Peak kilovoltage - Wikipedia
mA Patient dose is also directly proportional to the mA with exposure at a given kVp, we can divide the exposure by the mAs to get the mR/ mAs. . Used to describe the relationship of radiation. Milliampere-seconds more commonly known as mAs is a measure of radiation all other factors (mA, focal spot, SID, kVp) to reduce time to avoid motion blur. X-ray intensity increases dramatically with increasing accelerating potential. dose rate varies directly with energy fluence rate, another factor, the mass More complex relationships exist, but they will not be discussed here.
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.
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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.
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. One could go as far as to say clinically that if we wish to affect the contrast, we should only be concerned with the kVp.
The higher the kVp, the lower the contrast, but the longer the scale of contrast. For example, PA chest views are taken at kVp to demonstrate subtle differences between the soft tissues of the lungs.
A thoracic view is taken at 80 kVp to shorten the scale of contrast and demonstrate the osseous structures. Suppose we have taken a radiograph and wish to change the factors but maintain the same beam intensity.
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- Effect of mAs and kVp on resolution and on image contrast.
- Peak kilovoltage
This rule states that to maintain the same beam intensity, we can: With the first change in technique, increasing the kVp and decreasing the mAs, we will decrease the contrast and lengthen the scale of contrast; in the second, we will increase the contrast and shorten the scale of contrast while keeping the same film density. That's enough physics for today. I hope this little review helps you take the quality of films you want.
Just to summarize what we have been discussing: Physics and Technology in Routine Radiographs. National College of Chiropractic, An Introduction to the Physics of Diagnostic Radiology. 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 the creation of x-rays within it. During x-ray generation, surface electrons are released from a heated cathode by thermionic emission.
The applied voltage kV accelerates these electrons toward an anode target, ultimately producing x-rays when the electrons are stopped in the anode. Thus, the kVp corresponds to the highest kinetic energy of the electrons striking the target, and is proportional to the maximum energy of the resulting X-ray emission spectrum.
One standard way to measure pulsating DC is its peak amplitudehence kVp. Most modern X-ray generators apply a constant potential across the x-ray tube; in such systems, the kVp and the steady-state kV are identical. Each body part contains a certain type of cellular composition which requires an x-ray beam with a certain kVp to penetrate it.
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The body part is said to have "subject contrast" that is, different cellular make up: 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.