Contributors: Fraser McConnell

 Species: Feline   |   Classification: Miscellaneous

Introduction

Overview

  • See radiography: thorax Radiography: thorax for details of positioning technique.
  • Radiography allows assessment of lung expansion and density and the thoracic wall.

Radiographic considerations

  • Tabletop technique using high detail films and screens maximizes lung detail. The dorsoventral projection is often the most useful for identifying pathology involving the chest wall, and pleural and mediastinal spaces.
  • The lateral projection is often used as a sole projection for thoracic radiography but this in fact provides poor detail of the dependent lung.
  • Orthogonal views should be taken to locate the 3-dimensional position of a lesion.
  • Oblique views may be required to highlight or skyline a lesion - particularly chest wall swellings where an oblique is useful to skyline the suspected lesion.
  • The lung fields provide an inherent contrast within the thorax - A high KVp and 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.

  • Exposure is normally made at the point of maximal inspiration to provide the most contrast within the thorax but this will depend on the lesion under examination.

Restraint

  • Examination is normally performed under sedation.
  • Particular care is required with dyspneic animals.

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.

Chemical restraint (especially general anesthesia) may cause hypostatic congestion of the lungs and generalized megaesophagus.

Indications

  • Dyspnea.
  • Trauma.
  • Evaluation of masses adjacent to the chest wall.
  • As part of minimum database in the investigation of many medical conditions especially FUO, neoplasia, ascites.
  • Regurgitation.

Ultrasonography often more useful than radiography in the presence of pleural fluid.

Radiographic anatomy

  • The thoracic structures include the:
    • Mediastinum and particularly its contents, eg heart.
    • Pleural space.
    • Diaphragm.
    • Sternum.
    • Ribs.
    • Lungs   Radiology: lungs  .

Mediastinum

  • The mediastinum is the potential space located between the two layers of parietal pleura. The mediastinum extends from the thoracic inlet to the diaphragm and runs obliquely from the midline dorsally to right of the midline cranially.

    Cranioventral mediastinal reflection is difficult to see in cats as it is relatively narrow.

  • The mediastinum contains:
  • The cranial mediastinum is normally the same width as the spine Thorax: normal - radiograph DV .
  • The thymus in young cats is best visualized on the DV projection and extends caudolaterally to the left of the cardiac silhouette in the cranioventral mediastinum.
  • The mediastinum divides the thorax into left and right sides.
  • It is normally fragile but is in loose folds so can expand before rupture.
  • The only mediastinal structures that are normally visible are the thymus in young cats, heart, trachea, aorta and caudal vena cava.
  • The cranial vena cava, lymph nodes, brachycephalic trunk and subclavian artery give a combined appearance of a soft tissue zone running ventral to the trachea.
  • The esophagus is visible if air (dyspnea or sedation) or fluid-filled.
  • The mediastinum communicates with the cervical soft tissues via fascial planes and with the retroperitoneum via the aorta.

Pleural space

  • The pleural space is a potential space between the parietal pleura covering the chest wall and visceral pleura covering the surface of the lungs.
  • A thin film of fluid coats the pleura.
  • The right pleural cavity is larger than the left and there is no communication between the two.
  • The visceral (pulmonary) pleura covers the surfaces of the lungs and extends between the lobar divisions at the interlobar fissures to the mainstem bronchi.

Diaphragm

  • The diaphragm is a muscular tendinous sheet with a fibrous central tendon.
  • The surface of the diaphragm on the thoracic side is covered by the parietal pleura (costal pleura).
  • The diaphragm inserts, via 2 tendons, onto the ventral surface of the lumbar vertebrae (L3 and L4) via the left and right crus.
  • There are 3 openings in the diaphragm that allow the caudal vena cava, esophagus, aorta, azygous vein, thoracic duct and vagus nerve to cross it.
  • The caudal vena cava foramen is situated to the right of midline within the central tendon.
  • The esophageal hiatus is ventral to the aorta and slightly to the left of midline.
  • The aortic hiatus is immediately ventral to the spine and midline.

Ribs

  • There are 13 pairs of ribs in most cats which should be evenly spaced Thorax: normal 02 - radiograph lateral .
  • The ribs of T1-T10 articulate over the intervertebral disk at the cranial edge of the corresponding vertebral body and caudal edge of the cranial vertebral body.
  • The ribs of T11-T13 only insert at corresponding vertebral body.
  • Only the first 9 ribs are attached to the sternum via the costal cartilages.
  • The 10-12th costal cartilages form the costal arch.
  • Calcification of costal cartilages increases with age.

Interpretation

  • Care should be taken to assess all the thoracic contents carefully for concurrent disease.

Often one of the important differentiators between pulmonary, cardiac disease or structures outside these.

Mediastinum

  • The position of the mediastinum on the DV or VD projection is very helpful in differentiating pulmonary masses from consolidation or atelectasis:
    • In atelectasis   →   mediastinum moves towards the area of increased lung opacity.
    • With expansile lung masses the mediastinum moves away from the opacity.
    • If lung consolidation is present   →   mediastinum stays in midline.
  • It is convenient to divide the mediastinum into cranial, middle, caudal and dorsal and ventral sections for interpretation.
  • The mediastinum is normally visible on the radiograph because it contains the esophagus and perihilar lymph nodes, fat and major blood vessels.
  • Esophageal foreign bodies, megaesophagus, 'masses' and lymph node enlargement cause increased density and widening of the mediastinum.
  • Mediastinitis and hemorrhage may result in free fluid in the mediastinum Mediastinitis: pleural effusion - radiograph lateral .
  • Air in the mediastinum may be due to a perforated esophagus Esophagus: perforation , air migrating from a pharyngeal injury or jugular venepuncture , ruptured trachea or bronchus or transtracheal wash, cervical injury through fascial planes or very rarely, infection with gas-forming organisms in the mediastinum.

Pneumomediastinum may progress to pneumothorax but the converse CANNOT happen.

Ultrasonography can be very useful for examination of mediastinal structures.

Mediastinal abnormalities
Cranioventral mediastinal masses

  • Result in caudal displacement of the cranial lung lobe Thorax: mediastinal fluid - ultrasound  (easily assessed on DV) and widening of the cranial mediastinum Thorax: cranial mediastinal mass - radiograph DV  .
  • The cranial lung lobes should normally extend to the level of the 1st intercostal space.
  • On lateral projection mediastinal masses or fluid may show ' border effacement' or positive silhouetting with the cardiac silhouette (appear as a single soft tissue opacity).
  • Cranial mediastinal masses often result in elevation of the trachea towards the thoracic spine.
  • The elevation is usually cranial to the heart so the trachea appears kinked dorsally and dips ventrally towards the carina. Right or left deviation of the trachea may be visible on the orthogonal view.
  • Very large masses can cause caudal displacement of the heart   →   caudal shift of the carina from the normal 5th intercostal space.
  • Fluid can be detected by ultrasound Thorax: mediastinal fluid - ultrasound .

Pleural space

  • The pleural space is a 'potential' space in the normal animal.

Pleural fluid

  • Free pleural fluid results in lung lobe collapse Thorax: pleural effusion - radiograph DV .
  • In old animals, particularly if there has been previous pleural or pulmonary disease, there may be thickening of the pleura - this must be differentiated from pleural fluid.
  • Pleural fluid typically results in retraction and leafing ('scalloping') of the lung lobes (displacement of the lobes from the edges of the thoracic cavity).
  • Pleural fluid typically results in widening of the potential pleural spaces which form a triangle shape as they contact the body wall.

Only visible if x-ray beam is tangential to fluid.

  • There is often accumulation of fluid in the costophrenic angle on the DV/VD projection which is only visualized when it contains fluid or gas.
  • Effacement of fluid with mediastinum, heart, lateral aspect of diaphragm results in widening of interlobar fissure between left caudal and accessory lobe. Caudal lobe pulmonary parenchyma and pulmonary consolidation are better visualized in hilar region on DV projection.
  • Better visualization of cranial mediastinum, cranial lung lobes and heart on VD projection.
  • Central area of diaphragm indistinct and poor visualization of cranial and caudal lung lobes.

Ultrasonography is very sensitive for detecting small amounts of pleural fluid.

Pneumothorax

  • Separation of lung lobes from ventral and dorsal thoracic wall on lateral projection.
  • Separation of heart from ventral body wall on lateral projection.
  • Lungs separated from lateral body wall on DV projection .
  • Absence of pulmonary vessels extending to periphery of thoracic cavity (may require bright illumination to see peripheral vasculature).

Often due to trauma therefore examine for other lesions, eg rib fracture.

Diaphragm

  • The diaphragm should appear as a continuous line extending from the ventral body wall to the ventral aspect of the thoracic or lumbar spine.
  • The point where the diaphragm contacts the spine is dependent on the degree of inspiration. The lumbodiaphragmatic recess on lateral view is usually at T13.
  • Disruption to the smooth diaphragm line may indicate diaphragm rupture Thorax: ruptured diaphragm - radiograph lateral .
  • Rupture of the diaphragm is often associated with the presence of pleural fluid.
  • If masses in the pleural space or the caudal lung lobes impinge on the diaphragm it may be difficult to see if they are attached to the diaphragm.

Peritoneopericardial diaphragmatic hernia

  • Enlargement of cardiac silhouette due to presence of abdominal organs Thorax: pericardioperitoneal hernia - radiograph DV . Pericardial contour may be irregularly shaped.
  • Dorsal peritoneopericardial mesothelial remnant may be visible as a band of soft tissue running from heart to diaphragm ventral to caudal vena cava.

If gas filled bowel loops present diagnosis can be made from plain radiographs.

Ultrasound can be very helpful in making diagnosis.

Ribs

  • The appearance of the costal cartilages varies with age.
  • In young animals they have a uniform stippled appearance.
  • The cartilage becomes mineralized with age.
  • Mineralization starts caudally and progresses cranially.
  • At the costochondral junction in old animals there may be large irregular whorls of ossification which may mimic pathology.
  • The ribs bound the thoracic contents and pathology is easily overlooked, particularly on lateral projections and if the radiograph is underexposed.
  • A well positioned radiograph with no rotation facilitates a screening examination for rib lesions as alterations to the normal contour stand out.

It may be helpful to turn the radiograph upside down or rotate through 90° to help evaluate the chest wall. When reviewing the radiograph the correct way up the chest wall is often overlooked.

  • An oblique tangential view highlighting the affected rib is often useful for detailed interpretation once a lesion has been identified.
  • Masses are often only visible if the radiograph is taken centered on the lesion.
  • Radiographs of the thorax are often under-exposed for the ribs due to high relative kVp. A reduced kV and increased mAs technique may help to improve visualization of rib pathology.

Lesion-orientated obliques are often very helpful in showing the lesions.

Fractures

  • Usually the result of trauma.
  • Asymmetric chest wall and uneven rib spacing should prompt close examination for fractures.

Tumor

  • Usually grow into the chest and form mass lesions compressing the lungs.
  • Lysis of associated ribs may occur.
  • Metastatic tumors may be lytic or proliferative.

Osteomyelitis

  • Usually manifests as proliferative lesions.
  • May be blood bourne or locally spread from chest wall or pleural space.

Pitfalls

  • Nipples or skin masses overlying the lung fields may be mistaken for pulmonary metastasis.
  • Forelimbs should be pulled clear of the thorax so that soft tissue does not overlie the chest mimicking cranioventral lung or mediastinal pathology.
  • Anesthesia or sedation will often lead to esophageal dilation and should not be mistaken for megaesophagus.
  • Secondary hypostatic congestion and atelectasis should not be mistaken for pneumonia or other pulmonary disease.
  • Costochondral mineralization is often irregular and can mimic pulmonary masses.
  • Excessive mediastinal and pericardial fat can be mistaken for pleural fluid or mediastinal masses.