Check Your Physics
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December ) (Learn how and when to remove this template message). Peak kilovoltage (kVp) refers to the maximum high voltage applied across an X-ray tube during kVp controls the property called "radiographic contrast" of an x-ray image. kVp & mAs DEFINITIONS. kVp: Kilovolt Peak. kVp is the component that controls the QUALITY of the x-ray beam produced. It is also what controls the. mAs and kVp Relationship. Course: kVp that will be able to penetrate the body part, produce sufficient radiographic contrast and acceptable level of scattered radiation kVp is for tissue density/atomic number changes . Help Center.
A volt is defined as a unit of electrical force needed to move one ampere through a resistance of one ohm. Kilo means 1, so kilovoltage refers to a thousands volts.
The "p" stands for peak, therefore, kVp means kilovoltage peak. Peak denotes the highest voltage attained in a given electrical alternating current.
The kVp is the factor which controls the energy of the electrons as they move across the tube, or the speed of the electrons. It follows that higher energy electrons produce higher energy x-rays.Gray Scale of Contrast
Scientifically speaking, high energy photons or x-rays mean short wavelength photons. The shorter the wavelength, the higher the energy, for the wavelength and the easier it becomes to pass through matter. This is referred to as penetration.
kVp-mAs contrast relationship | Science | ShowMe
The greater the kVp, the greater the penetration. By controlling the energy of the x-ray beam, kVp controls the quality of the beam. Unfortunately, kVp is not as easy to work with clinically as far as controlling the beam intensity. While both mAs and kVp directly affect beam intensity, kVp has a direct and exponential relationship. If we wanted to double the intensity using kVp, we would not double the kVp but increase it by an amount determined by an equation in which beam intensity varies approximately according to the 2.
You'll need a chart to figure it out, so it is much easier to use mAs to change the density since it has a linear relationship. Now let's talk about film density and film contrast.
First, let's discuss film density. For this discussion, we are going to define film density as blackness of the film. The blacker the film, the more density it has. Both kVp and mAs have a direct affect on film density.
The mAs have a linear relationship to film density; kVP has an exponential relationship. It is hard to predict accurately and easily how much change the kVp will affect the change in density.
With mAs, the relationship is linear. For example, if we take a radiograph and feel that the contrast is adequate, but the density is about half what it should be, then we can easily correct this by doubling the mAs.
Effect of mAs and kVp on resolution and on image contrast.
If we define density as the blackness of the film, contrast is defined as the difference between the blackest part and the whitest part of the film. A film that is of high contrast is one in which parts of the film are either white or black, with very few shades in between.
A low contrast film is one in which there is not that great a difference between the black parts and white parts and there are many shades of gray.
The difference between how black the black is and how white the white is, is termed contrast. The number of shades in between the blackest black and the whitest white is termed scale. If there are many shades between black and white, it is termed a long scale; if there are fewer shades between black and white, it is termed a short scale.
Therefore, a high contrast film is one with a short scale, and a low contrast film is one with a long scale.
Film density is affected by both mAs and kVp, but film contrast is also affected, mainly by kVp. 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. As the energy which is proportional to the peak voltage of the stream of electrons in the x-ray tube increases, the x-ray photons created from those electrons are more likely to penetrate the cells of the body and reach the image receptor film or plateresulting in increased film density compared to lower energy beams that may be absorbed in the body on their way to the image receptor.
However, scattered x-rays also contribute to increased film density: Scatter adds unwanted density that is, density that does not bring pertinent information to the image receptor. This is why kVp is not primarily used to control film density — as the density resulting from increasing kVp exceeds what is needed to penetrate a body part, it only adds useless photons to the image.
Increasing mAs causes more photons radiation of the particular kVp energy, to be produced. This is helpful when larger body parts are imaged, because they require more photons.
The more photons that pass through a particular tissue type whose kVp is interacting at the cellular levelthe more photons reach the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information - the more useful the film density on the resulting image. Conversely, lower mAs creates fewer photons, which will decrease film density, but is helpful when you image smaller parts.