Contributors: Fraser McConnell

 Species: Feline   |   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.
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 or swallowing 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.

  • Detail screens and films should be used with a tabletop technique.
  • Exposure is normally made at the point of maximal inspiration.
  • Expiratory films are occasionally useful to document small pneumothoraces and pleural effusions, air trapping and bronchial and tracheal collapse.
  • Care should be taken to include the entire pulmonary field   Thorax: normal 01 - radiograph lateral  .

    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 truly 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.

    Megesophagus may occur secondary to chemical restraint. Hypostatic congestion of lungs occur with general anesthesia.

  • 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

  • Nasal discharge Nasal discharge.
  • Stertor.
  • Dyspnea.
  • Signs of upper airway obstruction.
  • Post-trauma.
  • Cough .

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   Skull: normal 01 - radiograph lateral  .
  • 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.
  • 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 head and neck results in the trachea bending in the thorax.
  • Typically there is dorsal bowing cranial to the heart.

This may mimic the presence of a cranial mediastinal mass.

  • To evaluate tracheal pathology the head and 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.

Cycle of respiration also changes tracheal diameter slightly.

  • Tracheal diseases may be dynamic therefore if tracheal collapse is suspected expiratory and inspiratory films should be obtained.
  • The trachea lies slightly to the right of midline at the thoracic inlet.
  • 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 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.

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 Homeopathy: overview.

  • Clinical examination is more important than radiology in differentiating cardiac and respiratory disease.

Upper airways

  • In very obese animals, 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 and soft tissue tumors in this region are easily overlooked.
  • In normal cats 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 there may be ventral depression of the larynx.

Trachea

  • Most pathologies cause narrowing of the trachea.

Generalized narrowing

  • 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.
  • Esophagus overlying trachea does not mimic narrowing of the trachea.

Segmental narrowing of trachea

  • Tracheal collapse - collapse of extrathoracic trachea occurs during inspiration .
  • Pathological collapse is rare in cats but has been reported.
  • Stenosis.

Focal intraluminal opacity

These should be visualized in orthogonal projections to confirm lesion is inside and 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   Thorax: cranial mediastinal mass - radiograph lateral  .

Typically cranial to heart so trachea dips ventrally towards carina.

  • In severe cases carina 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.

Lower airways

  • Radiographically bronchial disease is characterized by a bronchial pattern   Lung: chronic bronchitis  feline asthma - radiograph lateral  .
  • Bronchial pattern is an abnormal visualization of the bronchi due to bronchial wall thickening.
  • Patterns may be due to mineralization, peribronchial edema, or peribronchial, mural and/or luminal cellular infiltrate.

Bronchial mineralization is thinner, more opaque and more clearly defined than peribronchial cuffing.

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.

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.
  • 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 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.

Allergic bronchial disease (feline asthma)

  • Marked peribronchial infiltrate resulting in bronchial thickening   Lung: chronic bronchitis  feline asthma - radiograph lateral  .
  • Peribronchial fibrosis may develop.

Viral bronchitis rarely causes radiographic changes.

Bronchial dygenesis

  • Abnormal cartilage development causes abnormal bronchi.
  • Bronchial changes not visible radiographically without bronchography.
  • Result 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.
  • 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.