Principles of radiography

image of Principles of radiography
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Technique is a critical part of the radiographic study as in many conditions high quality images are required to detect subtle lesions and afford the clinician the confidence that a negative study genuinely means that no pathology is visible. The aim when taking a radiograph is to obtain an image that accurately represents the anatomy of the patient without distortion or unsharpness. for many anatomical regions, specific oblique views are required to visualize the anatomy and demonstrate pathology. The chapter discusses Patient preparation; Patient restraint; Patient positioning; Exposure factors; Contrast; Radiographic artefacts and film faults; and Establishing a practice radiographic facility.

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2.1 Radiolucent wedges and a sandbag are used to correctly position this anaesthetized dog for a lateral thoracic radiograph. The wedges are used to elevate the sternum since the thorax is narrower ventrally than dorsally (as is the case in most breeds). The distal limbs are supported by a wedge to avoid rotating the thorax towards the table top when the legs are extended and retracted from the cranial thorax.
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2.2 Incorrect positioning can result in images that display anatomical structures in an unfamiliar way, leading to missed or erroneous diagnoses or difficulty in localizing the abnormalities. DV view of the thorax in a dog. Note the marked rotation around the long axis making comparison with examples of normal anatomy in a radiological atlas impossible. Lateral view of the thorax in a cat. Superimposition of the forelimbs on the cranial thorax partially conceals the enlarged sternal lymph node (arrowed). Oblique view of the skull of a cat with an airgun pellet injury. This image provides no information about the location of the pellet. Two orthogonal views are required to localize the pellet.
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2.3 Geometric distortion. The X-ray beam diverges from the focal spot equally in all directions. The further a structure is from the centre of the beam, the greater the geometric distortion. VD view of the pelvis of a cat. Foreshortening of the left femur is due to the reduced range of motion in the left coxofemoral joint, preventing the femur from being extended parallel to the X-ray plate. Thus, the left femur appears shorter than the right femur. The foreshortened femur (arrowheads) and surrounding muscles (arrowed) appear more opaque as the depth of tissue penetrated by the X-ray beam is greater. Dental radiographs of the mandible in a dog. Note the geometric distortion of the teeth when (c) a conventional VD intraoral view is obtained compared with (d) an image taken using the bisecting angle technique.
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2.4 Effect of a small focal spot and a large focal spot on image sharpness. The larger the focal spot, the larger the penumbra around an object. The penumbra makes the margin of the object appear less sharp.
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2.5 Centring the X-ray beam. The area to be exposed is illuminated by light from the light beam diaphragm. The centre point of the radiograph is indicated by a central cross.
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2.6 Lateral thoracolumbar radiograph showing centring for the spine. The disc spaces appear progressively narrower (arrowed) the further away they are from the centre of the beam (+). This is due to divergence.
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2.7 Effect of object–film distance on the radiographic image. The object is located close to the film, resulting in an accurate representation of the object on the radiograph. Increasing the distance between the film and the object results in magnification of the object on the radiograph. If not all parts of the object are parallel to the film (i.e. are not equidistant) there will be uneven magnification of different parts of the same object. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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2.8 Magnification. The positional marker on the top has been imaged on the table top with the cassette placed under the table. The positional marker on the bottom has been imaged under the table placed directly on top of the cassette. The marker on the top is magnified because of the increased object–film distance created by use of the under table (‘Bucky’) grid. Lateral radiograph of the caudal abdomen of a dog. The effect of magnification is obvious when comparing the length and diameter of the lower (solid arrows) and upper (dashed arrows) limb.
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2.9 Radiographic markers. A marker of true size or a ruler (arrowed) with radiopaque measurements should be used where magnification of the image could lead to errors. When taking radiographs for orthopaedic templating, a marker of known length (arrowhead) is positioned at the level of the bone of interest to allow accurate measurements to be obtained.
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2.10 Exposure factors. Underexposed DV thoracic radiograph of a dog taken using conventional (analogue) film. The radiograph needs to be repeated as the amount of meaningful information that can be extracted is limited. For a region such as the thorax with high inherent contrast, the difference between soft tissue structures and air-filled lung is not appreciated. This would suggest that diffuse pulmonary pathology is present and penetration (kV) should be increased by a minimum of 15% (but possibly considerably more). Overexposed conventional lateral thoracic radiograph of a cat. The lungs are overexposed and no assessment of the pulmonary parenchyma is possible. Bronchial disease, infiltrate and metastases may all be overlooked. In the cat, pulmonary parenchymal detail is fine, so to assess any changes with confidence, high detail images are required.
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2.11 Inherent contrast. There is inherently less contrast in the abdomen than the thorax. In the abdomen, contrast is due predominately to abdominal fat (arrowed) between the organs. Using a low to intermediate kV and a high mAs maximizes this contrast. In the thorax, there is high contrast between the air-filled lungs and the heart and using a high kV and short exposure time (mAs) limits movement blur. Short exposure times are necessary where conscious radiography is unavoidable, as in this case of a dog with left-sided heart disease (the enlarged left atrium is arrowed) to assess for signs of congestive failure (pulmonary oedema, enlarged lobar veins).
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2.13 A grid comprises multiple lead strips, often angled so that they are parallel to the diverging primary X-ray beam. Scattered or off-focus X-ray photons (dotted lines) cannot pass through the lead strips and are absorbed. Primary beam photons, which are responsible for the useful radiographic image, pass between the lead strips.
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2.14 The effect of a grid. Lateral radiographs of the abdomen in a dog taken with the use of a grid and without a grid. Note the increased contrast of the image taken using a grid.
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2.15 Incorrect use of a grid. Lateral radiographs of the abdomen in a dog taken with a focused grid correctly positioned and with a focused grid positioned off-level (i.e. the central X-ray is not perpendicular to the grid). In (b) misalignment of the grid has resulted in an underexposed radiograph and reduced image contrast.
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2.17 Effect of exposure on digital images. Underexposure and overexposure. The wide dynamic range of digital radiography allows the structures within the abdomen to still be visible on both images. (a) This image has a grainy or ‘salt and pepper’ appearance due to quantum mottle. Insufficient information (number of X-ray photons) has reached the detection system. (b) In this image the thinner parts of the patient are displayed as completely black. All pixels in these areas have been assigned the maximum value. Altering image brightness or contrast will not generate any useful information about these regions.
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2.18 Lateral radiograph of the thorax in a dog. The motion blur (most evident in the mid-section of the ribs, the larger caudal lobar vessels and the diaphragm) is due to rapid panting.
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2.19 The underdeveloped film is too light across both the image and the background. When held to a light box, fingers can be seen through the background (arrowheads), which is not the case in an adequately developed radiograph.
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2.20 Overdeveloped film. The film became stuck during automatic processing and the top part of the radiograph is overdeveloped. It is too dark, even in the parts which have not been exposed (e.g. underneath the orthopaedic implant).
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2.21 Fixing and washing fault. This radiograph has become yellow/brown due to inadequate fixing or washing of the film at the time of processing. The fault can be corrected by reprocessing the film.
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2.22 Dark splash marks (arrowheads) are visible on this radiograph. This is due to developer splashing on to the radiograph prior to processing.
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2.23 Crimp or pressure marks. These curved dark lines (arrowhead) are caused by bending or kinking of the film due to careless handling prior to processing. This results in areas of the film being overdeveloped because the film emulsion is damaged, allowing developer to penetrate into the emulsion.
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2.24 Roller marks. The regularly spaced parallel lines (arrowheads) across the radiograph are due to dirt on the rollers. The areas of film under the dirt are undeveloped.
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2.25 The surface emulsion of the film has been scratched (arrowheads) during handling or processing. This leads to sensitized areas of the film and overdevelopment.
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2.26 Hair in the cassette. The fine white line (arrowed) is caused by a hair within the cassette located between the film and the intensifying screen. Any debris between the screen and the film results in a white mark on the image as it attenuates light produced by the intensifying screen. The artefact is sharply marginated due to the proximity of the dirt to the film surface. Note: hair attenuates light and not the X-ray beam directly.
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2.27 Light fogging. The black zone at the edge of the radiograph (arrowed) is due to light fogging. The protective plastic envelope that encloses the flexible cassette was damaged along the seam and white light has caused fogging of the film. Important diagnostic information has been lost. A similar appearance occurs along the top edge of film within an incompletely sealed storage packet. Any visible white light (e.g. from mobile phones being used in the darkroom or damaged indicator lights on the automatic processor) can lead to fogging of the film.
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2.28 Dirt on the imaging phosphor and the light guide. (a) The white spots (arrowed) across the image are due to dirt on the imaging phosphor, which prevents light from reaching the detector during processing. (b) The white line (arrowheads) across the radiograph is caused by dirt on the light guide of the computed radiography digitizer.
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2.29 Reader artefacts, data malfunction and data transfer artefacts may appear as geometric shapes or lines, which can mimic pathology. Reader artefact mimicking a bladder calculus. Magnified view of (a); the artefact is denoted by the arrow. Data transfer artefact. The central areas of the image have been incompletely digitized or corrupted during data transfer.
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2.30 Inadequate or no collimation. If no collimation is applied, the software defines the margins of the ‘patient’ (black arrows) based on pixel intensity and excludes or rejects useful information (white arrows). Manually reapplying the collimation allows the useful information in the image to be retrieved.
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2.31 Uberschwinger artefact. The radiolucent halo around the screws in the radius (arrowheads) is an artefact created by incorrect processing of the data due to large density differences between the metal and surrounding bone. The inappropriate filtering can be avoided by selecting algorithms that do not produce edge enhancement. Note: there is marked quantum mottle from underexposure.
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2.32 Moiré artefact. Alternating bands of dark and light are present across the radiograph where a static low frequency grid was used. During processing, alignment of the computed radiography laser with the grid lines results in this interference pattern. Moiré artefacts are more obvious when images are viewed on a low resolution monitor and can be avoided by using a high frequency or moving grid and viewing images on a high resolution clinical grade monitor.
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2.33 Illuminated warning sign outside an X-ray room containing a fixed X-ray machine. The yellow lettering is illuminated whenever the machine is switched on and the red lettering is illuminated during the preparation and exposure stages.
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2.34 X-ray room with a mobile lead screen (*) and a permanent lead screen (arrowed) behind which the operating console of the X-ray machine is mounted. The radiographer remains in the room behind the permanent screen when the exposure is taken and can observe the animal through the lead glass window. The exposure button is mounted on the operating console. The mobile lead screen is used to shield any person required to stay within the room close to the patient (e.g. for the purpose of manual inflation of the lungs for thoracic radiography).
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2.35 Correct storage of lead aprons and gloves.
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2.36 Radiograph of bones covered by a single layer of lead rubber: compare with the edge where there are two layers of lead rubber and all the primary beam appears to have been absorbed.
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2.37 Radiograph of gloves showing cracking of the lead rubber at the base of the fingers (typical site). These gloves should be discarded.
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2.38 Sources of radiation hazard, including both leakage of primary radiation from a damaged tube head and scattered radiation from the patient, table top and floor. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.

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