Radiography

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(Redirected from Radiograph)

Radiography is the creation of images by exposing a photographic film or other image receptor to X-rays. Since X-rays penetrate solid objects, but are weakened by them depending on the object's composition, the resulting picture reveals the internal structure of the object. Medical radiography is undertaken by a specially trained professional called a radiographer.

1 Uses
2 Theory
3 Further reading
4 External links

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Uses

The most common use of radiography is in the medical field (where it is known as medical imaging), but veterinarians and engineers also use it.

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Medicine

X-rays are the second most commonly used medical tests, after blood tests. Bone and some organs (such as lungs) especially lend themselves to X-ray imaging. It is a relatively low-cost investigation with a high diagnostic yield, although CT scans or other more specialised technologies may be necessary to delineate diseases. Ultrasound, by comparison, requires more expertise to perform.

Medical radiography can be divided into four main classes:

Dental - for teeth. A dentist may examine a painful tooth and gum using X-ray equipment. These examinations tend to give only a very small radiation dose.

Mammography - X-ray examination of female breasts and other soft tissues. This has been used on older women to screen for breast cancer, but implants designed to enlarge the breasts reduce the ability of mammography to observe changes within the breast. The radiation used for mammography tends to be softer (has a lower photon energy) than that used for the harder tissues. Often a tube with a molybdenum anode is used with about 50 000 volts (50 kV), giving X-rays with an energy of about 15 keV. Many of these photons are generated by the atoms in the anode (Mo-K radiation).

Hard tissues such as bone. After an accident, a patient's bones may be examined for breaks using X-rays. For this type of work a higher energy photon source is needed, and typically a tungsten anode is used with a high voltage (150 kV) to generate braking radiation. Depending on the part of the body which needs to be examined the dose can either be low or high. A chest x-ray is thought to be about the same as smoking one cigarette.

Double contrast technique. To examine the digestive system, a substance which is opaque to X-rays (barium sulfate) is fed to the person in food or as an enema. Barium sulfate coats the walls of the digestive tract (first contrast), which allows the shape of the digestive tract to be outlined on an X-ray after the introduction of air (second contrast). The barium meal is an example of a contrast agent swallowed to examine the upper digestive tract. Note that while soluble barium compounds are very toxic, the insoluble barium sulfate is non toxic because its low solubility prevents the body from absorbing it.

A number of substances have been used as contrast agents: Silver, bismuth, cesium, thorium, tin, zirconium, tantalum, tungsten and lanthanide compounds have been used as contrast agents. The use of thoria (Thorium dioxide) as an agent was rapidly stopped as thorium causes liver cancer.

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Airport security

Luggage both for the hold and carry on hand luggage, is normally examined by X-ray radiography. See airport security for more details.

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Industrial radiography

Industrial radiography is a nondestructive method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation to penetrate various materials. Either a high energy X-ray machine or a gamma radiation source (Ir-192, Co-60 or in rare cases Cs-137) can be used as a source of photons. In some rare cases radiography is being done with neutrons. This type of radiography is called Neutron Radiography (NR, Nray, N-Ray) or Neutron Imaging. Neutron Radiography can see very different things than X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils.

Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.

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Inspection of welds

The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Defects such as delaminations and planar cracks are difficult to detect using radiography, which is why penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, chloroform and diiodomethane. Choice of the penetrant is determined by the ease with which it can penetrate the cracks and also with which it can be removed. Diiodomethane has the advantages of high opacity, ease of penetration, and ease of removal because it evaporates relatively quickly. However, it can cause skin burns.

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Safety

Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong gamma sources (> 2 Ci) in remote sites with little supervision when compared with workers within the nuclear industry or within hospitals. Many of the lost source accidents commented on by the IAEA involve radiography equipment. Lost source accidents have the potential to cause a considerable loss of human life, one scenario is that a passerby finds the radiography source and not knowing what it is, takes it home. The person shortly afterwards becomes ill and dies as a result of the radiation dose. The source remains in their home where it continues to irradiate other members of the household.

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Theory

A photon is an X-ray when it is formed by an event involving an electron, while the photon is a gamma ray when it comes from the nucleus of an atom. In general, medical radiography is done using X-rays formed in an X-ray tube.

The types of electromagnetic radiation of most interest to radiography are X-ray and gamma radiation. This radiation is much more energetic than the more familiar types such as radio waves and visible light. It is this relatively high energy, which makes gamma rays useful in radiography but potentially hazardous to living organisms.

The radiation is produced by X-ray tubes, high energy X-ray equipment or natural radioactive elements, such as radium and radon, and artificially produced radioactive isotopes of elements, such as cobalt-60 and iridium-192. Electromagnetic radiation consists of oscillating electric and magnetic fields, but is generally depicted as a single sinusoidal wave. While in the past radium and radon have both been used for radiography, they have fallen out of use as they are irksome radiotoxic alpha radiation emitters which are expensive, iridium-192 and cobalt-60 are far better photon sources. For further details see commonly used gamma emitting isotopes.

Such a wave is characterised by its wavelength (the distance from a point on one cycle to the corresponding point on the next cycle) or its frequency (the number of oscillations per second). All electromagnetic waves travel at the same speed, the speed of light (c). The wavelength (λ, lambda) and the frequency (f) are all related by the equation:

f = c / λ

This is true for all electromagnetic radiation.

Electromagnetic radiation is known by various names, depending on its energy. The energy of these waves is related to the frequency and the wavelength by the relationship:

E = hf = h (c / λ)

Where h is a constant known as Planck's Constant.

Gamma rays are indirectly ionizing radiation. A gamma ray passes through matter until it undergoes an interaction with an atomic particle, usually an electron. During this interaction, energy is transferred from the gamma ray to the electron, which is a directly ionizing particle. As a result of this energy transfer, the electron is liberated from the atom and proceeds to ionize matter by colliding with other electrons along its path.

For the range of energies commonly used in radiography, the interaction between gamma rays and electrons occurs in two ways. One effect takes place where all the gamma ray's energy is transmitted to an entire atom. The gamma ray no longer exists and an electron emerges from the atom with kinetic (motion in relation to force) energy almost equal to the gamma energy. This effect is predominant at low gamma energies and is known as the photoelectric effect. The other major effect occurs when a gamma ray interacts with an atomic electron, freeing it from the atom and imparting to it only a fraction of the gamma ray's kinetic energy. A secondary gamma ray with less energy (hence lower frequency) also emerges from the interaction. This effect predominates at higher gamma energies and is known as the Compton effect.

In both of these effects the emergent electrons lose their kinetic energy by ionizing surrounding atoms. The density of ions so generated is a measure of the energy delivered to the material by the gamma rays.

The most common means of measuring the variations in a beam of radiation is by observing its effect on a photographic film. This effect is the same as that of light, and the more intense the radiation is, the more it darkens, or exposes, the film. Other methods are in use, such as the ionizing effect measured electronically, its ability to discharge an electrostatically charged plate or to cause certain chemicals to fluoresce as in fluoroscopy.

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