kV, mAs, and density

One of the more difficult topics for first year students is the correlation between kV and density. Once you think you have this concept down, including the 15% rule and the subsequent lab experiments, this topic gets revisited a number of times throughout the entire x-ray program, and some very good questions about kV, mAs, and density typically arise. For instance:

Does the number of photons increase as kV increases?

Well, yes... let me explain first by clarifying that the number of electrons produced at the cathode does not increase - that is controlled only by mA while the duration of production is controlled by the time.

Now, let's say I have a technique of 65 kV and 10 mAs for a knee x-ray. A certain number of electrons are converted to x-ray photons at the anode during that exposure, and then a certain number of primary photons are converted to secondary and tertiary photons and so forth when they interact with the patient until one of two things will happen to all photons:

1 - they will leave the patient as scatter or expose the film
2 - they will lose potential difference and become absorbed in the patient

Now let's focus on the photons in the latter category... when we increase kV, we know that more photons reach the film because they have increased energy to penetrate the patient, but something else happens. There will still be a percentage of photons that will be absorbed in the patient, but it will not be as high of a percentage as the 60 kV exposure. You will now, at 75 kV for instance, have more energy even in the photons that are absorbed, to ionize tissue before those photons deposit all energy into tissue.

To clarify, let's say we have a characteristic interaction between a primary x-ray photon and an atom of carbon in the patient. We'll also say that this photon carried 75 kV of potential difference. The binding energy of the k shell for carbon is .28 keV, so we're left with a secondary x-ray photon of 74.72 keV with the ability to produce the same reaction approximately 267 more times (75/.28) before it is absorbed and each reaction produces more photons that will be absorbed. If the same series of reactions occurred with the 65 kV exposure, then you would only have a possible 232 of these identical interactions before the photon's energy is absorbed.

Keep in mind that this is only one example of a photon's interaction with matter, and there is always that randomness applied to how they react. If you haven't studied compton, photoelectric, or characteristic interactions yet, don't feel bad if you didn't understand the last paragraph. Just remember that when a photon interacts with matter, other photons are typically produced, or electrons are ejected that can ionize adjacent atoms as well, and increasing kV will increase the number of interactions that occur before the photons are absorbed.

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