How can I understand the 15% Rule in Radiology? For a given number of x-rays incident on the patient the x-rays are much more likely to penetrate the patient and be measured on the detector as the kVp is increased. For an average patient and x-ray spectrum the number of x-rays coming out of the patient is roughly proportional to kVp^3. In addition to more x-ray photons being incident on the patient for higher kVp settings there is also a change in the x-ray penetration of the beam due to the kVp change, and hence a change in the remnant x-ray beam. How does the kVp influence the X-ray Penetration and the Remnant beam? The number of x-rays incident on the patient is the first of two major factors which are responsible for the 15% Rule. This is more strong than the mA or the time (s) which are both linearly proportional to the number of x-rays. The number of x-rays is proportional to kVp^2. The kVp changes the penetration and the contrast of the incoming x-ray beam, but also changes the number of x-ray photons that are incident on the patient. How does the kVp influence the incoming x-ray beam? There are two separate reasons why changes in kVp impact the number of x-rays measured at the image receptor.
Changes in the kVp are much more impactful than changes to the mAs to the number of x-rays that make it through the patient to be measured on the image receptor (i.e. The kVp is a very important technical parameter as it is the most direct way to influence the beam quality on x-ray systems. The 15% Rule is an approximation that Rad Techs can use to change the mAs to compensate for changes in the kVp. In some scenarios it is desirable to lower the kVp for increased contrast or to raise the kVp for increased penetration. Typically the kVp is selected first in an x-ray protocol as it influences the contrast and penetration of the x-ray beam (i.e.
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