Virtual Congress speaker Mike Herrtage – Diagnostic Imaging notes


Michael E. Herrtage
Department of Clinical Veterinary Medicine
University of Cambridge

Indications for thoracic radiography

Radiography of the canine and feline thorax is an invaluable aid to diagnosis of many thoracic diseases and lesions, but a poor radiograph can be worse then none at all, because it may provide false information.

The indications include the following clinical presentations:

• Coughing
• Dyspnoea
• Cardiovascular disease
• Thoracic trauma
• Assessment of primary and secondary neoplasia
• Lesions of the chest wall
• Regurgitation of food
• Investigation of other abnormalities detected by palpation, auscultation or percussion
• Miscellaneous

Restraint for thoracic radiography

The Ionising Radiations Regulations (1999) require that animals may only be manually restrained for radiography in exceptional circumstances when clinical consideration precludes the use of sedation or anaesthesia.

With patience and skill, most patients can be restrained using sedation with positioning aids such as troughs, sandbags, foam wedges and ties. General anaesthesia may be safer then frantic struggling and may prevent unnecessary repeat radiography.

Radiographic technique

Film/screen combination – Rare-earth screens reduce the exposure required and thus allow the shortest exposure times to be used. In addition, rare-earth screens are less sensitive to scattered radiation than are calcium tungstate screens. This may allow a grid to be dispensed with.

Cassettes – Large cassettes (up to 43 x 35 cm) should be available so that the whole chest can be included on a single film. It is very difficult to interpret a mosaic of smaller films.

Grids – A grid should be used to reduce the effect of scattered radiation if the chest is more than 15-20 cm thick. However this will require an increase in exposure time of approximately 2-4 fold.

Exposure factors – A high kV low mAs technique is usually employed. It has two advantages: the higher kV reduces the inherently very high contrast in the chest and produces a greater number of grey shades, and the low mAs reduces movement blur.

Exposure factors will need to be increased if pleural fluid or diffuse lung pathology are present and reduced if pneumothorax is present.

Collimation – Close collimation of the beam will reduce scattered radiation and improve detail, but the margins of the lung field should not be coned off as they may contain useful diagnostic information.

Radiation safety – If manual restraint is required then all measures to protect personnel must be taken.

Inspiratory films – The exposure should be made at full inspiration so that the lung fields are fully expanded and aerated. This may be difficult in animals that are panting or breathing shallowly. Sedation may reduce the respiratory rate. Temporary occlusion of the nostrils or blowing on the nostrils may cause the animal to inhale deeply. In the anaesthetised animal, the lungs may be held inflated by the use of the rebreathing bag taking care to avoid overdistension.

Darkroom Technique – Films should be developed with care to avoid artefacts. Underdevelopment is the most common film fault and results in loss of contrast.

Radiographic projections

Lateral view – The right lateral recumbent view should be taken as standard since i) the heart lies in a more consistent position ii) there is more air-filled lung between the heart and lower chest wall giving better cardiac detail and iii) the diaphragm obscures less of the caudal lung field. However, as the dependent lung field is less well aerated, small soft tissue densities may be masked. For this reason, it is often useful to take both right and left lateral recumbent views since lesions in the uppermost lobes will be better visualised.

The patient should be padded into the true lateral position and the forelimbs pulled well forwards. Centre to the caudal border of the scapula, midway between the scapula and the sternum.

Dorsoventral/ventrodorsal View – The DV projection is essential if the heart is under investigation. If the animal is in respiratory distress, a VD view must not be attempted. For both views it is important to ensure symmetrical positioning with the spine and sternum superimposed.

Centring is level with caudal scapular border (DV) or midway along the sternum (VD).

Evaluation of thoracic radiographs

A systematic and thorough evaluation of the thoracic and extrathoracic structures is essential:
• Consistent placing of radiographs on the viewing box enhances familiarity with normal anatomy.
• Evaluate for technical quality, particularly positioning and exposure factors.
• Determine degree of inspiration.
• Evaluate extrathoracic structures including spine, sternum, diaphragm, ribs, thoracic wall and liver.
• Evaluate position and diameter of the trachea and carina.
• Evaluate the cranial and caudal mediastinum for evidence of widening, abnormal density or the presence of a mass.
• Evaluate the aorta, aortic arch and caudal vena cava.
• Evaluate the cardiac borders (cranial, caudal, left and right) and the position of the apex of the heart.
• Evaluate the main pulmonary artery and the size and shape of the pulmonary arteries and veins.
• Evaluate the lung fields, which for descriptive purposes can be divided into three areas – central, middle and peripheral.

Normal lung pattern

The normal lung is composed primarily of air. The remaining portion of the lungs contains the pulmonary vasculature, bronchi, bronchioles, alveolar ducts, alveoli, interstitial tissue, lymphatics and pleurae. The major pulmonary vessels and bronchi are usually visible in the central zone near the hilus, but only the pulmonary vessels are seen in the middle zone. The other structures provide general background density.

The pulmonary arteries run dorsal and lateral to their bronchi and the pulmonary veins run ventrally and medially. The arteries and veins are normally of similar size. They should be smaller than the diameter of the proximal third of the fourth rib on the lateral view and should not exceed the diameter of the ninth rib where they cross on the DV view.

Pathological lung patterns

There are 5 pathological patterns; bronchial, vascular, alveolar, interstitial and mixed. However there are a number of factors, which can mimic these patterns:

• Underexposure
• Underdevelopment
• Poor inflation due to expiration, obesity, hepatomegaly, ascites or laryngeal paralysis
Ageing changes such as mineralisation of the tracheal and bronchial walls, increased interstitial markings, mineralised pleural plaques, which should not be mistaken for miliary tumours, pleural thickening, and hyperlucent lungs

Bronchial pattern – Mineralisation of bronchial walls occurs as an ageing process, especially in chondrodystrophic dogs. Other causes of increased visualisation of the bronchial walls include both soft tissue pathology and calcification:

• Chronic bronchitis (infectious, allergic, irritant, parasitic)
• Pulmonary infiltrates with eosinophils
• Neoplasia along airways e.g. lymphosarcoma
• Left-sided failure with peribronchial oedema around airways

The markings are linear (“tramlines”) and ring-like (“doughnuts”) with “signet rings” being doughnuts with adjacent end-on vessels. The size and clarity of the marking depends on the pathology with the calcified markings of Cushing’s disease being particularly striking. Take care not to confuse parallel blood vessels for tramlines.
Bronchiectasis may produce wavy or wide tramlines and large doughnuts.

Alveolar pattern – Patches of alveoli become flooded with fluid or neoplastic cells and so lose their air lucency and take on a soft tissue opacity. The usual types of fluid are oedema (many causes), haemorrhage and exudate, and they cannot be distinguished except by their distribution and by other radiographic signs (e.g. enlarged heart, fractured ribs). The flooded areas are not well-defined and so the appearance is of “fluffy” or “mottled” soft tissue markings with ill-defined aerated areas or air alveolograms between them. The vascular pattern and heart outline are blurred or lost. Lobar divisions may become apparent in cases with a more localised pattern

As the patches of flooding coalesce and become larger the airways may be seen running through the consolidated areas due to their contained air; a sort of natural negative contrast technique and so logically called air bronchograms. The blood vessels are completely obscured.

Localised alveolar pattern Generalised alveolar pattern
Bronchopneumonia Severe bronchopneumonia
Pulmonary oedema Severe pulmonary oedema
Pulmonary haemorrhage Severe pulmonary haemorrhage
Primary lung tumour Miliary metastatic disease
Pulmonary metastasis Near-drowning
Lobar collapse or atelectasis Smoke inhalation
Heartworm disease Terminal paraquat poisoning
Pulmonary infarct

Vascular pattern – The normal vessel size is very variable, but arteries and veins are usually similar in size. They are said to be 25 – 100 percent of the diameter of the proximal third of the 4th rib where they cross it on the lateral view (a huge range)(9th rib on DV).

• Hypervascular pattern: the blood vessels are increased in size (arteries or veins or both). Causes: left heart failure, left-to-right shunting, heartworm, pulmonary thromboembolism.
• Hypovascular pattern: the blood vessels are reduced in size and are very thready, giving the lung fields a hyperlucent appearance. Causes: shock, dehydration, haemorrhage, hypoadrenocorticism (Addison’s disease), severe pulmonic stenosis, Tetralogy of Fallot and other causes of right-to-left shunting.

Interstitial pattern – Interstitial patterns may be unstructured (hazy or reticulated) or nodular. Most unstructured interstitial patterns are incidental, but there are a number of specific causes:

Localised unstructured interstitial pattern Generalised unstructured interstitial pattern
Partial lung collapse Artifact
Pulmonary contusion Interstitial oedema
Pulmonary haemorrhage Interstitial pneumonia
Bronchial foreign body Diffuse pulmonary metastasis
Pulmonary thromboembolism Paraquat poisoning
Disease in transition bronchopneumonia, oedema Disease in transition bronchopneumonia, oedema

Pulmonary masses of various types are also classified as interstitial pathology, but are nodular rather than reticular. Small nodules are not usually visible individually when they are less than 0.5 cm diameter, but if a number are superimposed they are seen due to superimposition. Beware interpreting composite thoracic shadows, end-on vessels or calcified plaques as metastases.

Nodular interstitial pattern Nodular cavitary interstitial pattern
Pulmonary metastasis Pulmonary metastasis
Primary lung tumour Primary lung tumour
Lymphosarcoma Granuloma
Granuloma Abscess
Abscess Partially fluid-filled bulla
Haematoma, cyst Cyst
Traumatic bulla Bronchiectasis
Mucus-filled bronchus

Mixed pattern- Commonly, more than one pattern is present. Both may be pathological (e.g. alveolar and bronchial in bronchopneumonia or bronchial and interstitial in chronic bronchitis) or one may be an incidental finding (e.g. ageing bronchial markings). Generally, alveolar patterns are more serious than interstitial and bronchial patterns.

The “silhouette sign” – The silhouette sign is used to determine whether or not two soft tissue structures in the chest (or less commonly in the abdomen) are in contact, which aids in recognising their position and origin. Such structures may be normal, such as the heart or a major vessel, or abnormal such as a mass. If they are not touching and there is therefore air-filled lung between them, their margins are still visible even though they may be superimposed; this is the “negative for the silhouette sign”. If they are touching then the contact parts of their borders merge and they appear to be a single mass; the “positive for the silhouette sign”. Reference to merely the “silhouette sign” or “silhouetting” is confusing terminology and should be avoided.


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