Transcription
So we've now looked at the structure of the electromagnetic wave, or electromagnetic radiation, and at a specific frequency and wavelength, we classify that electromagnetic radiation as X-rays.
Now, in the coming module, I want to show you how we go about creating those X-rays and how we use those X-rays to create a useful image. So I thought it would be a good idea to take a step back here, have a bird's-eye view over the whole X-ray physics module before diving into these specifics.
Now, before we get into this, I just want to say a lot of your physics exams, especially if you're writing your part one Radiology physics exams, comes down to X-rays. I've gone through multiple past papers, collated the questions in those papers, and seen the mark value for those specific questions, and X-rays accounts for more than 40 percent of the marks on average over the last couple of years of radiology exams. X-ray not only covers radiographs, but it covers CT, fluoroscopy, mammography, digital subtraction angiography. The physics here underlies all of those modules. So knowing this well will give you great bang for your buck for your exam. And it's very tempting to get caught up in the weeds with MRI and nuclear medicine, and by the time you spend weeks fretting over that, you've forgotten these basic fundamentals of X-ray physics. So if you take anything away from this talk, it's that X-ray physics must not be neglected. It counts for a major portion of the exam on average.
So let's take a step back and look at this diagram that I've made to summarize our entire X-ray physics module.
Now, in order to create an X-ray, we need to go through all of these steps here. Firstly, we have what's known as our X-ray circuit. Now, the function of the X-ray circuit is twofold. One, we need to take our wall current that is alternating and low voltage and convert that current into a high voltage direct current. And that's the function of the primary and secondary X-ray circuits. And the second thing we do is we actually can manipulate some controls on the X-ray circuit in order to manipulate the X-rays that we create. We can change the average energy of those X-rays, we can change the amount of X-rays that we produce, as well as the time that we expose the patient to X-rays.
Now, once we've converted that low voltage alternating current into direct high voltage current, that electricity supply can then go to our X-ray tube, which is this double-edged structure that I've drawn here. Now, the X-ray tube has multiple different components that we will go through, but the function of the X-ray tube is to produce electrons at the cathode, this orange structure here. It's a negatively charged electrode of our X-ray tube. Produce electrons by the process of thermionic emission. Once we've created electrons through the process of thermionic emission, we accelerate those electrons by a tube potential that accelerates negative electrons towards our relatively positive anode here. Those electrons then strike our anode, and that energy is either converted into heat or electromagnetic radiation.
Now, the electromagnetic radiation that we create there is X-rays, and a certain portion of those X-rays will head out of our X-ray tube. Those X-rays will go through a window in the X-ray tube. The rest of the X-ray tube is highly attenuating to X-rays. No X-rays will spread out except through this window. And once they've gone through this window, they will come into contact with two separate structures. The first is a filter. We can place sheets of metal in our X-ray beam to preferentially filter out lower energy X-rays that contribute to patient dose but don't contribute to our image. And that process is called filtration. When we add filtration, that's called added filtration. And the inherent filtration of the X-ray machine, the constituents of the X-ray machine, is called inherent filtration. And we can use different types of filters in order to manipulate this X-ray beam that's exiting the patient.
The second thing that we can do to these X-rays leaving the X-ray tube is what's known as collimation. We can narrow down that X-ray beam to a specific area on our patient, an area that we want to image without exposing other areas of the patient to X-ray radiation. The X-rays will come off of our anode in very specific geometric orientations, and this is what's known as the line focus principle, which we will spend a whole talk going over.
Now, X-rays can head towards our patient and interact with matter within the patient. They can either penetrate or be transmitted through the patient, or they can be attenuated by the patient's matter. All of that X-ray energy is absorbed within the patient, or they can go through a process known as scattering, where an incident X-ray will deflect and come off at a different angle. Those transmitted and scattered X-rays then head to our X-ray detector here.
Now, there are multiple different types of X-ray detectors: screen-film radiography, computed X-ray detectors, and our indirect and direct digital X-ray detector systems, each of which we will go through in some detail.
Scattered X-rays provide a problem when trying to create a crisp image with good contrast and good spatial resolution because scattered X-rays are no longer congruent with where they interacted with the patient. So they reduce our spatial resolution in an image by increasing noise and decreasing contrast. And there are multiple different mechanisms that we can use in order to reduce the scatter within our image. And we also are going to spend some time looking at scatter and looking at how we can reduce the amount of scatter in an image.
Now, once we've gone through all of these processes, we would have covered the vast majority of what can come up in our exams when it comes to X-ray physics. So we're going to start with the X-ray tube itself and show how we can create X-rays within the X-ray tube. We will then move on to our X-ray circuit and then make our way down through these various different processes that I've discussed over the coming talks.
Now, each and every step within this process comes up over and over again in exams, and as I've said, it accounts for over 40% of the marks on average throughout the Radiology physics part one exams, and that's kind of the same worldwide. So if you are studying for your X-ray physics part one, in the first line of the description, I've linked a question bank below in which I go through past paper questions, showing you how I would answer those questions, going through not only the correct answers but the incorrect answers and telling you why those answers are incorrect.
So in our next talk, we're going to look specifically at the cathode, the process of thermionic emission, the tungsten filament creating those electrons, as well as the focusing cap focusing that electron beam onto our anode. So I'll see you all there. Goodbye.