plus scatter from the surrounding slices. CTDIvol or the volume CTDI is calculated using the weighted CTDI and dividing it by the beam pitch factor.

Cumulative exposure

The total amount of radiation dose a person has received throughout the course of his or her life. This includes exposure to natural background radiation and exposure from medical imaging and other medical procedures.

Cumulative risk

The risk to a person from the cumulative dose of radiation received during his or her life. The level of risk depends on a variety of factors such as the amount of exposure, what parts of the body were exposed, and how old the person was at various times of exposure.

Dose Area Product (DAP)

The result of the radiated surface area multiplied by the radiation dose at the surface. A DAP measurement is useful because it enables a direct calculation of the effective dose.

Dose-Length Product (DLP)

Helps indicate the integrated radiation dose of a CT examination. It incorporates the number of scans and the scan width. It is calculated by multiplying the CTDIvol by the scan length.


The amount of radiation exposure received. In medical imaging, the dose is typically expressed in millisieverts (mSv).


The amount of energy transferred due to the amount of radiation a person receives. The exposure and "dose" are roughly equivalent for medical radiation. A low level of exposure occurs through routine activities. Medical imaging procedures result in various levels of exposure to the patient. In addition, the clinician performing imaging exams can also receive radiation exposure.

Gray (Gy)

A unit of measurement of the absorbed dose of radiation. One Gy is equivalent to 1 joule of energy per kilogram of tissue. The average cumulative radiation absorbed dose from natural sources is estimated to be about 2-3 milliGy (or 0.002-0.003 Gy) per year for humans. The Gy replaces a former unit of measurement called the rad.

Medical radiation

Ionizing rays used to aid in diagnosis or therapy of medical conditions. Radiation is used in some medical imaging procedures such as computed tomography (CT) scans and nuclear medicine scans. Ultrasound examinations and magnetic resonance imaging (MRI) do not use radiation.


One one-millionth of a Sievert (see Sievert).

MilliCuries (mCi)

Indicates the amount of radioactivity that is present in a quantity of material.

Millirem (mrem)

One one-thousandth of a rem (see rem).

Millisievert (mSv)

One one-thousandth of a Sievert (see Sievert).


A measurement of absorbed dose. 1 rad is equal to .01 Gray (Gy).


A general term used for radiant energy that is emitted from radioactive elements, luminous bodies, radiographical tubes, and other items. (See medical radiation.)

Radiation Safety

Using sound practices to minimize radiation exposure when working with or being near radioactive material. These practices follow the ALARA principles - As Low As Reasonably Achievable - and involve increasing distance from the source of radiation, decreasing exposure time and using the right shielding, if appropriate.

A provider's commitment to patient safety should be reflected in its commitment to ALARA, and a strong, cross-disciplinary radiation safety team.

Rem (roentgen equivalent in man)

The traditional unit of radiation exposure. 100 rem = 1 Sv.

Sievert (Sv)

A unit of measurement of absorbed radiation, sometimes referred to as the effective dose. Radiation doses received during medical procedures or everyday activities are often measured in millisieverts (mSv), which is one one-thousandth of a Sievert. Radiographic procedures can range from 0.01-10 mSv, and computed tomography scans are typically 2-20 mSv. The annual dose of background radiation from being on earth is approximately 3 mSv/year, but may vary considerably depending on altitude and other environmental factors.

Tube potential (kV)

The use of optimal tube potential may improve image quality or reduce radiation dose. Its primary benefit is improved contrast enhancement, which may help offset an increase in noise. Decreasing tube potential essentially decreases the radiation dose.

Tube current (mA)

The flow of the electrical current or beam through the X-ray tube. Of several parameters that contribute to the radiation dose, tube current is one. Decreasing tube current essentially decreases the radiation dose.