The 15% Rule in Radiography (kVp impact to mAs)

The 15% Rule 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).

Select kVp First.

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 can use to change the mAs to compensate for changes in the kVp.

Where does the 15% Rule come from? 

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 remnant 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

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 tells us how we compensate the mAs for the change in kVp so that we can keep the x-ray exposure constant at the image receptor.

Why is it important to keep the detector exposure constant in X-ray imaging?

When you optimized a protocol on your system then keeping the exposure constant at the image receptor is important to maintain image quality. The image is dependent upon the exposure on the image receptor for all radiography acquisitions so keeping the exposure constant will help keep the image noise consistent, and the radiologists reading the exams will thank you. Also if you are using a film system then it is even more important to maintain a constant x-ray exposure on the film so that the density of the film is maintained, i.e. it is not overexposed or underexposed. 

The 15% Rule in x-ray radiography comes from the fact that (1.15)^5 ~2. 

This means that a 15% increase in the kVp will lead an exposure approximately 2 times higher at the image receptor (e.g. the detector or film). Therefore, if you raise the kVp by 15% then you need to reduce the mAs by a factor of two (i.e. mAs_new=mAs/2) to maintain the same exposure at the image receptor. 

Likewise, the 15% Rule in x-ray radiography comes from the fact that (1/1.15)^5 ~0.5. 

When you decrease the kVp by 15% the exposure incident on the image receptor goes down by a factor of two, i.e there is one half of the exposure.  When we want to compensate for this reduction in exposure we need to increase the mAs by a factor of two. Therefore, if you decrease the kVp by 15% then you need to increase the mAs by a factor of two (i.e. mAs_new=2*mAs).


Example Problem of the 15% Rule in Radiography.

If the kVp is changed from 80 to 92 and the product of the tube current and time was originally 50 mAs, what is the new mAs?

First calculate 10% of 80 which is 8, then divide that by 2 to get 5% of 80 (8/2=4). Then we add 8+4=12, which is 15% of 80.

Since 92 is 12 higher than 80 we see that the kVp has been increased by 15%.

Therefore, we need to divide the mAs by 2 to get the new mAs (50 mAs /2 = 25 mAs). Thus, the new value is 25mAs.

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