Diaphragm
The thoracic and abdominal cavities are separated by the diaphragm. The cranial surface of the diaphragm is easily visualised by its contrast to the adjacent radiolucency of the lungs. The liver and stomach project cranially against the dome of the diaphragm. These are more difficult to distinguish from the diaphragm, as they are of similar opacity. The right and left crura are the dorsal muscular parts of the diaphragm attaching to the ribs and ventral bodies of the lumbar vertebrae.

Mediastinum
The potential space formed between the serosal layers of the two pleural cavities. The mediastinum divides the thoracic cavity into two halves. Unlike the pleural cavities, it is not a closed cavity, but is continuous with the fascial planes of the neck cranially and the retroperitoneal space caudally. Therefore, air in the mediastinum can move up the neck or into the retroperitoneal space. The ventral mediastinum is very irregular in contour, due to the presence of the thymus, heart and vena cavae.
Visible structures of the mediastinum: most structures are not seen, except the air-filled trachea and tracheal bifurcation, the heart and the aorta.
The mediastinum is divided into parts:
1. Cranial mediastinum: The area in front of the heart. It contains the oesophagus, trachea, thymus (when present), vessels in front of the heart etc.
The oesophagus is generally not seen on a radiograph unless it contains swallowed air (aerophagia), because it is of similar radiopacity to the mediastinum. Air surrounding it, as in pneumomediastinum, renders it visible. Chest survey films of the oesophagus should include the cervical and thoracic portions, including the caudal pharynx and cranial abdomen.

Sail sign: An oblique soft tissue opacity seen in VD or DV views, due to the presence of the thymus in the ventral part of the cranial mediastinum. A sail sign is a normal finding in young dogs and in cats.

2. Middle part of the mediastinum: The region containing the heart, oesophagus, bifurcation of the trachea, etc.

3. Caudal part of the mediastinum: The region caudal to the heart, containing the aorta, oesophagus, caudal vena cava and vagal trunks.

Pleural reflection between the left caudal lung lobe and the right accessory lobe is seen on the left side extending from the base of the heart to the diaphragm. It forms the left side of the caudal mediastinum.

Dorsal and ventral parts of the mediastinum: Divided by the trachea and oesophagus.

Trachea:Found in the cranial mediastinum, and visible as a radiolucent line (air). It terminates over the base of the heart at the tracheal bifurcation. In a DV view, the trachea should be superimposed over the thoracic spine. The tracheal angle should be 15° to the vertebral column in a lateral view. TB, tracheal bifurcation or carina: appears as a black circular opacity above the base of the heart. The image darker than the rest of the trachea due to its greater cross-sectional area. The tracheal bifurcation is often incorrectly referred to as the carina by radiologists; the carina is actually a ridge at the base of this structure.

Primary bronchi: the two continuations of the trachea into the lungs. Usually only the air in the major bronchi is visible. The air in the caudal lobe shows better than the cranial ones, as the caudal lobes are thicker and more radiopaque. The right middle bronchus is the most ventral of the bronchi, making the right middle lung lobe the most common site for aspiration pneumonia. The second most common site is the cranial right lung lobe.

Pulmonary triad: a lobar bronchus with its associated lobar pulmonary artery and vein. Radiographically, the vessels appear as a pair of soft tissue ‘lines’ separated by a radiolucent air line (bronchus). Normally the blood vessels will be of approximately equal size. Pulmonary arteries arise from the right ventricle, which is cranially located.

Lungs
Normally not visible on radiography because they are air-filled tissues and hence highly radiolucent. The left lung has two lobes, cranial and caudal. The cranial lobe, divided into cranial and caudal parts, is considered one lobe by anatomists. Radiologically we refer to three left lung lobes; cranial and caudal, with the cranial lung lobe being divided into cranial and caudal segments. The right lung has four lobes: cranial, middle, caudal and accessory. The cranial lobe of the right lung is seen in front of the cranial lobe of the left lung in a lateral view. It appears as a defined, round, air-filled structure, which can be confusing.

Pleural Cavities
Potential rather than real spaces, because serous tension keeps the visceral pleura of the lungs in apposition with the parietal pleura of the thoracic wall. For this reason they are generally not visible radiographically.

Cupula of pleural cavity: The cranial extremity of the pleural cavity, extending past the first rib.

Skin folds: Easy to confuse with fluid lines. Skin folds can usually be traced past the boundaries of the thorax. They may also be mistaken for the boundaries of the lungs, leading to a false diagnosis of pneumothorax.

Lung Fields
Interstitium or parenchyma: The framework of the lungs, consisting of muscle tissue, alveolar cells, lymphatics, connective tissue, vessels and nerves. Normally this is a very dark air-filled network because the fine structures do not show up on the radiograph. Disease processes can increase the radiopacity of the parenchyma – the lungs appear whiter in colour.

Heart
Lateral view
1. Caudal vena cava: located caudally between the heart and the diaphragm

2. Right side heart: cranial part of the heart

3. Right atrium: upper part of the cranial heart

4. Right ventricle: lower part of cranial heart

5. Pulmonary trunk or main pulmonary artery (MPA): cannot be seen on a lateral projection as it is superimposed

  *  Left heart: makes up the caudal aspect of the heart

6. Left atrium: the dorsal part of the caudal aspect of the heart

6’. Left auricle: superimposed over the middle of the heart so is not seen in lateral or VD views

7. Left ventricle: makes up the ventral part of the caudal aspect of the heart and the apex

8. Aortic arch: leaves the base of the heart

9. Descending aorta: travels caudally high in the thorax against the vertebral bodies to the abdomen. Clearly visible.

10. Cranial waist: located at the junction between the cranial vena cava and the right atrium. Enlargement of the aortic arch, right auricular appendage or pulmonary trunk could lead to disappearance of the cranial cardiac waist.

11. Caudal waist: the coronary or atrioventricular groove between the left atrium and left ventricle.

Cat thorax
Similar to the dog, but more triangular and more lucent than that of the dog. The heart is also somewhat more upright in the lateral view.

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