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So in order to create an X-ray signal, we need an X-ray tube.

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The fundamental principle of an X-ray tube are the following.

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These are the requirement. The X-ray tube need to have an evacuated chamber.

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It needs to have a source of free electron,

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because those are the electron which is going to interact with the target surface,

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which we call it as anode. And that target surface has to be a heavy

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metal because it should not be easily melted out or

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vaporized. So that again defines the choice of the target

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material we choose. We also need to have a way of accelerating the

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electron from the cathode to anode and hitting it.

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And in the process, we also have a means to getting rid of the heat,

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because the X-ray tube, the process is quite

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inefficient because 99% of the time it's the heat which is

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produced, only 1% of it is X-rays produced.

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So there's a challenge to push out or remove the heat as soon as possible.

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This is one of the fundamental X-ray tube design.

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Pretty much from old days to nowadays, it's still the principal

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schematic diagram works. So what are the different factors in this particular

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X-ray tube? The X-ray tube is a filament here.

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This is made up of a tungsten filament, and it is heated by a power

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supply. It's almost like a light bulb.

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When the light is turned on, the current passes through the filament and the

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filament will glow, and that's exactly here.

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The filament will release electron.

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We call this photoelectron. Electrons are negatively

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charged. They are now attracted towards the positively charged

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anode, and the way it is done is maintained by a voltage.

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That's the one which determines the tube voltage we set on X-rays.

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And that amount of current, the electron produced, is determined by the tube

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current, and that is called the mA, which is the protocol setting of the mA of the

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protocols. So these electron, they are

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accelerated towards the anode, and when they hit that anode,

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that's when X-rays are produced, both Bremsstrahlung radiation or characteristic

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X-rays. And some of it is absorbed here, some of it is coming out

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of a window here, which is what is positioned on the patient

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to capture this X-ray passing through the patient to get an image.

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Apart from this, there is also what is called as the anode.

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On top of it is a tungsten target is embedded in the anode, and this

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rotates or different methods are applied to take the heat off.

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And this is the fundamental diagram of an X-ray tube.

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Here are some images of X-ray tube.

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This is the early cold cathode X-ray tube,

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probably back in 1920s. This is one of those 1940s X-ray tube housed

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in a glass casing, which is no longer the case because the X-ray tube are

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well kept inside a sealed and a lead capsulated chamber

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so that you're trying to avoid any of these radiations coming out of the X-ray

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tube to be captured, absorbed. We call this a leakage radiation.

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Now, if you slice through the X-ray tube, which is currently well housed in a

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very big container, this is a side view of an X-ray tube.

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So you have a cathode, which is the filament element here, and you have

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an anode, which is here, which is actually fixed on a

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rotating cup, and this rotates quite fast.

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By doing so, you can take the heat out of the X-ray production.

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A cathode, anode, you have a rotor, which rotates at almost 10,000

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revolution per minute. You have a glass and a metal envelope usually

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covering everything inside, and then you have a tube housing.

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The whole thing is housed inside this.

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And this housing, this is a X-ray tube, and this is a tube housing, can

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have air passing through to take the heat off, or it can be immersed

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in oil, because oil is also another way of conducting the

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heat out and so forth.

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And this is the output port where you are directing the X-rays to come out.

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If you look examine it closely with respect to cathode, shown here is the

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filament. This is the filament. This is called a filament cup or a

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focusing cup. There are two different sizes of a filament, and I'll

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tell you why, because we have different filaments, and that will also

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determine what the best spatial resolution you can get in imaging.

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Smaller the filament, greater the spatial resolution.

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However, smaller filament cannot operate continuously, so for

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a fluoroscopy or interventional radiology, we use a larger

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filament. Apart from that, you have filament made up of tungsten

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wire, because tungsten wire has a very high melting point, so it can

5:19

sustain large amount of heat it produced.

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The filament current is typically three to six mA, and that when it does,

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you have a process called thermionic emission, which releases the electron out of

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the filament.

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You have a focusing cup. Focusing cup is basically to focus this

5:39

electron towards the anode. And most X-ray tube have two X-ray filament,

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and there may be X-ray tube which has three filaments in some special procedure.

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So if it is not focused, these electron can wander around not

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going close towards the anode. If it is focused, it will focus in a

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beam of electron towards the anode, which will result in a higher spatial

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resolution, and most of it is efficiently utilized.

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So there is also a structure on the anode. The anode looks like this.

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It is called a target electrode, which is maintained at a positive

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potential with respect to cathode.

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Electron from cathode, upon impact, transfer energy by

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collision and radiative interaction with anode producing

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heat and X-rays. So when these electron hit

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the anode here at a certain speed,

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99% of the time there's a heat produced, and 1% of the time there's an X-ray

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produced, and this X-ray is then captured towards the outside of the

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window, which is what is used for imaging.

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The most common target is tungsten because it has very high melting

6:55

point and high atomic number. And this anode

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is fixed into a rotor, so the rotor can rotate faster so that the

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electron don't hit only one spot on the anode.

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If that happens, that anode can get burnt off or get a pit, which can cause more

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artifact. So when it is rotating, the X-rays are exposing a track

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on the particular anode, and that is created.

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And we're going to discuss a little bit more about the focal spot and so forth

7:25

later.

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One other key factor in radiography is called the heel effect.

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Heel effect is the heel similar to our heel in the foot.

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Imagine the X-rays are produced here.

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The electrons are hitting the anode, and the electron creating X-rays, X-rays

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coming out in all different way. Some of it is also

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absorbed by the anode itself.

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Because of that, the X-ray intensity measured in this

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direction is much lesser than the X-ray measured in this

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direction. And we call this as a heel effect.

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Means the heel of the foot is absorbing some of the X-rays, therefore, the

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intensity in that direction is lesser compared to the other direction.

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It's the maximum here, lesser here, and it's less on the other side.

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So X-ray emitted on the anode side

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must pass through a greater thickness of anode, which results in a

8:29

reduced intensity.

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Heel effect is less important with larger focal spot size.

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If the focal spot is very small, you can see this heel effect.

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So you can see here, shorter distance to exit the beam, there is less

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attenuation it comes out, whereas here, the X-ray produced, they're also

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absorbed by the anode material itself, so therefore, the intensity is less.

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Why I'm sharing this heel effect? This becomes really important when we are

8:57

discussing mammography X-ray tube and the way it's positioned with respect to the

9:02

patient. For most radiography, the heel effect is now negated to some

9:07

extent in terms of the averaging the signals in a digital detector.