The x-ray beam is heterogeneous which means that it is made of several different wavelengths. Higher kVp has a shorter wavelength and a higher frequency. The half value layer (HVL) also increases with an increase in kVp. Kilovoltage is defined as a measure of the maximum electrical potential across an x-ray tube, expressed in kilovolts. Kilovoltage controls the penetrability of the x-ray beam. Kilovoltage affects tube current. As kVp increases, electron kinetic energy is increased and electrons accelerate across the x-ray tube. Using higher kVp allows you to use less mAs, which decreases tube load and therefore decreases tube heat.
The x-ray beam is heterogeneous which means that it is made of several different wavelengths. Higher kVp has a shorter wavelength and a higher frequency. The half value layer (HVL) also increases with an increase in kVp.
Contrast and density are affected by kVp. Contrast decreases with increased kVp due to an increase in x-ray energy, penetration, and scatter radiation. A low kVp technique results in a high contrast, short gray scale image while a high kVp results in a low contrast, long gray scale image. On the other hand, density increases with increased kVp due to an increase in x-ray energy, penetration, and scatter radiation. As more photons reach the film, image density is increased.
Occasionally after a radiographic exposure, an adjustment (±) must be made to the previously used technical factors. At higher kilovoltage a difference of one kVp will make little if any difference radiographically; however, at lower kilovoltage, an error of a few kVp can make a very noticeable difference radiographically. A high kVp technique results in wide latitude whereas a low kVp technique results in narrow latitude.
A fifteen percent change in the kVp setting will double or half the density, just like doubling or halving the mass; however, contrast will also be affected. The table below can be used to compare changes in kVp to changes to mAs regarding image density.
As an x-ray photon passes through a patient, it will undergo total absorption, total penetration, or partial absorption. Total absorption occurs with low kVp techniques. Total absorption (photoelectric absorption) is caused by x-ray photons interacting with inner shell electrons of dense body tissues. An increase in mAs is required to achieve proper density. Total penetration occurs with high kVp techniques. In this instance, photons pass through tissue without interacting. A decrease in mAs is required to achieve proper density. Partial absorption is caused by x-ray photons interacting with outer shell electrons resulting in scattered radiation.