Factors of Radiographic Exposure

Tuesday, July 24, 2018


Exposure Time

Exposure time is a measure of how long the exposure will continue and is measured in units of seconds, fractions of seconds, or milliseconds. Electronic timers provide a wide range of possible settings, allowing the operator to precisely control the length of exposure. Together with the milliamperage, exposure time determines the total quantity of radiation that will be produced. When a variation in the quantity of exposure is desired, the exposure time is varied. Because a longer exposure time results in the production of more xrays, when all other factors are equal, a longer exposure time will produce a darker radiographic image. A decrease in exposure time will result in less radiation exposure and a lighter image. Patient dose is directly proportional to exposure time.

Exposure time settings may vary from a short as 1 millisecond to as long as several seconds. Some units have AECs. These automatic exposure timers terminate the exposure when a specific quantity of radiation has reached the image receptor. Machine with AEC have special controls related to this process.

Milliamperage

Milliamperage (mA) is a measure of the current flow rate in the xray tube circuit. It determines the number of electron available to cross the tube and thus the rate at which xrays are produced. You can think of mA as an indication of the number of xray photons that will be produced per second. Thus the mA setting will determine how much time is required to produce a given amount of xray exposure. High mA settings are used to shorten the needed exposure time when motion during a longer exposure would like cause blurring of the radiographic image.

The number of possible mA settings is limited and is usually in whole number that are divisible by 50 or 100. For example, a typical radiographic unit may have the following mA settings: 50, 100, 200, 300, 400 and 500 mA. Some xray machine are capable of producing as much as 1000 or 1500 mA.

The relationship between mA and exposure time is simple. The product of mA and time is milliampereseconds (mAs), which is an indicator of the total quantity of radiation produced in the exposure. This relationship is presented by the mAs formula:

mA x Time (seconds) = mAs

Most control consoles today provide the option of setting the mAs directly, while older models usually require the operator to set mA and exposure time separately. The mAs settings for varous applications commonly range between 1 and 300.

Changing the mA has other effects as well. In dual focus tubes, specific mA stations control each filament. In general, mA settings of 150 or lower utilize the small filament and the small focal spot, while mA settings of 200 or higher are associated with the large filament and large focal spot. On controls that permits the operator to select the mA setting, each setting will have an indication of which focal spot is associated with it. Controls that provide mAs selection without specific mA settings will have a separate mean of selecting focal spot size.

In addition to varying the focal spot size, changes in mA will affect the amount of heat that accumulates in the anode during the exposure and will be a cause for concern when large exposures are required. As a rule, an xray tube can handle larger exposures when the desired mAs is obtained with a lower mA setting and a longer exposure time.

Kilovoltage

The kilovoltage or kilovoltage peak (kVp) is a measure of the potential difference across the xray tube and determines the speed of the electron in the electron stream. This determines the amount of kinetic energy each electron has when it collides with the target and therefore determines the amount of energy in the resulting xray beam. This energy is expressed by the wavelengths have more energy and are more penetrating than those with longer wavelengths. For this reason, an increase in kVp results in a more penetrating xray beam. This will cause more exposure to the image receptor, because a higher percentage of the xrays produced will pass through the patient and reach the IR. An increase in kVp will produce a darker image, while a decrease in kVp will produce a lighter image.

Changes in kilovoltage will also cause other changes to the image. Because the differential penetration of the xray beam will be affected by wavelength, the contrast of the image will also change. This means that the degree of difference between the darker and lighter areas of the image will be affected. Somewhere between no penetration and total penetration of the subject is the optimum amount of diffetential penetration that will show a contrast in exposure between the various features of the subject. The amount of kVp that produces optimum penetration varies with the examination.

Kilovoltage settings for typical radiographic units range between 40 kVp and 150 kVp in increments of 1 or 2 kilovolts. Low kVp settings are used for small body parts. For example, 50 to 60 kVp is commonly used for radiographic examinations of the hand, wrist, or foot. Spine radiography typically utilizes settings between 75 and 100 kVp, while settings above 100 kVp may be used for chest radiography and for studies of the digestive tract that employ barium sulfate as a contrast agent.

Distance

The distance between the source of the xray beam (the tube target) and the image receptor is referred to as the source-image distance (SID). This distance is a prime factor of exposure because it affect the intensity of the xray beam. Radiation intensity might be thought of as the number of photons per square inch striking the surface of the image receptor. Because the xray beam diverges from its source, the size of the beam expands as the distance from the source increases. As the total quantity of xray photons in the beam spread out, there are fewer photons in any given area.

Source-image distance affects radiation field size and intensity

The change in xray beam intensity that results from changes in the SID is expressed by the inverse square law, which states that the intensity of the radiation is inversely proportional to the square of the distance. The inverse square law is expressed mathematically in this equation:


You will note in the picture above that, as the distance is double, each dimension of the radiation filed is doubled; so the radiation field is four times greater in area. Therefore, the intensity, the number of photons per unit area within the field, is one fourth of the original amount. Likewise, if the distance were tripled, the field area would be one ninth of the original amount.

Of course, as the radiation intensity decrease, exposure to the image receptor will also decrease. In order to maintain the same optical density (degree of image darkness) when the SID changes, the mAs must be adjusted corresponding. The formula for this adjustment is:


As you learn later when you study xray technique calculations in more detail, this formula will enable you to maintain a given radiation intensity, and therefore a given radiographic appearance, when changing the SID. For example, this formula will result in a fourfold increase in mAs compensates for the reduction in radiation intensity that occurs with the SID increase, with the result that the radiation intensity is unchanged.

Technique Charts

A technique chart located near the control console usually provides the radiographer with a listing of recommended mAs and kVp settings, as well as the SID, for each of the various body parts for different sizes of patients. Some control consoles have “anatomical programming.” These computerized units are preprogrammed with the required exposure settings for the selected body part and size.

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