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Introducing X-radiation

John Holroyd introduces what atoms and x-rays are, and includes key definitions.
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In this step, we are going to look at what X-rays are and how they are generated.
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X-rays are a type of electromagnetic radiation, much like visible light or radio waves. X-rays have particularly high frequencies and energies, which means they are a special type of radiation known as ionising radiation. It is this property of ionisation that is important when considering how X-rays are harmful, as we will find out later on in this course.
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But first, let’s look how the X-rays are produced. In order to understand this, we first need to consider how an atom is made up. An atom consists of a central nucleus containing protons and neutrons and is circled by rings or shells of electrons. The positively charged protons in the nucleus are positive charge and the electrons have a negative charge. Normally, an atom will have an equal number of protons and electrons, so it has no overall electrical charge.
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Every dental X-ray set contains an X-ray tube. It comprises an electrical circuit, as shown in the diagram. At one end is a thin metal filament made of tungsten, and at the other a tungsten disc, which is called the target anode and is set into a copper block.
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The filament is heated by an electrical circuit which provides enough energy for electrons to leave the surface of the metal filament. These electrons are then focused and accelerated towards the tungsten disc by a high voltage applied across the X-ray tube, which makes the filament negatively charged and the tungsten disc positively charged. The electrons collide with the atoms in the tungsten and this produces X-rays. To stop the X-rays being emitted in all directions, the X-ray tube will be encased in lead with a small window to produce an X-ray beam in only one direction and at the exact size and shape required.
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There are two atomic interactions that occur in the tungsten anode to create X-rays. The first is where an electron emitted from the filament travels close to the nucleus of a tungsten atom. The negatively charged electron is bent and slowed down as it passes close to a positively charged nucleus. The energy lost by the electron during this process appears in the form of an X-ray. This is known as bremsstrahlung radiation from the German word meaning breaking. Depending on how close the electron is to the nucleus, this will affect how much it is slowed and therefore the energy of the X-ray.
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The other type of interaction is where an electron collides directly with an electron in a tungsten atom. This can cause the electron to be removed from the tungsten atom, leaving a vacancy in the electron shell. In order to fill the vacancy, an electron from an outer electron shell falls down to fill the gap. To move to an inner shell, the electron needs to lose energy and it does so by emitting an X-ray. As there is a fixed energy loss required to move to an inner electron shell, the energy of the X-ray emitted will be fixed at this value. This is therefore known as characteristic radiation.
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This chart shows the range or spectrum of extra energies emitted from the X-ray tube. The higher the X-ray energy, the further it can travel, and it takes more shielding material to absorb it. Low energy, or soft X-rays, to the left of the spectrum, would not contribute to producing a radiograph but do deliver a radiation dose to the skin of the patient. I mentioned there are two types of effects creating X-rays. If we look at the spectrum of X-ray energies being emitted from the X-ray tube, we see a wide range of energies from bremsstrahlung radiation with some spikes corresponding to the characteristic energies. The maximum energy of the X-rays produced is determined by the operating voltage, or KV.
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So if we increase the voltage across the tube from, say, 60 KV to 70 KV, we increase the energy and the penetration power of the X-rays. The amount of X-rays that are produced is directly proportional to the tube current, VMA, and the exposure time. As these are increased, so too will the number of X-rays generated. As we’ve seen, a wide range of X-ray energies are produced from the X-ray tube, and it’s important to only expose the patient to those which are useful in producing a radiograph. Low energy X-rays will be absorbed by the patient’s skin but will not contribute to the radiograph and so have no benefit.
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These lower energy X-rays can be removed by adding what is known as filtration. Filtration is simply the addition of metal, usually aluminium, in the path of the X-ray beam to absorb the low energy X-rays and stop them reaching the patient. The more filtration that is used, the more the lower end of the X-ray energy spectrum will be absorbed. So it is important that the correct amount of filtration is used to help get the best quality image while reducing the dose to the patient.
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When we talk about the primary or main X-ray beam, this means the useful part of the X-ray beam that is used to produce the X-ray image. However, whenever X-rays strike a person or object, some will be scattered in all directions. When we talk about scatter or scattered X-rays, this is what we mean. It is important to ensure you stand in a suitable position that is protected from both the primary beam and scattered X-rays when taking a radiograph. And finally, just to end this step with a couple of important facts about X-rays. X-rays cannot induce radioactivity. In other words, they cannot make people or objects they expose radioactive.
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X-rays can also be switched off, just like a light switch turns off a light. Once the exposure is complete, there are no more X-rays.

The video above will refresh your knowledge of what X-radiation is and how it is produced. This will ensure you have a solid understanding before we move on to the hazards and health effects of radiation.

The video first looks at how X-rays are produced. The workings of an X-ray tube are described as well as the two types of effects that create X-rays. The X-ray energy spectrum is introduced and the process of filtration to remove low-energy X-radiation is explained. The concept of the primary X-ray beam and scattered X-radiation is also covered in this step.

A PDF version of these slides is available in the downloads section below.

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Dental Radiography: Radiation Protection in Dental Practice

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