The 15% Rule is fairly straight forward. Generally, if you want to change contrast of your image, you alter the kVp. If you want to make an image lighter or darker, you change the mAs. kVp, however, can be used to alter radiographic density (darkness) as well. We know that doubling the mAs doubles the number of x-rays (quantity) being produced, which doubles the density of the image. If you increase your kVp without changing anything else, you can also increase your radiographic density (it will also alter the contrast of an image, but that is unimportant in this explanation). This is because as you increase the kVp, you increase the quality (overall energy of the beam). When you increase the energy of the x-ray beam, you increase its ability to penetrate the body. This means more x-rays can make it through the body and onto the image receptor. Beam (a) could be 40mAs and 50 kVp and beam (b) could be 40 mAs and 100 kVp. Both beams contain the same number of x-ray photons, but more x-rays will make it to the image receptor in beam (b) due to the higher energy.
The 15% rule states increasing or decreasing the kVp by 15% is equiavalent to doubling or halving the mAs when observing radiographic density. Again, this does not mean they will appear the same. Increasing the kVp will lower the overall contrast of the image, making it appear more grey. They will, however, have the same radiographic density (overall darkness).
Example:
A radiograph of the hand is too dark and the contrast is too low. If you were using 60 kVp and 4 mAs, what would your new mAs be if you wanted to lighten the image while increasing contrast? All you do is find out what 15% of 60 kVp is, and since you want the image to be lighter, subtract that 15% from the original kVp.
60 kVp * 15% = 60 kVp * 0.15 = 9 kVp
60 kVp - 9 kVp = 51 kVp
Decreasing kVp from 60 to 51 will reduce radiographic density by half and increase contrast (more black and white)
You may have a situation where your image will have proper radiographic density, but still does not look right because the contrast is not correct. This can happen when imaging large patients. You might have the correct number of x-rays being produced (mAs), but don't have sufficient penetration ability in the x-ray beam. In this case you could increase your kVp by 15%, cut your mAs in half, and have an x-ray beam that will penetrate the patient while still maintaining the same radiographic density. For instance:
An abdomen x-ray is taken at 70 kVp and 40 mAs. What would you do to increase penetration of the x-ray beam while still maintaining radiographic density.
70 kVp * 15% = 70 kVp * 0.15 = 10.5 kVp (we'll round it up to 11)
70 kVp + 11 kVp = 81 kVp
Since we want to maintain radiographic density, we must reduce our mAs by half to 20 mAs.
Our new technique is 81 kVp and 20 mAs. Essentially:
70 kVp and 40 mAs will equal the density of 81 kVp and 20 mAs.
This leads to an important concept to minimize patient dose. All radiological exams have a kVp range that you should be at for that specific exam. You should use the highest kVp and lowest mAs to minimize patient dose. In the above example, using 81 kVp and 20 mAs on this patient will adminster much less dose than 70 kVp and 40 mAs.
We've learned another concept that qualifies under ALARA!