Basics of thoracic radiography and radiology

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Please note. The new 2nd edition of the BSAVA Manual of Canine and Feline Thoracic Imaging is now available here.

Thoracic radiography is an essential tool in the investigation of both thoracic and systemic disease. […] Despite the fact that radiography is easy to perform, careful technique is required to ensure that high-quality films are obtained. Poor technique is a common reason for misdiagnosis. The first part of this chapter outlines the methods that should be used to obtain thoracic films and special techniques to enhance their diagnostic yield. […] The second part of this chapter introduces some of the basic principles of thoracic radiology that apply to all anatomical regions. The following topics are discussed: Restraint and patient preparation; Technique; Digital radiography; Contrast radiography; Fluoroscopy; Systemic evaluation of the thoracic radiograph; Variations in radiographic anatomy; Essential principles of interpretation.

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1.2 Fractious cats can be difficult to restrain while conscious. A cat muzzle is often extremely useful and many cats can be positioned using tape and sandbags once this is applied.
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1.3 Many positioning aids are available. Sandbags should be heavy enough to provide adequate restraint. The internal sand or beans should be shifted to either end to avoid airway compromise.
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1.4 A dyspnoeic animal should be handled with care. If a thoracic radiograph is considered essential in a stressed cat or small dog then a screening DV radiograph can be obtained through a cardboard box. The lid can be closed (provide airholes or oxygen supply) for further calming effect.
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1.5 In the UK manual restraint should only be used in circumstances. Two people may be required to hold the animal in order that each is a maximal distance from the primary beam. Protective clothing and personal dosimeters must be worn.
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1.7 A dog positioned in left lateral recumbency for a thoracic radiograph. Note the full extension of the forelimbs and the use of a sandbag across the neck. The head and neck are gently extended. A foam pad has been placed under the head to make the dog comfortable. Some dogs will require a small foam wedge under the sternum to ensure that it is parallel to the spine.
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1.8 The use of foam wedges to achieve a parallel spine and sternum. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.9 Location for centring point of the beam for a lateral thoracic radiograph. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.10 Positioning for the DV view. Positioning for the VD view. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.11 Positioning for a DV radiograph. The elbows are to either side of the chest and a sandbag has been used across the front feet and over the neck. It can be helpful also to support the hindlegs with a sandbag. The radiograph is centred at the caudal aspect of the shoulder blades in the midline. A measurement is made from the centring point to determine the exposure to be set.
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1.12 A dog positioned in a trough for a VD thoracic radiograph. Sandbags are used to pull the forelimbs cranially. A small foam wedge has been placed under the head to make the dog comfortable. Care has been taken to make sure that the sternum is vertically above the spine.
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1.13 In order to take a decubitus view a horizontal X ray beam is required. Consideration should be given as to where the beam is aimed (e.g. thickness of wall behind). A cassette can then be held in an upright position using sandbags. An area of collimation is established. The dog is then positioned in lateral recumbency with the spine adjacent to the cassette. A foam wedge may be needed to elevate the patient. Normal decubitus radiograph in a mature dog. The dog is in left lateral recumbency and the right lung is uppermost. The lung markings extend to the periphery of the lung and there is no evidence of pneumothorax.
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1.14 Positioning for the decubitus view. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.15 Positioning and set-up for a horizontal beam standing lateral radiograph. A horizontal beam standing view in a dog. In this example the cassette has been attached to a vertically suspended platform, but in practice it can be taped to a wall or suspended in a carrier bag from a drip stand. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.16 An oblique DV view. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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1.17 A CR image reading device. Many different brands and types are available.
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1.18 The final CR image on a viewing monitor. The resolution of the monitor is extremely important. Post-processing, such as enlargement, annotation and windowing, can be performed at this time.
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1.19 A normal bronchogram of the left cranial lobar bronchus. Bronchography is now an obsolete technique.
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1.20 An X-ray suite with an image intensifier for fluoroscopy. The image intensifier is pulled forward for use and the corresponding X-ray tube is underneath the table. The image is shown on the television screen. Many different types of fluoroscopy set up exist.
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1.21 The simple technique of rotating a radiograph 90 degrees to an unfamiliar position immediately enables the observer to assess the ribs and other skeletal structures more thoroughly.
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1.22 Lateral and VD radiographs of a French Bulldog. The dog has multiple spinal abnormalities, including wedge and butterfly vertebrae, lordosis and kyphosis. A Lhaso Apso with a severe pectus excavatum. Such abnormalities make perfect positioning impossible and interpretation harder.
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1.23 Left lateral radiograph of an 11-year-old Labrador Retriever. Note the spondylosis deformans and degenerative changes in the sternum and fine interstitial lung pattern. These are age-related changes.
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1.24 DV radiograph of the thorax of an extremely fat dog. The large amount of pericardial fat is evident as a separate structure in this dog (between black and white arrows) but in many animals is not distinguishable and may mimic cardiomegaly. The cranial mediastinum is very wide due to the presence of fat and the lung fields are generally increased in opacity.
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1.27 Normal right lateral and left lateral thoracic radiographs. Note the difference in appearance of the diaphragmatic crura (right crus = pink line, left crus = blue line). They appear parallel to one another in the right lateral view and form a ‘Y’ shape on the left lateral view. The caudal vena cava (green lines) merges with the right crus as it passes through the diaphragm. This is the more cranial crus on the right lateral view and the more caudal crus on the left lateral view (the abdominal contents push the lowermost crus forward). Right lateral view. Left lateral view. (c, d Adapted from with permission.)
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1.29 Normal DV and VD thoracic radiographs. Note the differences in the diaphragmatic outline, which appears as three humps on the VD view but as one smooth curve on the DV view. This principle is shown in the two schematics. (c, d Adapted from with permission.)
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