What is kvp

Photon quantity output is approximately proportional to a power function, kVp n , where n is approximately 2 in radiography and 2. In reality, so as to not increase dose unnecessarily, the mAs setting is typically adjusted down to compensate for the increased photon quantity caused by increasing kVp.

Modern digital imaging systems achieve this through automated methods. The ability to reduce mAs by a fifth-power relation is more than adequate to balance the second to third power direct effect of increasing kVp on photon quantity. The difference is related to how the probability of energy absorption in various biologic tissues declines dramatically over the range of photon energies used for diagnostic imaging 9. Thus, absorbed dose can be reduced while keeping the image receptor exposure constant by choosing a higher kVp and lower mAs.

As this reduction is achieved with higher energy photons, however, the tradeoff is lower image contrast. Lower kVp techniques may be employed at lower dose when contrast is more important e. CT angiography of the abdomen 10 or when noise related to attenuation is less of a problem e. Please Note: You can also scroll through stacks with your mouse wheel or the keyboard arrow keys. Updating… Please wait.

Unable to process the form. Check for errors and try again. Thank you for updating your details. Log In. Sign Up. Become a Gold Supporter and see no ads. Log in Sign up. Articles Cases Courses Quiz. About Recent Edits Go ad-free. Edit article. Chest newborn. Chest 1 year old. Chest 6 year old. Lateral decubitus. KUB 2 year old. Half Value Layer. The penetrating ability of an x-ray beam quality is dependent on the kVp selected for the exposure.

The material generally used to determine half value layer is aluminum. Only a thin layer of a dense material, such as aluminum, lead, barium or iodine, would reduce the half value layer. A thicker layer of less dense material wood, glass, paper, cardboard, etc. In a modern imaging department, the half-value layer has two important applications. A diagnostic x-ray beam produces a wide range of energies.

Although we only mention the maximum energy of the beam for example, we say we are using 80 kVp for an exposure , the beam is made up of photons 80 kVp and lower. We don't want lower energy photons in our beam- we really just want those with an energy of There is no way to get a perfect x-ray beam with every photon possessing 80 kVp.

However, the half-value layer helps filter out low energy photons less than 80 kVp in our example. Basically, if the half-value layer for a given x-ray beam is low thin piece of aluminum filtration , then the x-ray beam contains more low energy photons that are less than 80 kVp.

They also have less penetrating power because of their lower energy. If the half-value layer is high thick aluminum added , the x-ray beam contains more high energy or highly penetrating radiation because the lower energy photons could not penetrate through the thick added aluminum. This is important because x rays used for medical x-ray must have enough energy to penetrate the body part of interest and expose the film sufficiently.

Lower-energy radiation is absorbed into the patient's tissues or scattered by the body and may not reach the film, contributing nothing useful to the image. Adding more filtration to the beam, which is typically done by the manufacturer of the unit prior to installation, will remove the undesirable low-energy x rays while allowing the desirable higher-energy x rays to pass through the patient to the film.

On the other hand, if there is too much filtration in the beam, there is a loss of contrast in the x-ray image differential absorption is reduced. This is why a physicist evaluates all x-ray equipment on a regular basis, typically once a year, and measure half-value layer as part of that testing. A second application of half-value layer in an imaging department has to do with room shielding. Rooms that contain x-ray equipment are typically shielded with lead-lined walls to reduce the radiation exposure to workers and the public from the use of x rays within the department.

When designing the shielding for a room, the physicist will perform calculations based on the half-value layer of the x-ray beam. In general, the design will call for enough half-value layers of shielding to reduce radiation exposure outside the room to acceptable levels. To produce an optimal image, a certain amount of contrast is necessary so that differences in various tissues can be distinguished, along with abnormalities or pathologic processes.

This is referred to as short-scale contrast, or narrow contrast latitude. The image looks "flat". This is referred to as long-scale contrast, or wide contrast latitude. Images below: Top row: compares the contrast differences in three black and white images. Bottom row: compares contrast differences in three color pictures. Low contrast long scale. Moderate contrast. High contrast short scale. High Contrast short-scale. In the images above, differences between high and low contrast can easily be seen.

The density darkness was never changed- only the contrast was adjusted in each picture. Below: In the x-ray images of a finger, differences in contrast can easily be seen. The differences between light and dark areas on the film are great. There are very few shades of gray. The image looks flat and gray. There are few differences in the lightest and darkest areas of this image.

Low contrast images are usually the result of using higher kVp settings or due to thick, dense body tissues. There are several types of contrast which influence the radiographic image: Radiographic contrast is defined as the overall differences in optical densities seen in the radiographic image. Radiographic contrast enables us to see fine details within the image clearly. This is referred to as subject contrast.

For example, a radiographic image obtained on a very heavy patient will have less subject contrast than a patient who is thin because adipose tissue decreases subject contrast significantly. There are four factors which together determine the overall amount of subject contrast in an image:. The length of the actual x-ray exposure can be set by the technologist on most x-ray consoles.

Time plays an important role in producing a quality image. Time directly affects the density of a film because it determines how long the image will be exposed. An exposure time of 10 milliseconds. It has no effect on the strength penetrating power of the x-ray photons. In this example, mA multiplied by 0. Calculating mAs. Time in sec. There are many combinations of mA and time that, when multiplied together, equal the same mAs.

For example, all of the following combinations of mA and time equal mAs. Time selected. Any of the above exposures will produce the exact same density on the resultant x-ray film. Faster exposure times are needed if the patient is having trouble holding still, as in the case of small children. When using a short exposure time and a high mA setting, be sure to refer to the tube rating chart to verify that the tube will be able to stand up to the extreme temperatures generated by such an exposure.

Regardless of which of the five possible combinations is selected in the example above, each will produce exactly the same number quantity of x-ray photons. An image produced by any of these combinations will have the same radiographic density amount of image blackening. Rules About mAs. If you just obtained an image of a hip, using 20 mAs, and the film is too light, increasing the mAs will make the film darker.

If the image was very light, you may need to double the mAs 40 mAs from the original exposure. Remember that any change in kVP affects image density, but primarily it changes image contrast. The primary role of kVp is to control strength and power of the x-ray beam and control contrast. Whenever possible, mAs should be used to increase or decrease the overall density darkness of an image- not kVp.

When adjusting technique settings, initially only one or the other kVp or mAs should be adjusted rather than making changes in both. The background of an under penetrated image and some tissues will have acceptable densities, but the area of interest for example, a bone will look washed out with little or no detail. In such an image, the x-rays were so strong that most penetrated right through the patient, and no differential absorption took place.

Reduce kVp. Changing Density of an Image. When taking a repeat film for over or under exposure, the minimum amount of change in technique should be as follows: - At least double the mAs for an image that is being repeated because the film was too light - At least cut in half the mAs for a film being repeated because the film was too dark.

Quantum Mottle. This increases the chance of a passing incident x-ray to have an interaction with the crystal. A larger crystal will give off more light than a small crystal, thereby darkening a greater are of film. Using low mAs for an exposure may also lead to quantum mottle. If mAs is set too low, there are not enough x-ray photons in the beam to interact with a sufficient number of crystals.

It is similar to comparing a light rain to a big thunderstorm. Distance from x-ray tube to film source to image distance, or SID is one of the key components in producing a quality x-ray. All exposures were performed using phototiming.

This image was generated at 60 kV, and required a relatively high radiation intensity of mAs. The L number generated by this imaging system was 2.

The skin dose for this examination was estimated to be 7. The image shown above was generated at 75 kV and required an exposure of 36 mAs. The increase in x-ray tube voltage increases the amount of radiation coming out of the x-ray tube, as well as the average photon energy i. Accordingly, the tube current exposure time product value mAs is reduced to 36 mAs; whereas at 60 kV, the value was much higher mAs.