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Figure 1- X-ray radiograph of two hands source: Google images searchOne of the greatest medical advances in recent years is in the field of Radiography- the use of electromagnetic radiation in imaging. Projectional radiography is the use of 'hard' x-rays, to produce two dimensional images. It is used heavily in medicine, as it can be used to internally image the human body. It involves shining an x-ray beam onto the target area, above a photographic plate, and capturing the resulting 'shadow', the remnant beam left over. A portion of the x-rays are absorbed or scattered by the internal components of the body. Different tissues in the body can be seen, as different density tissues made up of different elements all absorb different amounts of x-ray radiation. The Main use of this Technique is in orthopaedic, chiropractic and dental examinations and evaluations, as it incredibly easy to see tissue lesions and bone fractures and breaks, not to mention tumours and changes in tissue, such as fluid build up.
The aim of this project is to understand how x-rays are generated for medicinal use, given how they are used a lot in general practise, and to examine the possible risks associated with it. High energy Ionising radiation can be very damaging on a cellular level. However, is the risk posed strong enough to outweigh the benefits of having a medical X-ray Radiography procedure?
The X-rays used in radiography is a form of ionising electromagnetic radiation, and like all electromagnetic radiation, it exhibits the following properties;
It does not require a medium for propagation
They travel with the same velocity of m/s in a vacuum
They interact with matter and may be absorbed or scattered
They can exhibit characteristics of both a transverse wave and a particle (a photon)
The energy of the photon is equal to the frequency multiplied by Planck's constant, and the frequency is equal to the speed of light over the wavelength of the light. This means that the shorter the wavelength, the greater the frequency, and the greater the frequency, the greater the energy.
The atoms that make up your body tissue absorb visible light photons very well. The energy level of the photon fits with various energy differences between electron positions. Radio waves don't have enough energy to move electrons between orbitals in larger atoms, so they pass through most stuff. Both of these forms of electromagnetic radiation have longer wavelengths than X-rays. X-ray photons also pass through most things, but for the opposite reason: They have too much energy.
The X-ray machine
Figure 2 - a diagram showing the parts of an X-ray machine1
Inside an x ray machine (figure ()), there is an anode and a cathode, inside a glass vacuum tube. The cathode is heated up, which causes electrons to get loose from its surface, which then speed towards the positively charged anode, due the positive-negative attraction. The Electrons collide with the tungsten anode and x-rays are generated. The entire mechanism is surrounded by a thick shield made of lead. This keeps the X-rays from escaping in all directions. A small window in the shield lets some of the X-ray photons escape in a narrow beam. The beam passes through a series of filters on its way to the patient. The collision between the generated electrons and the tungsten anode generates a lot of thermal energy, as well as the x-rays, so a cool oil bath surrounding the envelope also absorbs heat.x ray machine.jpg
Electrons are produced inside the x ray machine by a process called thermionic emission. Thermionic emission is the movement of charged particles, in this case electrons, due to heating. The surface of the metal cathode is heated, giving the electrons in the metal more kinetic energy. When the electrons have enough kinetic energy, they will break free from their nuclear bonds. Instead of returning to the surface of the metal, the negative electrons speed towards the positive anode. The force of this attraction can be worked out by using coulomb's law;
Where the force of attraction is equal to k times the value of the first charge, Q1, times the value of the second charge Q2, divided by r2, where r is the distance between the two charges. If they are like charges, then the force repels them, and if they are opposite charges, the force will attract them. k is worked out as follows;
Epsilon is the permittivity of the material between the charges. In the vacuum in the chamber, the permittivity of the empty space is 8.85 x 10-12 C2 N -1 m-2 , which means k= 8.99 x 109
Coulomb's law means that the greater the two charges, the greater the force. Also, the shorter the distance between the charges, the greater the force.
It is also possible to calculate the kinetic energy of the electron using Ek=qV. An average dental x-ray tube possesses a voltage of around 64000V, and the charge of an electron is 1.6x10-19
therefore J or in Electron volts 63750 eV, which is roughly 64000 electron volts.
Since E=0.5x mv2, the velocity of the electron can be worked out as
This gives a speed of about 1.5x108 m s-1, roughly half the speed of light.2
The anode is usually made of tungsten and rotates very quickly, from 3,300 rpm to 10,000 rpm, so that the electron beam is not transferring energy to the same spot for too long, as this would cause the anode to melt. The reasons that the anode is usually made of tungsten is that it has a very high melting temperature of 3370 CËš and high Atomic number, of 74.this is beneficial because elements with higher atomic numbers improves the efficiency of generating Bremsstrahlung x-rays, which will be explained later.
As mentioned above, the difference in voltage between the cathode and the anode is very large, so the electrons fly through the tube with a great deal of force. X-rays are generated due to both electron collisions and the Bremsstrahlung (German for 'Braking Radiation') effect.
Figure 3- a diagram showing characteristic x-ray generation in tungsten4
Figure (- a diagram showing how characteristic x-rays are generated from electrons falling down energy levels.If the electrons collide with electrons from the tungsten atoms with sufficient energy, the impacting electron may give the electron enough energy to overcome its bonding to the nucleus. When this occurs, it is ejected from the atom. The shell it was in previously becomes energetically unstable. An outer shell electron with a lower binding energy moves down to fill the space in the lower shell, which is a lower energy state. This change in electron orbits causes the release of a photon of energy. The energy given off is equal to the difference in the energy level of the outer and inner orbital electrons. This is why there is a higher chance of x-rays being released by atoms with greater atomic numbers, which possess K, L, m, or N shells, and s one of the reason tungsten, is used.
As can be seen from figure (), the more shells the electron can jump down, the greater the energy of the photon that is given off. Since binding energies are unique to elements, the x-rays emitted are characteristic of individual elements.
Figure 4 - A diagram of a tungsten atom being hit by electrons at different places, causing Bremsstrahlung effects4Bremsstrahlung X-rays occur when the high speed free electron interacts with the nucleus of the tungsten atom. There is no actual collision between the electron and the nucleus, but the electron interacts with the coulombic nuclear forces, and its direction and speed are changed. Since kinetic energy is derived from velocity, the kinetic energy of the electron also changes. The energy lost as kinetic energy from the electron is released as a photon. The Change in direction and velocity is dependent upon how close the electron comes into contact with the nucleus. As can be seen in Figure (), the closer the electron comes to the nucleus, the more its path is changed, and the more energy is given off in a photon.breh.jpg
When generating x-rays, about 99% of the energy is given off as heat, through collision like reactions. About 0.5-1% of the time, the electron collides with a low energy shell electron, and characteristic x-rays are emitted, or it is a nuclear interaction and Bremsstrahlung.3
Humans are exposed to ionising radiation constantly, at low levels, due to background sources, such as food, the ground, argon in the air, and space. Ionising radiation can be harmful to the human body. Ionising radiation is called ionising radiation due to its ability to ionise atoms. Since it can be penetrating, it can cause damage to our cells by corrupting the sequence of DNA in our cells responsible for making proteins. If certain parts of the DNA sequence are damaged, then these cells can turn cancerous, which can be fatal. It seems that extended exposure to ionising radiation increases the likelihood of developing cancer. During an x-ray examination, x-rays, which are a form of ionising radiation, pass through the body and are absorbed or scattered to different extents, due to the different densities of tissues. There is a lot of debate as to whether or not X-ray examinations are responsible for significant increased cancer risk. According to Berrington de Gonzalez a, Darby S. Risk of cancer from diagnostic x-rays: estimates for the UK and 14 other countries. Lancet 2004, "radiation exposure from medical imaging may be responsible for 1-3% of cancers worldwide". However, other reports state that an X-ray of your chest, teeth, arms or feet is the equivalent of a few days' worth of background radiation and has a less than 1 in 1,000,000 chance of causing cancer. An X-ray of your skull or neck is the equivalent of a few weeks' worth of background radiation and has 1 in 100,000 to 1,000,000 chance of causing cancer. An X-ray of your breasts (mammogram), hip, spine, abdomen or pelvis is the equivalent of a few months' to a year's worth of background radiation and has a 1 in 10,000 to 100,000 chance of causing cancer. An X-ray that uses a contrast fluid, such as a barium meal, is the equivalent of a few years' worth of background radiation and has a 1 in 1,000 to 10,000 chance of causing cancer. It is generally agreed in the medical community that X-rays do not pose a threat due to increasing the cancers of cancer, as the chance of developing cancer through an x-ray is very low.5
In medicine, X-rays are generated by releasing electrons due to the thermionic effect, then accelerating them in an electric field, then using them to bombard a tungsten anode. The collisions from these interactions result in heat and X-ray emission. Although x ay sae a form of ionising radiation, and increase the chances of developing cancerous cells, the risk is still very low from exposure in a medical exam.