Which exposure factor is primarily used to control radiographic contrast?

The primary exposure factor that controls radiographic contrast is kilovolt peak (kVp). Contrast in radiography refers to the difference in density between various areas of the image, which is crucial for distinguishing between different tissues and structures.

kVp determines the energy of the x-ray photons produced. A higher kVp results in a greater penetrability of the x-rays, allowing them to pass through denser tissues more easily. This increased penetrability results in a wider range of exposures on the film or digital receptor, thus enhancing the tonal range and overall contrast of the image. Conversely, a lower kVp may lead to inadequate penetration, resulting in a loss of detail and poor contrast, as denser structures can appear similar in density.

On the other hand, milliampere-seconds (mAs) influences the overall exposure or the quantity of radiation reaching the receptor, which affects image density but not contrast directly. The exposure time can be part of the mAs factor but does not specifically control contrast. Lastly, focal spot size pertains more to image sharpness and detail than to contrast control.

Thus, kVp is the key factor in influencing radiographic contrast by affecting the energy and penetrability of the x-rays used during the imaging process.