Radiography

From Freepedia

Radiography is making of radiographs, photographs made by exposing a photographic film or other image receptor to X-rays. Since X-rays penetrate solid objects, but are weakened in force by them, the picture resulting reveals the internal structure of the object.

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

Theory

The type of electromagnetic radiation of most interest to radiography is 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. It is generally pictured as a single sinusoidal wave.

It is characterised by its wavelength (the distance from a point on one cycle to the 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 utilizing its effects onto a photographic film. This effect is the same as that of light, and the more intense the radiation is, it will produce a darker film, or a more exposed 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.

Radiograph production

X-ray machines are the primary source of X-rays used in radiography.

Uses

Medicine

X-rays are, after blood tests, the most commonly used medical tests. Bone and some organs (such as lungs) especially lend themselves for imaging by X-ray. 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.

Industrial radiography

Industrial radiography is a nondestructive method of inspecting materials for hidden flaws by utilising the ability of short wavelength electromagnetic radiation to penetrate various materials. The value of this ability lays in the fact the material to a degree dependent upon its composition and thickness absorbs penetrating radiation. 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 wave length less about 10 nanometres.

The beam of radiation shall be directed to the middle of the section under examination and shall be normal to the material surface at that point, except on special techniques where known defects would be 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%. So, to follow up on what has just been said, 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 (becoming greater by successive additions) in its response, relatively weak radiation can be detected by prolonging the exposure until the film can record an image, which 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 metal is too irregular, it may be desirable to grind it to obtain a smooth finish, however this is likely to be limited to those cases 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 the reason that penetrants are often used to enhance the contrast in the detection of such defects. Penetrants used include silver nitrate, zinc iodide, trichloromethane 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.

References

• Kodak. (website?)
• Agfa. (website?)
• Encyclopaedia Dictionary, Chemistry and Physics. (ISBN needed)
• Composite Materials for Aircraft Structures by Alan Baker, Stuart Dutton (Ed.), AIAA (American Institute of Aeronautics & Ast) ISBN 1563475405

External Links

• Radiology Web Site Directory
• Radiology Job Outlook
• Radiology Resources for Students and Professionals
• Major John Hall-Edwards, British radiography pioneer