Contributors: Fraser McConnell

 Species: Canine   |   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

  • The dorsoventral projection is often the most useful for identifiying 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 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.
  • Rare earth screens, preferably fast screens, are required for medium to large dogs.
  • Use of a grid will provide better images if the depth of tissue is > 10 cm.
    If the X-ray machine is low-powered it may not be possible to achieve a sufficiently short exposure time with a grid and image quality may have to be sacrificed.
  • 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.

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

  • 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.
  • The mediastinum contains:
    • Heart.
    • Trachea.
    • Esophagus.
    • Lymph nodes.
    • Thymus.
    • Thoracic duct.
    • Phrenic and vagus nerves.
    • Major vessels.
  • The cranial mediastinum is normally 1-1.5 x the width of the spine (but may be 2 x the width in fat dogs).
  • It is wider in brachycephalic dogs which often show bowing of the trachea and esophagus to the right of midline on the DV projection.
  • The thymus in young dogs (< 6 months) is best visualized on the DV projection and extends caudolaterally to the left of the cardiac silhoutte.
  • The mediastinum divides the thorax into left and right sides.
  • It is normally fragile but is in loose folds so can expand before rupture. There is debate as to whether it is fenestrated in the normal dog.
  • The only mediastinal structures that are normally visible are the thymus in young dogs, heart, trachea, aorta and caudal vena cava Thorax normal - radiograph lateral.
  • The cranial vena cava, lymph nodes, brachycephalic trunk and subclavian artery give a combined appearance of a soft tissue band 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 3rd and 4th lumbar vertebrae 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 foramen is situated to the right of midline within the central tendon.
  • The esophageal hiatus is first 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 dogs which should be evenly spaced Thorax normal - 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 Esophagus foreign body - radiograph lateral , megaesophagus Esophagus megaesophagus - radiograph lateral , 'masses' and lymph node enlargement Mediastinum presternal lymphadenopathy - radiograph lateral cause increased density and widening of the mediastinum.
  • Mediastinitis Mediastinal disease and hemorrhage may result in free fluid in the mediastinum.
  • Air in the mediastinum Thorax pneumomediastinum - radiograph lateral may be due to a perforated esophagus Esophagus: perforation , air tracking from a pharyngeal injury Pharynx: stick injury , ruptured trachea or bronchus, 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 retraction of the cranial lung lobe (easily assessed on DV) and widening of the cranial mediastinum.
  • 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' 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.
  • Very large masses can cause caudal displacement of the heart ’ caudal shift of the carina from the normal 5th intercostal space.

Pleural space

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

Pleural fluid

  • Free pleural air results in lung lobe collapse.
  • In old animals, particularly if there has been previous plural or pulmonary disease, there may be thickening of the pleura - this must be differentiated from pleural fluid.
  • Pleural fluid typically results in scalloping of the lung lobes (displacement of the lobes from the edges of the thoracic cavity).
  • Pleural fluid typically results in widening of the pleural tissues 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 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 tissue between left caudal and accessory lobe widest caudal lobe pulmonary parenchyma better visualized and pulmonary consolidation in hilar region on DV projection.
  • Better visualization of cranial mediastinum, cranial lung lobes and heart on VD.
  • Central area of diaphragm indistinct and poor visualization of and caudal lung lobe.
  • Ultrasonography very senstive for detection of small amounts of pleural fluid.
    100 ml fluid is visible on lateral and VD in medium size dog.

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 thorax.
  • Often due to trauma, therefore examine for other lesions, eg rib fractures.

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.
  • Disruption to the smooth diaphragm line may indicate diaphragm rupture Thorax ruptured diaphragm - radiograph DV 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.

Pericardial diaphragmatic hernia

  • Enlargement of cardiac silhouette due to presence of abdominal organs.
    If gas filled bowel loops present diagnosis can be made from plain radiographs.
  • Ultrasound very useful for 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 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 so a higher exposure with reduced kV and increased mAs 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 fracture.
  • Increased intercostal space may result from torn intercostal muscle, eg as result of dog bite. Do not confuse healed rib fracture with rib trauma.

Tumor

  • Usually grow into chest and form mass lesions.
  • Lysis often occurs.
  • Secondary tumors may be lytic or proliferative.

Infection

  • May be blood borne or local from chest wall or pleural space.
  • Results in bony proliferation.

Pit falls

  • 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 eosphageal dilation and should not be mistaken for megaesophagus.
  • Costochondral mineralization is often irregular and can mimic pulmonary masses.
  • Excessive mediastinal and pericardial fat can be mistaken for pleural fluid or mediastinal masses.

Additonal studies

Ultrasonography

  • Can be very useful in presence of pleural fluid for visualizing thoracic structures.

Contrast

  • Esophageal swallows can help differentiate sophageal from tracheal or pulmonary masses.