Contributors: Fraser J McConnell
Species: Feline | Classification: Miscellaneous
- Radiography allows assessment of lung size, position and density.
- The lateral projection Radiography: thorax is often used as a sole projection for thoracic radiography but this in fact provides poor detail of the dependent lung.
- If lateral projections are used, eg in the identification of pulmonary metastases, both laterals should be taken.
- Orthogonal views should be taken to locate the 3-dimensional position of a lesion.
- Ventrodorsal projections allow better visualization of the lung fields (especially accessory lobe) than the DV.
- DV projection has a more consistent shape to the cardiac silhouette and gives better visualization of the caudal lobe blood vessels.
- The lung fields provide an inherent contrast within the thorax - a high KVp mAs should be used to maximize the range of densities available of pulmonary radiographs.
Use as short an exposure time as possible to minimize movement blur.
- A table top technique and detail film-sheet combination should be used.
- Exposure is normally made at the point of maximal inspiration.
- Expiratory films are occasionally useful to document small pneumothoraces and air trapping.
- Care should be taken to include the entire pulmonary field.
- Examination is normally performed under sedation but heavily sedated animals may have poor inspiratory volumes making it impossible to obtain a view of a truly inflated lung.
- The VD projection should be avoided if there is a large volume of pleural fluid.
Most dyspneic animals will lie quietly in sternal recumbency for a DV projection with minimal restraint and no sedation.
- A horizontal beam may be used in dyspneic animals to give a lateral projection.
Stressful handling of dyspneic animals may result in fatal decompensation.
- Chronic cough .
- Screening for metastasis in the presence of malignant neoplasms elsewhere.
- Pre-anesthetic check.
- As part of minimum database in the investigation of many medical conditions.
- Ultrasonography is often more valuable than radiography in the presence of pleural fluid.
Dyspnea in the face of normal thoracic radiographs should prompt consideration of upper airway obstruction, pulmonary thromboembolism and acute viral pneumonia Pneumonia .
Not all serious lung pathology will result in radiographic abnormalities.
- There are 6 lung lobes:
- Right cranial.
- Right middle - small triangular shaped lobe overlying cardiac silhouette.
- Right caudal.
- Accessory - midline, ventral to caudal vena cava and between heart and diaphragm.
- Left cranial - extends cranially to thoracic inlet (sub-divided into cranial and caudal segments).
- Left caudal.
- The position of the lobes can be identified by the mainstem bronchi supplying each lobe.
- The right caudal lobe crosses the midline caudal to the heart.
The right middle lobe is the smallest lobe and is predisposed to collapse by its larger surface area to volume ratio.
- The bronchi are situated between the corresponding pulmonary artery (lies dorsal and lateral) and pulmonary vein (lies ventral and medial).
Do not mistake the pulmonary blood vessels for blocked bronchi.
- In the normal animal only the bronchi in the hilar region are visible.
- Bronchi should taper as they extend towards the periphery of the lung.
- Care should be taken to assess the rest of the thoracic contents carefully for concurrent disease.
Often one of the important differentiators is the between pulmonary and cardiac disease Homeopathy: overview .
- Clinical examination is more important than radiology in differentiating cardiac and respiratory disease in many cases.
- Interpretation should define the location of lung affected and the component of the lung (bronchi, alveoli etc) to help produce a list of useful differentials.
Identification of the lung component involved may assist selection of further diagnostic tests, eg BAL may not provide diagnosis if interstitial disease present.
- Lesions may be described as being located in the periphery, perihilar area or according to lung lobe involved.
- Lung lesions are often described relative to other structures within the chest, eg 'dorsal to the trachea in the 3rd intercostal space'.
- Lung size can change according to pathology:
- Normal size- occurs due to consolidation of lung lobe or infiltration.
- Reduced volume- atelectasis and as a result there is increased opacity.
- Increased volume- masses, ie neoplasia Lung: pulmonary neoplasia or granuloma.
- The concept of mediastinal shiftis important when differentiating lung pathology:
- Consolidation - does not alter position of midline.
- Atelectasis - midline moves towards the affected lung.
- Large pulmonary masses - move midline away from lesion.
- Lung patterns are helpful in producing a list of differential diagnoses but distribution is important.
- In many diseases more than one lung pattern is visible and as disease progresses or resolves the pattern can change.
- Radiographically bronchial disease is characterized by a bronchial pattern .
- Bronchial pattern is an abnormal visualization of the bronchi due to bronchial wall thickening but has poor sensitivity and specificity.
- Patterns may be due to mineralization, peribronchial edema, or cellular infiltrate.
Bronchial mineralization is thinner, more opaque and more clearly defined than peribronchial cuffing.
- Differential diagnoses for bronchial patterns:
- It is important to realize that there is a relatively poor correlation between radiographic changes and bronchial pathology:
- Many cats with chronic bronchitis have minimal radiographic changes.
- Many cats (particularly older animals) have asymptomatic bronchial mineralization.
- Bronchial patterns consist of linear opacities ' tramline' if the bronchi are seen in long axis or ' do-nuts' if seen in short axis. Care should be taken not to mistake blood vessels for bronchial markings.
- Widening of the bronchi indicates bronchiectasis- usually as a result of chronic and significant bronchial disease.
- In severe bronchiectasis especially congenital cases there may be mucus filling the dilated bronchi which mimics pulmonary nodules.
- Lesions within the lung fields may be classified by location, and lung pattern:
- Alveolar- occurs due to fluid or cells replacing air within the alveoli.
- Interstitial- increase in opacity of interstitial tissue by cells, fibrous tissue or fluid.
- Focal radiolucent lesions are less common than opaque lesions:
- diseases of the alveoli may result in loss of aeration of alveoli by collapse or flooding (fluid or cellular infiltrate).
- In normal lung alveolar air provides good contrast between blood vessels and parenchyma.
- Increased opacity of alveoli results in lung having soft tissue opacity → effacement between blood vessels, bronchial walls and alveoli lumen.
- Branching air-filled bronchi may be surrounded by soft tissue/fluid opacity → air bronchograms.
- If severe the entire lung lobe may become dense.
- More commonly focal patches of fluffy, poorly marginated, soft tissue/fluid opacity.
- Pathology causing edema may pass through interstitial lung pattern prior to, and following, treatment of alveolar pattern.
- Clinical signs often change before lung pattern.
- Certain disease may primarily affect characteristic areas of lung.
- Bronchopneumonia Pneumonia - usually affects cranioventral and right middle lobes preferentially .
- May by generalized involvement in severe cases.
- Usually concurrent bronchial pattern.
- Hemorrhage- may affect any part of lung and often mobile so apparently affects different parts of the lung in subsequent examinations.
- In coagulopathy may have submucosal hemorrhage causing tracheal narrowing.
- Bronchial obstruction- inhaled foreign bodies preferentially affect right caudal lung lobe.
- Usually focal and subtle increase in opacity but if totally occludes bronchus will lead to consolidation or atelectasis.
- Pulmonary edema- usually fluffy patches of soft tissue opacity in perihilar area . Left-sided cardiomegaly and pulmonary veins may also be seen. In cats, cardiogenic edema can occur anywhere within the lung.
- Non-cardiogenic edema- often causes pattern in atypical location, eg peripheral caudodorsal lung lobes.
- Differentials for non-cardiogenic edema:
- Primary lung tumor- usually appears as a solid mass .
- Lung lobe torsion- usually solitary lobe, Very rare in cats.
- May be abrupt termination of bronchus near hilus or abnormal orientation of bronchus.
- Usually concurrent pleural effusion.
- Made of lymphatics, connective tissue and alveolar septae.
- Interstitial lung patterns are caused by fluid or cellular infiltrates into interstitium.
- May be unstructured or nodular .
- Many disease causing alveolar lung pattern will first show interstitial pattern before fluid floods alveoli.
- Following successful treatment the alveolar pattern will progress to an interstitial pattern before resolving.
Unstructured interstitial pattern
- Generalized, diffuse increase in pulmonary opacity with reduced contrast and blurring of normal blood vessel and bronchial margins .
Appearance of net curtains superimposed on lung.
- The most common causes are artifactual:
- Obesity Obesity .
- Under-exposure of film.
- Under-development of film.
- Excess scatter.
- Other causes:
- Viral pneumonia.
- Early edema (cardiogenic or non-cardiogenic).
- Early hemorrhage.
- Neoplasia, eg lymphosarcoma Lymphoma .
- Solitary nodules cannot be see until >5 mm .
- Smaller nodules visible if multiple and superimposed .
- Differential diagnoses:
- Granulomata - mycoses, mycobacterial.
- Neoplasia - metastatic most common.
- Fluid-filled cysts, bullae or bronchus.
Inflammatory diseases often have indistinct margins and hemangiosarcoma metastases often have characteristic poor margination.
- Pulmonary nodules must be differentiated from:
- 'End-on' blood vessels- usually <1-2 mm smaller (therefore would not be visible if a nodule) and taper towards the periphery.
- May coallesce with blood vessels.
- Skin masses, eg ticks and nipples may mimic lung masses.
- Orthogonal views should locate them as extrathoracic structures.
If in doubt apply a spot of barium to mass to locate easily on radiograph.
- If animals are anesthetized, sedated or simply have been recumbent for some time there will be marked atelactasisof the dependent lung lobe.
- This can lead to the false impression of alveolar disease.
- Ideally the DV or VD projection should be taken before the lateral projection.
- Nipples or skin masses overlying the lung fields may be mistaken for pulmonary metastasis.
If in doubt barium can be painted on to nipple or a paper clip attached around an object to provide accurate location.
- Forelimbs should be pulled clear of the thorax so that soft tissue does not overlie the lung field.
Triceps muscle overlying the cranioventral thorax on the lateral projection may mimic pathology in mediastinum or cranioventral lung.
- Skin folds on DV or VD projections can mimic lung edges and imitate pneumothorax.
- The most common artifact is the taking of an expiratory radiograph resulting in false impression of an interstitial lung pattern.
- Poor inspiratory films may be part of the pathology, eg as a result of muscle weakness, ascites or severe hepatomegaly.