Technique charts are based on part thickness. A caliper is used to measure the body part and from the measurement appropriate technical factors are chosen. Proper measurement is required for correct utilization of technique charts. Technique charts are based on part thickness. A caliper is used to measure the body part and from the measurement appropriate technical factors are chosen. Proper measurement is required for correct utilization of technique charts.
With a fixed kVp technique chart an optimal kVp is determined for each body part based upon contrast and penetrating quality; mAs is then selected to achieve optimal density. A fixed kVp technique chart is commonly used with automatic exposure control (AEC) units. A variable kVp technique chart is one in which the mAs is the fixed technical factor and the kVp is chosen (in 2 cm increments) based upon part thickness as determined by a caliper measurement. Several disadvantages of using a variable kVp technique chart include inconsistent contrast and density on the radiographic image and increased scatter radiation production.
A normal body is composed of 62% water, 15% fat, 23% bone, and 40% muscle. When choosing technical factors it is important to understand the radiation absorption hierarchy. In the body, air is the least absorbing of radiation. Fat is a little more absorbing of radiation followed by muscle. Finally, bone is the most absorbing of radiation.
The atomic number of tissues contribute to radiographic contrast. The degree of muscle development and any pathologic processes also contribute to radiographic contrast. Pathologies can either reduce or increase tissue thickness and therefore affect contrast. As patient thickness increases, contrast decreases (caused by increased scatter).
Tissue density and radiographic density is inversely related. As tissue density decreases, radiographic density increases. Age, body habitus, and pathologic processes can all affect tissue density. As people age, the body goes through several changes. Decreased water content causes an increase in subject contrast; decreased muscle mass causes an increase in subject contrast; and decreased calcification of bones causes a decrease in subject contrast. Body types (hypersthenic, sthenic, hyposthenic, asthenic) have different densities which affect radiographic density. Pathologies can either reduce or increase tissue density and therefore require an adjustment in technical factors.
Taking a good history is essential for technologists to determine technical factors. Technologists must also have a good understanding of how pathologies affect attenuation. Some examples of pathologies that result in an increased attenuation and require an increase in technical factors―usually kVp―include:
Some examples of pathologies that result in an decreased attenuation and require a decrease in technical factors―usually mAs―include:
Orthopedic patents and Emergency Department patients wearing casts can create a challenge when choosing technical factors. Plaster casts require an increase of about 5% to 10% in technical factors to penetrate the dense plaster material. If the plaster cast is wet, a 15% increase in technical factors is required. Fiberglass casts do not require an increase in technical factors. Because contrast media attenuates the x-ray beam, an increase in technical factors is also required when they are used. Therefore, procedures that utilize contrast media (barium, iodinated contrast) also require an increase in technical factors.