Kilovoltage Peak (kVp or Kv) is the maximum voltage applied across an X-ray tube. It determines the kinetic energy of the electrons accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum. The actual voltage across the tube may fluctuate. kVp controls the resulting photographic property known as "radiographic contrast" of an x-ray image (the amount of difference between the black/whites). Each body part contains a certain type of cellular composition which requires an x-ray beam with a certain kVp to penetrate it. The body part is said to have "subject contrast" (that is, different cellular make up: some dense, some not so dense tissues all within a specific body part). For example: bone to muscle to air ratios in the abdomen differ from that of the chest area. So the subject contrast is said to be higher in the chest than in the abdomen.
For example: maximum energy (keV) numerically equals the maximum tube voltage (kVp). Basically, 100keV [can range between 0 to 100kVp] or equal 100kVp. If a radiological device reads the maximum tube voltage as 100 kVp, its understood that maximum energy of the photons could be 100 keV. Thererfore, the average energy of the x-rays will be approximately 1/3rd of the maximum energy 100 keV which equals 33keV.
Factors Affecting the X-Ray Emission Spectrum
kVp is the "Quality of X-Rays" - radiation quality, the penetration power of an X-ray beam which is determined by the kVp and employed filtration. The penetrating power may be measured by the half value layer measured in milil roentegen (mR) units.
Effect of kVp
A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum. When you change the kVp the quantity and quality of the x-rays increases. A few things to notice about the diagram below are: 1. the peak (highest part) of the graph moves to the right as kVp goes up, (quality goes up). 2. the peak goes up, (quantity goes up). 3. the characteristic x-rays do not occur at kVp lower than 70 keV.
A change in kVp has no effect on the position of the discrete x-ray emission spectrum. In the diagnostic range, a 15% increase in kVp is equivalent to doubling the mAs. This is the 15% rule that you need to know for x-rays.