Transcription
In our previous two talks, we've looked at x-ray production at the anode. We've seen that when we accelerate electrons towards the anode, and those electrons collide with our target material in the anode, they either produce heat or x-ray radiation.
And of that x-ray radiation production, there are two separate mechanisms that produce x-rays. The first being Bremsstrahlung, or breaking radiation, which accounts for the majority of photons within our x-ray beam. And the second being characteristic radiation, where x-rays are released in discrete energy levels that are dependent on our target material.
Now, the combination of Bremsstrahlung and characteristic radiation forms what is known as our x-ray spectrum, which you can see here. Now, if we all radiology exams, you need to be intimately familiar with the x-ray spectrum. And perhaps most importantly, we need to note that this is a spectrum. It's not a monoenergetic beam. We've got heterogeneous energy levels here, a vast spectrum of different energy levels.
Now, later on in this course, we are going to look at x-ray attenuation in tissues. We're going to use something called the linear attenuation coefficient, as well as the half-value layer of a material, to calculate the penetrability of an x-ray beam. Now, when we do those calculations, we are going to make the false assumption that our x-ray spectrum is a monoenergetic beam. And it's good to keep in your back of your mind that the x-ray spectrum is actually a spectrum. It's not a monoenergetic beam.
Now, there are two separate axes to this graph. The first being photon number here, or on our y-axis. And the second being photon energy on our x-axis. Now, when we look at the x-ray spectrum, we often refer to the x-ray spectrum as having two separate properties. The first being quality, and the second being quantity, both of which I want to cover briefly today.
Now, quality refers to the x-axis here, the photon energy. And quantity refers to the y-axis, the photon number. So, what exactly is quality and quantity of our x-ray beam?
Now, before we get into these, we can manipulate multiple factors when we are creating x-rays. We can manipulate our kVp, our filament current, and exposure time. We can change the target material, or the type of waveform that is powering our x-ray machine. And all of these changes will have an impact on our x-ray spectrum. And this is a common question in exams. It comes up over and over again: how does manipulating one of those factors affect our x-ray spectrum? And when we talk about how it affects the x-ray spectrum, we generally talk about it in two different ways: x-ray beam quality and x-ray beam quantity.
So, x-ray beam quality is the ability of an x-ray beam to penetrate a tissue. And we use what is known as a half-value layer as a proxy for x-ray beam penetration. Now, that refers to the energy of that x-ray beam. And when we look at an x-ray spectrum like this, we will average out those energies and assign a value to this specific x-ray spectrum. And we call that value the x-ray beam quality, the average energy of that spectrum here. So, this green line here represents the quality of this specific x-ray beam spectrum.
Now, when we refer to x-ray beam quantity, we are talking about the number of photons within the x-ray spectrum. So, it goes without saying that the number of photons here is the area under the curve. Our y-axis is our photon number. The area under this curve represents the x-ray beam quantity, the number of photons within that x-ray beam.
Now, we can manipulate the number of photons within the x-ray beam by changing the number of electrons that we accelerate towards our anode, by changing our target material. A higher atomic number will give us more x-rays. And we can expose the anode to electrons for a longer period of time, giving more x-rays.
Now, in the next talk, I'm going to show you the five main factors that we can manipulate in order to change this x-ray spectrum. And I want to show you how changing those factors will result in a change in x-ray beam quality and x-ray beam quantity.
Now, if there's one thing that comes up in every single x-ray physics exam, it's this: how various factors influence the x-ray beam quality and the x-ray beam quantity. And I've linked in the first line of the description below a question bank that I've created from past papers. I've used actual past paper questions, and about 10 of those questions are related to x-ray beam quality and quantity and the various factors that we can manipulate in order to change the x-ray spectrum. So, if you want to practice those, go and check out that question bank below. Otherwise, I'll see you in the next talk where we will examine the five different factors that manipulate the x-ray spectrum. I'll see you there. Goodbye.