How mAs and kVp affect IR Exposure

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Hi.

In response to your trail guide,

I've created a video that's gonna focus

on IR exposure in the 15% rule.

So, let's start off with IR exposure.

What are you really measuring here?

We're just measuring how many photons

interact with the image receptor.

So, that's key.

Let's draw an image receptor.

IR for image receptor.

So, when you think about exposure, once again,

we're thinking about how many photons

are actually striking this thing.

Now, let's put a body part in front of it.

And, let's think about the X-rays themselves.

So, here's our X-ray tube

pointing at the IR.

When we set what we call a "technique,"

we're really setting a combination

of two controlling factors;

our mAs, which again is our quantity

or number of photons we're gonna create,

and then our kVp, which is essential

because that's going to determine

how those X-rays behave, right?

So, we can't have one without the other.

First we decide how many X-rays we're gonna create,

and then we decide,

what kind of energy are we gonna give them?

And that's going to determine

how they penetrate through the tissues,

and how they interact with those tissues.

So, two very important parts of our "technique"

that we set as X-ray techs.

The thing to recognize too,

is you can have a certain amount of mAs

and a certain amount of kVp for one technique,

and then another tech can come along,

do the exact same exam on an exact

duplicate of the previous patient,

but use a different mAs and a different kVp,

and still achieve an adequate radiographic image.

So, there's no set prescribed

technique for each exam,

and each patient size or type of patient.

There's flexibility involved in the process.

That's a real important thing to recognize.

Our job as Radiographers,

is try to come up with an optimal technique,

and what does that look like?

What is an optimal technique?

You want to make sure you have enough kVp,

so that you have adequate penetration

of all the tissues involved.

For example, if you did a chest X-ray,

and you did not have enough kVp,

it wouldn't matter how much mAs you had

or quantity of photons.

If you weren't enabling those photons

to penetrate the tissues adequately,

you would wind up with a heart that would be,

maybe pure white, on the image

because the photons were absorbed in the tissue

and didn't actually penetrate through it,

strike the image receptor,

and therefore, demonstrate the areas around the heart

and kind of behind the heart.

So, kVp is just as important as mAs,

or the number of photons.

Now, let's look at that from a different perspective.

Let's say you have adequate kVp.

You have enough kVp that you're going to properly

penetrate the tissues just fine,

however, you must calculate it

on the quantity of photons to be produced.

So, your mAs was too low.

So, again, it doesn't matter if your kVp is just right.

If you don't have enough photons to go around,

then you will not capture all the anatomic detail

necessary for a good radiographic image.

So, both are important.

Let's take a quick look at how each can influence

IR exposure.

So, again, here's our IR, here's our patient.

Let's look at mAs first.

So, again, mAs is all about the quantity of X-rays.

When we say X-rays, we mean X-ray photons.

When we say X-ray beam, we're talking about

all the X-ray photons in that particular beam,

so we'll use these terms interchangeably.

For now, let's just say mAs is the quantity of X-rays.

Okay, so that's going to determine

how many X-rays you produce, okay.

And, let's just draw X-rays out here.

Now, of course we can't see X-rays,

but for this illustration,

we're gonna pretend they look like little, tiny circles.

So, this is what mAs controls;

your quantity of X-rays.

kVp is not up there.

It's gonna be right here where I write.

kVp controls

the energy

of the beam, or of the X-rays.

And remember, that it's that energy that determines

how they're gonna penetrate through the tissues.

If your X-rays

have a low energy,

then they're going to be attenuated more

or absorbed in the tissues more.

And we'll just draw that out.

They have a lower energy, which comes with low kVp,

then few of them are gonna make it all the way

through the body to the image receptor,

and more will be absorbed by the tissues.

If they have a higher energy, which comes with higher kVp,

and we can think of that wave form like this.

Those X-rays will be more likely

to penetrate through the tissues,

and reach the image receptor, okay.

So, when we set that technique of mAs and kVp,

because that's how you need to think

of the technique as a Radiographer.

When it's time do an exam

and it's time to set that technique,

you're thinking of kVp and mAs,

what am I gonna use?

You look at the size of the part,

and you wanna make sure that you have enough

photons to go around,

and then you also consider the size of a part,

and what type of part it is,

and you wanna make sure you have a kVp that's high enough

it's going to allow these X-ray photons

to penetrate the tissues adequately,

and reach the image receptor, okay.

So, you adjust your kVp

so that when these take off and go through the body,

enough of them, not all of them,

you don't want all of them to penetrate through the body

without any attenuation,

or your X-ray image would be solid black.

But you want enough of them

to penetrate through the tissues,

and reach the image receptor.

Remember, to make an X-ray image,

we have to have a combination of three things going on.

X-ray photons that make it all the way through the body

without any attenuation.

X-ray photons that scatter a little bit outside the body.

We want to prevent that remember, as much as possible,

but we can't get rid of scattering altogether.

Scatter is one of the reasons X-rays look the way look.

Kind of ghostly, and that black and white look

that is indicative of an X-ray.

You can recognize it in an instant.

You know what an X-ray looks like.

Well, in large part, that's because of scatter.

It's just part of the process.

We do want to keep it to a minimum.

And then, we want some of these photons

to be absorbed right in the patient's tissues.

Those are gonna be the areas that look white,

varying shades of white on the gray scale.

So, those three things, we want them to happen,

and we want them to happen in an optimal way

that's gonna produce a radiograph

that demonstrates the anatomy properly.

And again, if your kVp is too low,

not enough photons will reach the image receptor.

If your mAs is too low,

you won't have enough photons to go around

even if your kVp is high enough

to push them through to the image receptor,

there still won't be enough photons to

demonstrate the anatomy properly,

to demonstrate all the detail.

So, that's kind of how you have two parts

of your "technique" that are very important,

and both are able to contribute to image receptor exposure.

Now, as I said,

this isn't a black and white, set in stone process.

And that's why you're out in clinical for 16 months.

Because it really takes that long

to get a feel for how to use

both of these primary factors, kVp and mAs,

to make sure you get adequate IR exposure,

and a beautiful X-ray image.

Okay, more videos coming soon,

but try to answer these questions

before you watch the next videos.