Thoracic Radiography

When performing thoracic radiography it is important to include all of the thorax in the film, including part of the neck, part of the abdomen, the vertebral bodies and the sternun. Thoracic films are taken at the peak of full inspiration. The chest cavity is expanded during inspiration, resulting in less contact between the diaphragm and the heart. In expiratory films the lungs are deflated, appearing more opaque which may suggest pathological change. Indications for thoracic radiography include some cardiac problems, dyspnoea, and abnormal lung sounds.

Viewing the radiograph: Place the film so that the right side of the animal is to your left for both VD and DV views.

Lateral views:Place on the viewer so that the a cranial side of the animal is to the left.  R or L markers on a lateral film of the trunk indicate the side on which the animal was lying when the film was taken (right or left lateral recumbency.). In VD/DV films, these markers orientate the right and left sides of the animal. A quick check of the markers is the apex of the heart, located to the left in the VD or DV views.

Evaluating and reading thoracic films: The area approach can be done diagonally from the upper caudal to lower cranial or starting at the centre and moving outward, or from the outside to the inside. The organ approach looks at individual organs and organ systems systematically.

A) Borders of thorax:
chest wall
thoracic spine
presence of thoracic limbs in the film

B)  Heart:

C) Great vessels:  shape, size, opacity
Caudal vena cava

D) Trachea:  position and diameter

E) Tracheal carina

F) Bronchial tree

G) Pulmonary vessel: size, shape, position, opacity

H) Mediastinum: width, lateral shift and any abnormal density or masses

I) Lungs: increased or decreased opacities

J) Pleural cavities: only seen when they contain fluid or air

Pulmonary vessels: The pulmonary artery and veins should be equal in size.  The artery to the cranial lung lobe is located dorsal and cranial to the vein in the lateral view. In the DV or VD views the pulmonary arteries are at the 4 and 8 o’clock positions and are lateral to the adjacent pulmonary veins.  The bronchi lie between the artery and vein.

Lung Fields:
Basically, lungs are evaluated for the presence of fluid and air. Radiographically, air is black. A normal lung field has a black background with soft tissue structures (vessels) passing through it. Lungs must be evaluated for an increase or decrease in the radiopacity of the parenchyma. These changes can be generalised or localised, diffuse or focal. The two sides of the thorax must be compared in a DV view; they should be of equal opacity. There are four lobes on the right - cranial, middle, caudal, and accessory.  There are two left lung lobes - cranial and caudal.  The left cranial lobe is further divided into cranial and caudal components.
If an animal is dyspnoeic, do not compromise respiration further by taking a VD view; instead take a DV.

Heart:  The shadow is actually the pericardium and the heart should be termed the cardiac silhouette.  The cardiac silhouette fills about 3/4 of the thorax in a lateral view and 2/3 in a DV/VD view. Minimal heart enlargement will be hard to determine. The heart appears as a solid globe, so the boundaries are evaluated rather than the chambers. The cardiac silhouette should be less than half the width of the VD thorax at the level of the ninth rib.

Chambers of the heart: Can be imagined on a lateral view by drawing two crossing perpendicular lines. The first one follows the axis of the heart and passes from the heart base at the tracheal bifurcation through the apex. This gives a rough estimate of the location of the chambers. The clock-face analogy may be useful, but in cardiac enlargement it can be deceptive. This is because the right ventricle wraps almost completely around the left ventricle, except for the caudal side. In right heart enlargement it can even project caudal to the left ventricle. However the analogy is still useful in a 3D interpretation of the radiographs of a normal heart. The locations of the pulmonary trunk, aortic arch, apex and left auricle are important.

Clock-face Analogy of the Heart
VD view
11-1 o’clock: aortic arch
1-2: pulmonary trunk (PMA)
2-3: left atrial appendage (left auricle)
2-6: left ventricle
5: apex
6-9: right ventricle
9-11: right atrium
The left atrium is located in the centre of the cardiac silhouette at the level of the tracheal bifurcation.

Lateral view
1-2 & 9: cranial and caudal ‘waist’ of heart
11-12: aortic arch
1-2: left atrium
2-6: left ventricle
5: apex
6-9: right ventricle
9-11: right atrium

DV/VD view:
1. Cranial vena cava: not seen because of all the other structures in the cranial mediastinum
2. Caudal vena cava: extends from the right side of the heart to the diaphragm
3. Right atrium: on the cranial heart
4. Right ventricle: comprises cardiac silhouette from the apex along the right side
5. Pulmonary trunk: (called the main pulmonary artery by radiologists) leaves the left cranial side of the heart at the 1-2 o’clock position
6. Left atrium: summated over the caudal heart directly above the left ventricle. Slightly caudal to the tracheal bifurcation
6’. Left auricle: superimposed over the middle of the heart so not visible unless it is enlarged, when it projects out at the 2-3 o’clock position.
7. Left ventricle: comprises the caudal half of the left silhouette of the heart.
7’. Apex: part of the left ventricle at the 5 o’clock position. Angled to the left in the DV/VD view.
8. Aortic arch: not seen in the DV view since it is summated over the cranial mediastinum. It is located at the 11-1 o’clock position.
10. Descending aorta: a line representing the left edge of the aorta is all that is seen due to overlapping densities. This edge should be seen in a good radiograph.

Positioning terms & techniques