15% Rule Radiology (mAs compensation for kVp changes for Rad Techs)

The 15% is a useful approximation for Radiologic Technologists / Radiographers to adjust the mAs when changes to the kVp are desired in the x-ray protocol. The 15% Rule states: when the kVp is lowered by 15% the mAs needs to be increased by a factor of 2, and when the kVp is increased by 15% the mAs needs to be multiplied by 0.5 (i.e. divided by 2).

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. the beam quality). In some scenarios it is desirable to lower the kVp for increased contrast or to raise the kVp for increased penetration. The 15% Rule is an approximation that Rad Techs (Radiologic Technologists) can use to change the mAs to compensate for changes in the kVp.

For more information on physics for x-ray technologists see:
https://howradiologyworks.com.

Where does the 15% Rule in Radiology come from and how can x-ray Radiologic Technologist use it clinically?

The kVp is a very important technical parameter as it is the most direct way to influence the beam quality on x-ray systems. 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 remant beam). There are two separate reasons why changes in kVp impact the number of x-rays measured at the image receptor.

How does the kVp influence the incoming x-ray beam?
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. The number of x-rays is proportional to kVp^2. 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 incident on the patient is the first of two major factors which are responsible for the 15% Rule.

How does the kVp influence the X-ray Penetration and the Remnant beam?
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. For an average patient and x-ray spectrum the number of x-rays coming out of the patient is roughly proportional to kVp^3. 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.

How can I understand the 15% Rule in Radiology?
The two major components of the 15% Rule are:
1) The number of x-ray photons incident on the patient is approximately ~kVp^2
2) The likelihood of the x-rays to pass through the patient and be measured on the image receptor is approx. ~kVp^3

Therefore for x-ray imaging the number of photons measured at the image receptor is proportional to kVp^5.

Even physics folks don’t like to raise things to the 5th power in their heads. Therefore, we need an approximation that we can use so that we can have an understanding on how the number of photons at the detector is changing as a function of kVp, and most importantly how we can compensate by changing the mAs.

The 15% Rule in x-ray radiography comes from the fact that (1.15)^5 ~2. Therefore, a 15% change in the kVp will lead to approximately twice the

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