Radiographic Exposure Factors

Radiographic exposure factors determine the visibility of the anatomic structures on an X-ray. They also affect patient radiation dose. Although most digital X-ray systems provide the ability to manipulate the amount of blackness or various shades of gray on an X-ray after it has already been processed, administering an acceptable amount of exposure to the patient remains a priority. The X-ray operator, rather than a computer, should be responsible for radiation exposure and film quality.

The Prime Factors

• Although there are some factors outside of an x-ray operator's control, three aspects of radiation emission usually can be adjusted by the operator. These include milliamperage-seconds, kilovoltage peak and the distance from the source of the X-rays to the device that captures the image. These factors dictate both the number of X-rays produced as well as their capacity to pass through the human body. This is often described as the quantity and quality of the X-ray beam.

Milliampere-Seconds

• The product of milliamperage and time, mAs is primarily responsible for the quantity of X-rays produced during an exposure. Milliamperage is an electrical term for the current, or the rate of electrons flowing through the tube per second. Time is the duration of the exposure. The quantity of X-rays that reach the film or image receptor determine the overall blackness or darkness of the image.

Kilovoltage Peak

• Kilovoltage peak controls the penetrating power and quality of the X-rays. X-rays that are unable to pass through the body are visualized as white areas on the image. It is normal for dense tissue structures, such as bone, to absorb more radiation. On the image, kVp controls contrast, or the difference between the light and dark areas of the X-ray. An X-ray beam produced with higher voltage will result in lower contrast, causing the image to possess many shades of gray.

Distance

• As distance decreases from the X-ray tube to the image receptor and patient, the intensity of the X-ray beam increases. This is similar to a light bulb seeming brighter when a person is near, whereas light intensity is diminished at an increased distance. Usually, distance is predetermined for any X-ray examination. However, certain circumstances may necessitate a modification in distance, and a compensation in mAs is typically used to account for the change.