Contributors: Fraser McConnell

 Species: Canine   |   Classification: Miscellaneous

Introduction

Overview

  • Radiography has limited value in assessment of dynamic disease of upper airways, eg laryngeal paralysis and soft palate disorders unless fluoroscopy available.
  • In most cases additional investigations, eg bronchoscopy and tracheal wash Transtracheal wash will be required for diagnosis.

Radiographic considerations
Larynx and trachea

  • The lateral projection is generally the most informative as the trachea and larynx are obscured by the spine on the VD projection.
  • The skyline projection of the thoracic inlet may be helpful in identifying tracheal collapse.
  • For evaluating cervical trachea and larynx the ET tube should be removed when exposure made as this will hinder evaluation and may result in displacement of the trachea mimicking a cervical mass.

Upper airways

  • The neck should be in a neutral position and head carefully padded to avoid rotation when evaluating the pharyngeal region.
  • Rotation will result in the soft palate appearing thickened.

Bronchi

  • Left lateral and DV projections allow best visualization of bronchi.
  • The lung fields provide an inherent contrast within the thorax - a high KVp, low 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 grid is necessary if the depth of tissue is >10 cm.
  • Exposure is normally made at the point of maximal inspiration.
  • Expiratory films are occasionally useful to document small pneumothoraces, air trapping and bronchial and tracheal collapse.
  • Care should be taken to include the entire pulmonary field.

Restraint

  • Examination is normally performed under sedation but heavily sedated animals may have poor inspiratory volumes making it impossible to obtain a view of a truely inflated lung.
  • General anesthesia is required to obtain a true lateral projection of head and neck. The endotracheal tube often hinders evaluation as it distorts soft palate and laryngeal position - if possible it should be removed for exposure.
  • 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.Stressful handling of dyspneic animals may result in fatal decompensation.

Indications

Radiographic anatomy

Larynx and pharynx

  • Air surrounding the epiglottis and within the lumen of the larynx and pharynx delineates the normal boundaries.
  • The soft palate is recognized as a linear band of soft tissue extending caudally from the palatine bone.
  • The soft palate divides the pharynx into nasopharynx and oropharynx.
  • There are 9 hyoid bones which attach the tongue and larynx to the skull.
  • The hyoid apparatus is mobile and its position depends on the position of the head.
  • There are breed variations in the shape of the hyoid apparatus with brachycephalic dogs having more a 'V' shaped hyoid apparatus, compared with the normal 'U' shape in other dogs.
  • The short axis of the single basihyoid bone is seen on a lateral projection and appears very opaque.
    This should not be mistaken for a foreign body.
  • The larynx lies ventral to C1-C2 when the head and neck are in a neutral position.
  • Extreme flexion will result in caudal displacement of the larynx to the level of C3/C4.

Lower airway radiography
Trachea

  • The trachea is easily recognized as a linear structure filled with gas extending from the larynx to carina.
  • The trachea has a relatively fixed length and altering the position of the neck results in the trachea bending in the thorax.
  • Typically there is dorsal bowing cranial to the heart Thorax: normal anatomy 01 radiograph lateral.
    This may mimic the presence of a cranial mediastinal mass.
  • To evaluate tracheal pathology the neck should be in a neutral position.
  • The diameter of the trachea is slightly less than or the same as the larynx.
  • Slight variation in tracheal diameter is normal with extension of the neck resulting in mild narrowing of the trachea at the thoracic inlet.
  • As an approximate guide in non-brachycephalic dogs the trachea should be approximately 3 x the diameter of the proximal third of the 3rd rib.
  • A ratio of tracheal diameter to diameter of the thoracic inlet may also be used and is:
    • 0.2 for non-brachycephalic dogs.
    • 0.16 in non-bulldog brachycephalics.
    • 0.13 in Bulldogs.
  • There are breed variations in tracheal diameter with severely brachycephalic dogs having narrower trachea than other types.
  • Tracheal diseases may be dynamic therefore if tracheal collapse is suspected expiratory and inspiratory films should be taken.
  • The trachea lies slightly to the right of midline at the thoracic inlet - this is most marked in brachycephalic dogs where there is often mild bowing of the trachea to the right.
  • There is slight variation in tracheal diameter with phase of respiration.
    Severe dyspnea, especially due to upper airway obstruction, may result in tracheal widening and dilation of esophagus/stomach with gas.

Bronchi

  • The bronchi are situated between the corresponding pulmonary artery (lies doral 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.

Interpretation

  • Care should be taken to assess the rest of the thoracic contents carefully for concurrent disease.
    Often one of the important differentiators is between pulmonary and cardiac disease Radiology: cardiac examination.
  • Clinical examination is more important than radiology in differentiating cardiac and respiratory disease.

Upper airways

  • In adult dogs mineralization of laryngeal cartilages allows identification of individual parts of the larynx.
  • In very obese animals, brachycephalic dogs and with extreme flexion of the neck, the pharyngeal lumen is reduced and appears overcrowded.
    Do not mistake this appearance for retropharyngeal masses.
  • The normal air-filled nasopharynx should be evaluated carefully as foreign bodies in this region are easily overlooked.
  • In normal dogs the roof of the pharynx should be straight or slightly concave, fascial planes between retropharyngeal muscles are normally clearly visible.
  • Retropharyngeal swelling may result in ventral displacement with convex bulging of the roof of the pharynx.
  • There is usually loss of visualization of fascial planes in severe cases and may be ventral depression of the larynx.

Trachea

  • Most pathologies cause narrowing of the trachea.

Generalized narrowing

  • Hypoplastic trachea Trachea: hypoplasia Trachea hypoplastic - radiograph lateral - occurs in severly brachycephalic breeds, eg Bulldog.
  • Thoracic inlet:tracheal diameter ratio <0.13 Trachea hypoplastic - radiograph lateral.
  • Submucosal hemorrhage - especially coumarin poisoning Anticoagulant rodenticide poisoning.
  • May also see pleural or mediastinal fluid and interstitial/alveolar pattern.
  • Submucosal edema - inhalation of toxic chemicals, smoke.
  • Severe tracheitis.

Segmental narrowing of trachea

  • Tracheal collapse - collapse of extrathoracic trachea occurs during inspiration Trachea collapse - radiograph lateral.
  • May also see concurrent collapse of mainstem bronchus (important to recognize as poorer prognosis).
  • Smooth and marginated segmental narrrowing of bronchial lumen.
    Redundant infolding of dorsal ligament may mimic this (it is an incidental finding in large dogs).
  • Esophagus overlying trachea does not cause apparent narrowing of the trachea.
  • Hyperflexion of the neck may cause laryngeal muscle to indent trachealis muscle and mimic tracheal collapse.

Focal intraluminal opacity

  • Soft tissue masses within the tracheal lumen are outlined by gas.
  • May be due to parasitic nodules ( Oslerus osleri Oslerus (Filaroides) osleri ), neoplasia (most commonly chondrosarcoma Chondrosarcoma , foreign body , tracheal stenosis Trachea: stenosis (secondary to trauma).
    These should be visualized in orthogonal projections to confirm lesion is inside (not overlying) trachea.

Extraluminal masses

  • Most commonly cause deviation of trachea rather than compression.
  • Cervical masses - abscesses, granulomata, thyroid tumors cause ventral displacement of cervical trachea.

Cranial mediastinal masses

  • Usually located ventrally so cause dorsal deviation of trachea Mediastinum: mass 01 radiograph lateral.
    Typically cranial to heart so trachea dips ventrally towards carina.
  • In severe cases trachea may be displaced caudally (normally 5th or 6th intercostal space) or mass compresses the trachea.
  • Less commonly craniodorsal mediastinal disease (esophageal disease, aortic body tumors, paravertebral masses) cause ventral deviation of trachea Thorax: megaesophagus radiograph lateral.

Lower airways

  • Radiographically bronchial disease is characterized by a bronchial pattern.
  • Bronchial pattern Lung bronchial pattern (close up) - radiograph is an abnormal visualization of the bronchi due to bronchial wall thickening.
  • Patterns may be due to mineralization, peribronchial edema, or cellular infiltrate.
    Bronchial mineralization is thinner, more opaque and more clearly defined than peribronchial cuffing.
  • Bronchial mineralization is often seen as an aging change.
  • It may also occur secondary to hyperadrenocorticism Lung bronchial pattern (Cushings disease) - radiograph lateral and, if there is concurrent pulmonary parenchymal mineralization, it may result in impaired gas exchange.

Soft tissue opacity

  • If the bronchus remains aerated an 'air bronchogram' may be visible where the branching gas density of the bronchus is surrounded by soft tissue/fluid opacity Lung: alveolar pattern (close up) radiograph.
    More typically alveolar patterns are rather patchy and give the appearance of fluffy, ill-defined soft tissue opacities.
  • It is important to realize that there is a relatively poor correlation between radiographic changes and bronchial pathology:
    • Many dogs with chronic bronchitis have minimal radiographic changes.
    • Many dogs (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.
  • Widening, or wall irregularities, of the bronchi indicates bronchiectasis Lungs bronchiectasis - radiograph lateral - 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.
  • Bronchial dysgenesis:
    • Abnormal cartilage development causes abnormal bronchi.
    • Bronchial changes not visible radiographically without bronchography.
    • Results in hyperlucent lung fields due to lobar emphysema.

Pitfalls

  • 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 Thorax: false pneumothorax radiograph DV.
  • 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 due to hyperadrenocorticism, ascites or severe hepatomegaly.

Additional studies

Ultrasonography

  • Allows evaluation of laryngeal function, cervical masses and mediastinal disease.

Contrast

  • Fluoroscopy especially useful to assess swallowing disorders.

Scintigraphy

  • Assessment of mucocilary clearance function.