Peak kilovoltage
Encyclopedia
Peak kilovoltage is the maximum voltage
applied across an X-ray tube
. It determines the kinetic energy
of the electron
s 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. In order to image the body so that the maximum information will result, higher subject contrast areas require a higher kVp so as to result in a low radiographic contrast image and vice versa.
Although mAs is the primary controlling factor of radiographic density, kVp also affects the radiographic density in a round about way. As the energy (kVp) of the stream of electrons in the x-ray tube increases, the more likely the x-ray photons created from those electrons will penetrate the cells of the body and reach the image receptor (film or plate), resulting in increased radiographic density (compared to lower energy beams that may be absorbed in the body on their way to the Image Receptor). However, "scatter radiation" also contributes to increased radiographic density; in that, the higher the kVp of the beam, the more scatter will be produced. Scatter is unwanted density (that is, density created without bringing any pertinent information to the image receptor). This is why kVp is not primarily used to control density - as the density resulting from increasing kVp passes what is needed to penetrate a part, it will only add useless information to the image.
Increasing mAs causes more photons (radiation)of the particular kVp energy, to be produced. This is helpful when larger parts are imaged, because they require more photons. The more photons you can get to pass through a particular tissue type (whose kVp is interacting at the cellular level) will result in a statistically increased amount of photons reaching the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information - the more useful the density is created on the resulting image. Conversely, lower mAs creates less photons, which will decrease density, but is helpful when you image smaller parts.
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
applied across an X-ray tube
X-ray tube
An X-ray tube is a vacuum tube that produces X-rays. They are used in X-ray machines. X-rays are part of the electromagnetic spectrum, an ionizing radiation with wavelengths shorter than ultraviolet light...
. It determines the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
. 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. In order to image the body so that the maximum information will result, higher subject contrast areas require a higher kVp so as to result in a low radiographic contrast image and vice versa.
Although mAs is the primary controlling factor of radiographic density, kVp also affects the radiographic density in a round about way. As the energy (kVp) of the stream of electrons in the x-ray tube increases, the more likely the x-ray photons created from those electrons will penetrate the cells of the body and reach the image receptor (film or plate), resulting in increased radiographic density (compared to lower energy beams that may be absorbed in the body on their way to the Image Receptor). However, "scatter radiation" also contributes to increased radiographic density; in that, the higher the kVp of the beam, the more scatter will be produced. Scatter is unwanted density (that is, density created without bringing any pertinent information to the image receptor). This is why kVp is not primarily used to control density - as the density resulting from increasing kVp passes what is needed to penetrate a part, it will only add useless information to the image.
Increasing mAs causes more photons (radiation)of the particular kVp energy, to be produced. This is helpful when larger parts are imaged, because they require more photons. The more photons you can get to pass through a particular tissue type (whose kVp is interacting at the cellular level) will result in a statistically increased amount of photons reaching the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information - the more useful the density is created on the resulting image. Conversely, lower mAs creates less photons, which will decrease density, but is helpful when you image smaller parts.