Physics and equipment

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X-rays are produced by X-ray machines when electricity from the mains is transformed into a high voltage current, following which some of the energy in the current is converted to X-ray energy. The intensity and penetrating power of the emergent X-ray beam varies with the size and complexity of the apparatus and the exposure settings used. Portable X-ray machines are capable only of a relatively low output, whereas larger machines are far more powerful. X-rays form part of the electromagnetic spectrum, which defines different types of radiation by wave-length and frequency. X-rays have some additional properties which mean that they can be used to create images of the internal structures of biological material. The chapter covers the Production of X-rays; Interaction of X-rays with tissues; Recording and displaying the image.

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1.1 The electromagnetic spectrum.
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1.2 A stationary or fixed anode X-ray tube. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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1.3 The X-ray spectrum representing radiation emission at the target. The K and L peaks of characteristic radiation for tungsten are superimposed over the continuous spectrum. Effect of increased tube voltage on the X-ray spectrum. The quality of the beam is increased (peak of the curve has shifted to the right), the intensity is increased (area under the curve) and the maximum energy of the beam is increased (maximum velocity on the right-hand side of the curve). Effect of increased tube current on the X-ray spectrum. The intensity is increased (area under the curve) but the quality of the beam and characteristic radiation remain unchanged.
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1.4 The line focus principle. Angulation of the target (anode) results in a large actual focal spot and a small effective focal spot. The larger actual focal spot increases the heat capacity of the anode and the smaller projected focal spot increases radiographic definition (image sharpness).
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1.5 A rotating anode X-ray tube. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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1.6 A portable X-ray machine.
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1.7 A mobile X-ray machine.
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1.8 A fixed X-ray machine.
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1.9 Cross-section of a thorax showing the formation of an X-ray ‘shadowgraph’. X-ray photons passing along path C are largely absorbed, resulting in pale areas on the radiograph, which represent radiopaque structures. X-ray photons passing along path B are only partly absorbed, creating intermediate shades of grey on the radiograph, which represent more radiolucent structures. X-rays passing along path A are outside the patient and so are not absorbed, leading to black areas on the radiograph. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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1.10 The photoelectric effect. An incident X-ray photon displaces an electron from an inner orbit and in doing so is absorbed. An electron from an outer shell falls into the vacancy created by the photoelectron and in doing so emits a photon which is characteristic of the absorbing material and the orbit shells involved. The characteristic photon is absorbed within the patient.
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1.11 The Compton effect. An incident X-ray photon collides with a loosely bound electron and ricochets in a different direction at lower energy (longer wavelength).
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1.12 Section of an X-ray film showing the emulsion coats bound to the base by subbing layers and protected by supercoats.
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1.13 Cross-section through an X-ray cassette (note that the lead backing is variably present). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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1.14 Screen unsharpness. The arrows show how visible light emitted from each phosphor crystal may affect several silver halide grains, resulting in some loss of definition of the image.
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1.15 Essential features of an automatic processor.
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1.16 Automatic processor with the lid removed showing the rollers and tanks.
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1.17 A darkroom for an automatic processor.
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1.18 Digital radiography. Lateral radiograph showing the abdomen and spine in a dog. Magnification of the circled area in (a) allows the individual pixel elements constituting the matrix, as well as the greyscale, to be recognized.
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1.19 Characteristic curve of radiographic film. The sigmoidal curve reflects the relationship between relative exposure and optical density of the film. Contrast is highest along the linear part of the curve. The toe and shoulder areas of the curve are regions of low contrast. For radiographic film, the linear response extends over a narrow range of exposures. Digital film has a linear response over a much greater range of radiation exposures. Therefore, structures with a large range of attenuation can all be viewed on the same exposure.
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1.20 Lateral abdominal radiographs demonstrating the wide latitude of digital images. The bones and thicker soft tissues are visible but the thinner soft tissues and fat along the ventral abdominal wall are overexposed (arrowed). Adjustment of the contrast and brightness (window levelling) allows these soft tissues to be seen (arrowed).
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1.21 Computed radiography acquisition workstation.
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1.22 Imaging plate used for computed radiography. The plate is placed within a cassette containing intensifying screens. Digitizer for computed radiography with cassette ready to be inserted for reading.
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1.23 Digital image viewer simultaneously showing multiple images.

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