Imaging of the thorax (Proceedings)

The normal thorax is well suited to radiographic evaluation because there is marked inherent contrast between the air-filled, fluid-filled, soft tissue, and bony structures that comprise the thoracic viscera and thoracic wall. As has been stated before, at least 2 orthogonal views of the thorax are required for complete and accurate interpretation. For routine evaluation of the thorax, either a right or left lateral projection, and a dorsoventral or ventrodorsal projection of the thorax are required.

Because the thoracic viscera are asymmetrically positioned, the appearance of the diaphragm changes with patient position. The appearance of the thorax will vary slightly when comparing left and right lateral thoracic radiographs, and dorsoventral and ventrodorsal radiographs because of variable magnification artifacts associated with patient position. In general, the position of the cardiac silhouette is most consistent on a right lateral projection. When attempting to characterize a pulmonary lesion, the lesion is best defined when surrounded by well-aerated lung. The down side, or dependent lung decreases in volume due to atelectasis when a patient is in recumbency for even short periods of time. Therefore, when attempting to assess a pulmonary lesion, a view which places the pulmonary region of concern in the most nondependent position is best for evaluation. In addition, thoracic radiographic images should be obtained while the animal is in the peak inspiration phase of respiration. The dorsoventral projection provides better consistency in cardiac position and usually provides a better evaluation of the caudal lung lobe vessels.

Trachea and Bronchial Tree

The trachea can be identified on a lateral radiographic view as a well-defined radiolucent tube extending from the larynx to the central region of the thoracic cavity. The trachea should have well-defined, thin, radiopaque margins, and a uniform tracheal lumen diameter. The trachea terminates at the carina, which represents the bifurcation of the major airway into the two main stem bronchi. The trachea can be seen on a dorsoventral or ventrodorsal view positioned centrally within the cervical and cranial thoracic regions ending at the carina in the central thorax. Typically, the intrathoracic trachea is positioned slightly to the right of midline on the DV or VD views. In the dog, the tracheal rings, which provide some rigidity to the tracheal wall, are incomplete dorsally. The dorsal tracheal membrane often protrudes into the tracheal lumen, particularly in obese, small breed animals. This can produce an apparent decrease in tracheal diameter on lateral projections, but can and should be differentiated from the clinically more important abnormalities, the hypoplastic trachea and the true collapsing trachea.

The main stem bronchi can be seen arising from the carina, both on the lateral and the DV or VD projections. In addition, in many instances, the major lobar bronchi can be identified coursing through their respective lung lobes. Frequently, the lobar bronchi are not well visualized, but when seen follow the path of the adjacent lobar arteries and veins. Often the definition of the bronchi is accentuated by mineralization of bronchial walls (often a clinically insignificant aging change) or due to peribronchial cellular or fluid accumulations associated with a variety of disease processes.

Pulmonary Parenchyma

In normal animals, the pulmonary parenchyma should be radiolucent but normal lung parenchyma and pulmonary vascular anatomy should still be appreciated. As mentioned before, thoracic radiographs obtained to evaluate the lungs should be obtained with the animal in full end inspiration, which provides maximal aeration of the lungs. In general, the nondependent portions of the lung will be better aerated than the dependent regions, which is in turn dependent on the position of the patient at the time the radiograph is taken. In general, the major lobar airways and the secondary and tertiary pulmonary blood vessels to the lobes should be identifiable within the lung parenchyma. The parenchyma itself should have a fine mesh-like cross-hatch radiolucent appearance. The visceral pleural margins in the normal animal should be seen. Keep in mind however that, although often the pulmonary lobar margins cannot be defined, lobar divisions are present within the thorax and may become apparent with the presence of pleural fluid or pulmonary lesions that involve a single lung lobe.

Cardiac Silhouette – Size and Position

Descriptions of the radiographic appearance of the heart are usually associated with a number of rules of thumb that people use to determine whether the size and shape of the cardiac silhouette and cardiac chambers are normal or enlarged. These rules can sometimes be of value, however be cautioned that very often, due to the wide range and variation in body size and conformation in veterinary patients, often times these rules can be inaccurate.

For small animals, the cardiac silhouette is positioned centrally within the thoracic cavity at an approximately 45 degree angle, with the apical region of the heart positioned adjacent to the sternum. In deep-chested dogs, the cardiac silhouette is positioned more vertically on the lateral projection. Conversely, in shorter, stockier breeds, the angle of the long axis of the heart may be less than 45 degrees. In addition, the long axis of the feline cardiac silhouette decreases with age.

Barrel-chested breeds (Bulldogs, Dachshunds)

      -Cardiac width 3 ½ intercostal spaces

      -Cardiac shape rounded on lateral view

      -Reduced tracheal-spinal angle

      -Greater cardiophrenic and cardiosternal contact

      -Acute lumbodiaphragmatic recess

      -Distribution of bronchovascular markings-severe overlap on lateral views

      -Rightward deviation of trachea on DV view

Deep-chested breeds (Setters, sight hounds)

      -Cardiac width 2 ½ intercostal spaces

      -Pear-shaped, upright heart on lateral view

      -Greater tracheal-spinal angle

      -Reduced cardiophrenic and cardiosternal contact

      -Obtuse lumbodiaphragmatic recess

      -Distribution of bronchovascular markings-severe overlap on lateral views

      -Round heart on DV view

Canine Cardiac Silhouette

Usually positioned between the 3rd and 8th thoracic vertebrae, with the middle of the heart base at the level of the carina near the 5th or 6th intercostal space.

Usually measures from 2.5 to 3.5 times the width of the intercostal spaces. Deep-chested dogs have cardiac silhouettes that measure closer to 2.5 intercostal spaces due to the more upright conformation of the heart. Barrel-chested dogs typically have cardiac silhouettes measuring closer to 3.5 intercostal spaces due to the more horizontal position of the heart.

The cardiac silhouette occupies approximately 70 percent of the distance from the sternum to the thoracic spine on lateral radiographs.

On the dorsoventral projection, the cardiac silhouette is approximately 60 to 65 percent of the thoracic width.

Vertebral Heart Score: 9.7 ± 0.5 thoracic vertebrae.

Feline Cardiac Silhouette

The feline cardiac silhouette on the lateral projection is usually positioned at an angle less than 45 degrees. This angle decreases with age.

     • The feline heart is typically positioned from the 4th to the 6th ribs on the lateral projection.

     • The cardiac silhouette usually measures approximately 2 intercostal spaces in width on the lateral projection.

     • On the dorsoventral projection the cardiac silhouette often represents less than 50 percent of the overall thoracic width.

     • Cardiothoracic ratio less than dogs

     • Thorax similar to deep-chested breeds

     • Vertebral Heart Score: 7.5 ± 0.3 thoracic vertebrae.

     • Age Related Changes in the Feline Cardiac Silhouette (Cats >10 years of age)

     • -Exaggerated horizontal alignment of the heart

     • -Tortuous, redundant aorta

Vertebral Heart Score (VHS)

The cardiac height (L. long axis) is measured along the thoracic vertebrae, beginning at T4. The width (S, short axis) is also measured.

L + S = VHS

Canine VHS = 9.7 ± 0.5 vertebrae

Feline VHS = 7.5 ± 0.3 vertebrae (Adapted from Buchanan JW, Büchler J. Vertebral scale system to measure canine heart size in radiographs. JAVMA 1995;206(2):194-199)

Cardiac clock face diagram and anatomical overlay for the lateral thoracic radiograph. Ao, aortic outflow tract. Rau, right auricle (appendage of right atrium). Pa, main pulmonary artery. Lpa, left pulmonary artery. Rpa, right pulmonary artery. La, left atrium. Lau, left auricle. Pc, caudal vena cava (post cava). Cv, cranial vena cava. It, internal thoracic artery/vein. Br, brachiocephalic artery. Ls, left subclavian artery. Rv, right ventricle. Lv, left ventricle. Ldc, location of interventricular septum (descending branch of the left coronary artery-never seen radiographically, shown for illustrative purposes only).

Cardiac clock face diagrams and anatomical overlays for a dog VD/DV radiograph.

The four chambers of the heart and the great vessels are shown. RA, Right atrium. RV, right ventricle. PA and MPA, main pulmonary artery. LA, left atrium and auricle. LV, left ventricle. Ao and AA, aortic outflow

Cardiac Disease

Radiographic guidelines to characterize cardiac size were included in the section on normal radiographic anatomy of the heart. Apparent generalized cardiomegaly can occur in normal animals for a variety of reasons, the most common of which are listed below:

     • Cardiac size relative to thoracic volume is greater in young animals, as compared to mature animals.

     • Shallow-chested animals may have a cardiac silhouette that appears large in comparison to total thoracic volume.

     • Athletic animals may have a cardiac size that is at the upper limits of normal.

     • Obese animals may have a significant volume of adipose tissue deposited within and around the pericardium, producing an apparent increase in the size of the cardiac silhouette. Often because the adipose is less dense than the heart, the cardiac margin can be defined within the pericardial sac. In some instances however, definition of the cardiac margin is poor.

Generalized cardiomegaly can occur radiographically when one or more chambers of the heart are enlarged. In addition, the cardiac silhouette can become enlarged with the presence of a pericardial effusion. Pericardial effusion is defined as a fluid accumulation within the pericardial sac and surrounding the heart. Because the fluid is evenly distributed throughout the pericardial sac, the cardiac silhouette takes on a very rounded, globoid appearance on both lateral and DV or VD projections.

Most often, when cardiac enlargement is present, it is due to enlargement of one or more cardiac chambers. Characteristic radiographic findings are associated with enlargement of each of the cardiac chambers.

Specific Cardiac Chamber Enlargement

Left Atrial Enlargement

On the lateral radiographic projection, the enlarged left atrium elevates the distal portion of the trachea and the carina dorsally. The left bronchus often becomes displaced dorsally. Usually, there is a straightening of the caudal cardiac margin and a loss of the caudal cardiac waist. On the dorsoventral projection, a rounded summation opacity is identified superimposed on the cardiac centrally and is produced by distension of the left atrium. This summation shadow is positioned between the two main stem bronchi, and may displace the bronchi abaxially. When marked left atrial enlargement is present, there may be a bulge of the cardiac margin at approximately the 2 o'clock to 3 o'clock position on the dorsoventral view. This finding is due to distension of the left auricular appendage.

Left Ventricular Enlargement

On the lateral projection, the caudal cardiac margin may be straighter than normal with a loss of the caudal cardiac waist. The caudal cardiac margin may also be more vertically oriented than normal. The trachea and carina may be elevated due to an overall increase in the apical to basilar length of the heart. The cardiac silhouette may be wider than normal. On the dorsoventral projection, the left ventricular margin may be rounded, and there may be a decrease in the space between the left cardiac margin and the thoracic wall.

Right Atrial Enlargement

On the lateral projection, there may be a bulge associated with the cranial margin of the cardiac silhouette and an attendant loss of the cranial cardiac waist. The trachea may be elevated dorsally, as it courses over the right atrial region of the cardiac silhouette. On the dorsoventral projection, prominence of the cardiac margin may be evident at the 9 to 11 o'clock position.

Right Ventricular Enlargement

On the lateral projection, there may be increased sternal contact of the right ventricular cardiac margin, with an associated elevation of the cardiac apex. One needs to be cautious in over-interpreting this finding however. There is usually a much greater degree of apparent sternal contact and there is typically an elevation of the cardiac apex when a left lateral projection is obtained. A right lateral view of the thorax is usually more consistent in regard to the position of the right ventricular and apical margins of the cardiac silhouette. Due to cardiac enlargement, the trachea may be dorsally elevated. There may be a rounding of the right cardiac margin and an elevation of the caudal vena cava. On the dorsoventral projection, rounding of the right cardiac margin is also evident. There may be a shift of the cardiac apex to the left. When marked right ventricular margin is present, the cardiac silhouette has a reversed "D" appearance.

Lung Disease

Radiographic evaluation of the lungs is often complex and confusing initially. One radiographic classification scheme is to describe generalized pulmonary abnormalities into one of four categories. This classification system is intended to characterize an abnormality by the subgross anatomical structures involved. The four categories generally considered are listed below.

The pulmonary arteries can be identified arising from the main pulmonary artery on the lateral projection. They are course dorsal to the lobar bronchi, while the pulmonary veins are ventral to the bronchi. The cranial and middle lung lobe arteries and veins can usually be differentiated on the lateral radiograph, but the caudal lobar vessels cannot, as they overlap one another. On the DV or VD radiographs, the lobar arteries are lateral to the major bronchi, the pulmonary veins are medial. The DV view is the best view to assess the caudal pulmonary artery and vein pairs. Tertiary branches of the pulmonary vasculature can often be seen in the larger breeds of small animal species and in the large animal species.

Schematic of cranial lobar artery, bronchus. and vein relationship on lateral and VD/DV radiographic image. Although not illustrated, the same holds true for the cranial pulmonary structures, though they can be more difficult to routinely see on a radiograph.

Vascular Pattern

The pulmonary arteries and veins are easily seen coursing through the lung parenchyma in normal thoracic studies. The vessels are larger and therefore more easily seen centrally near the hilar region, and as they taper and branch peripherally, they become more difficult to see.

Decreased Pulmonary Vessel Size

The pulmonary vessels, both arteries and veins, can appear smaller in diameter than usual due to hypovolemia. Hypovolemia can be due to a variety of abnormalities, including dehydration, blood loss, and shock. The decrease in pulmonary vascular size and number in hypovolemic animals is usually seen associated with a concurrent decrease in the size of the cardiac silhouette and the caudal vena cava.

Increased Pulmonary Vessel Size

Pulmonary Vein Enlargement

Enlargement of the pulmonary veins is often times seen with pulmonary venous congestion. This abnormality typically results from left ventricular insufficiency or failure.

Pulmonary Artery Enlargement

Pulmonary artery enlargement can occur due to obstruction of the smallest pulmonary arterial branches causing dilation of the more proximal larger arterial segments. Heartworm disease, which causes inflammation and occlusion of smaller pulmonary arteries, is one example of this finding. Pulmonary thromboembolism, or blood clots within the pulmonary arteries, are another example.

Enlargement of Pulmonary Arteries and Veins

Enlargement of both the pulmonary arteries and veins is typically associated with pulmonary over-circulation. This is usually the result of blood flow being diverted from the left side, or systemic side, of the circulation to the right side, or pulmonary side of the circulation. This results in an increased total blood flow through the blood vessels causing an apparent increase in size radiographically.

Bronchial Pattern

The trachea and major bronchi are normally seen on thoracic radiographs. Tracheal and bronchial walls are well-defined, opaque, thin-walled structures which appear as parallel linear opacities when seen in long axis, and as thin-walled circular opacities when seen end-on. Bronchial and tracheal walls will occasionally partially mineralize in older animals. This latter finding is typically not clinically significant.

A bronchial pulmonary pattern occurs when abnormal cells, fluid or fibrous tissues accumulate within or around the bronchial walls. This accumulation results in a thickening and sometimes irregularity of the bronchial margins radiographically. The bronchi are more conspicuous than normal. The circular opacities representing end-on airways will appear also as thicker walled, referred to as "donuts". The parallel linear opacities of the bronchi in long axis will appear thicker and more irregular. Because of similar terminology, the bronchial pattern may be confused with air bronchograms seen with an alveolar pattern described below. Keep in mind that the airway pattern is just that; a radiographic pattern produced by diseases that either arise from or involve the airway walls.

When an airway-oriented pattern is present, the alveoli surrounding the airways are still air-filled; therefore, the lung parenchyma surrounding the airway margins is still radiolucent. As a result, both the internal and external margin of the airway is clearly visible, as are the pulmonary blood vessels.

Interstitial Pattern

Interstitial patterns are very common and associated with a wide variety of pulmonary diseases. In general, the interstitial pattern can be subdivided into two major types; the unstructured interstitial pattern, and the nodular interstitial pattern.

Unstructured Interstitial Pattern

The unstructured interstitial pulmonary pattern is characterized radiographically by an overall increase in pulmonary opacity. This is produced by a summation of ill-defined linear or ill-defined opacities that arise from the pulmonary interstitium. This ill-defined increase in pulmonary opacity often times obscures the margins of the major airways and pulmonary vasculature coursing through the lung parenchyma. Think of this as internal silhouetting of pulmonary blood vessels by whatever pathology is present within the interstitial space. This pulmonary pattern can be caused by a variety of categories of disease, including inflammation, neoplasia, edema, and fibrosis.

Nodular Interstitial Pattern

As the name implies, the nodular interstitial pattern is produced by a generalized increase in pulmonary opacity due to the presence of multiple small soft tissue opaque nodules arising from the pulmonary interstitium. These nodules may be very small (miliary), or up to a few millimeters in diameter. In addition, nodules may either be well-defined or ill-defined, depending on the cause of the nodules. As with the unstructured interstitial pattern, the increased pulmonary opacity results in a diminished definition of airway and pulmonary vascular margins. The most common causes for nodular interstitial pulmonary patterns are diffuse and widespread pulmonary neoplasia, abscesses, or granulomas.

Alveolar Pattern

The alveolar pattern is caused by accumulation of cells or fluid within the alveoli of the lung. Keep in mind that the radiolucency of the normal lung parenchyma is due to the presence of air within the alveoli. When the alveoli fill with fluid or cells, the lung will take on a soft tissue opacity. When this occurs, there is complete internal silhouetting of pulmonary blood vessels; they can no longer be seen. Another term for alveolar pattern is CONSOLIDATION. When this occurs, portions of the lung affected are airless and therefore nonfunctional.

In addition to the marked increase in pulmonary opacity, a radiographic hallmark of the alveolar pattern is the presence of air bronchograms. Air bronchograms are produced by an air-filled bronchus being surrounded by fluid or cellular-filled alveoli. Radiographically, this appears as radiolucent bronchi, and in most instances the bronchial tree arborization can be seen. Air bronchograms may appear as circular radiolucencies when seen end-on. If disease progresses further, even the larger bronchi can bedcome infiltrated and airless. In this case, there is no air bronchogram; the lung is simply soft tissue opaque, completely consolidated.

The alveolar pattern must be differentiated from a bronchial pattern. Consolidation, the hallmark of an alveolar pattern, is not present with bronchial disease unless concurrent pathology is present (e.g., ventral lung consolidation (pneumonia) secondary to bronchiectasis). With consolidated lung, you cannot see the pulmonary blood vessels (by definition); with bronchial disease, the pulmonary blood vessels are still seen, at least to some degree.

Normal terminal lung schematic. Interstitial lung disease; thickening of the interstitial tissues.

Alveolar lung disease Lung patterns in summary

Pleural Disease

The pleural space is in general only considered a potential space. That is, under normal circumstances, there is very little actual volume and pleural space contains only a few mils of pleural fluid for lubrication. Abnormalities of the pleural space most commonly involve abnormal accumulations of air or fluid, or the presence of mass lesions.

Pneumothorax

A pneumothorax is defined as free air trapped within the pleural space. Major causes for pneumothorax include the following:

     • Injury or disease of one or more lung lobes, resulting in intrapulmonary gas escaping into the pleural space.

     • Injury to the trachea or bronchi, in which the mediastinum is also ruptured. This will result in pneumothorax, as well as pneumomediastinum, or air trapping within the potential space of the mediastinum produced by the parietal pleural reflections.

     • Trading chest wounds which lead to an open pneumothorax, or a pneumothorax in which a direct conduit between the pleural space and the external environment is present.

     • Rarely, the intrathoracic esophagus can be damaged with ingested air accumulating within the pleural space. Disruption of the surrounding mediastinal margins is also required for this rare cause of pneumothorax to occur.

Radiographic Findings

When air fills the pleural space, the visceral pleura of the lung lobes is retracted away from the thoracic wall. Radiographically, the lung lobe margins can be identified peripherally. Lung contours consistent with the demarcations of the various lung lobes are often seen. When a pneumothorax is severe, significant lung volume reduction occurs, and this is associated with an increase in pulmonary opacity.

In mild or moderate pneumothorax, the lung margins are most easily detected in the caudodorsal region on a lateral projection, and in the caudolateral recesses of the thorax on a DV or VD projection. Sometimes fissures representing the demarcation between lung lobes are also evident. A second radiographic finding that is commonly present with pneumothorax is an elevation of the cardiac margin away from the sternum. This is associated with a loss of pulmonary volume and resulting passive displacement of the heart away from the sternum. Other radiographic findings that may be seen secondary to pneumothorax are:

     • Pulmonary lesions that may be the source of the escaping air.

     • Tracheal or esophageal lesions which may be the source of pleural air.

     • Marked pulmonary atelectasis with associated increased pulmonary opacity.

     • A shift of the mediastinum and cardiac silhouette toward the side of the thorax with the greatest degree of pulmonary volume loss.

     • A loss of bronchial or vascular pulmonary markings in the periphery of the thoracic space.

Pleural Effusion

Pleural effusion, or an accumulation of fluid within the pleural space, can result from a wide variety of diseases and the character of pleural fluid may be that of a transudate, an exudate, or frank blood. Some of the more common causes of pleural effusion are heart failure, chylothorax (leakage of lymphatic fluid into the pleural space), pyothorax, and hemothorax.

Radiographic Findings

Radiographically, pleural effusion causes an overall increase in opacity within the thoracic cavity. Fluid accumulation within the pleural space displaces lung volume, with resultant pulmonary volume loss and increased pulmonary opacity. Generally, aerated lung lobe margins can be identified within the pleural fluid. Often lobar margins are well defined and separated by the fluid. When a moderate volume of effusion is present, positioning the patient in dorsal recumbency for a ventrodorsal (VD) thoracic image provides a better evaluation of remaining aerated lung and better visualization of the heart. This is because the lungs "float" to cradle the heart. HOWEVER, a VD view can mask the presence of small or even moderate pleural effusions. On a dorsoventral view (DV), the lungs "FLOAT AWAY" from the heart, thus the fluid silhouettes with the heart margin. It is this silhouetting of the heart margin (or border effacement) that allows detection of smaller pleural effusions on a DV view when compared to a VD view. The DV view also allows a more nature and consistent placement of the heart, thus allowing easier assessment for size and shape. The pleural fluid can also cause complete collapse of lung lobes.

Cross-sectional illustrations of the thorax depicting the differences in pleural fluid distribution between a dorsoventral image (left, patient in sternal recumbency) and ventrodorsal image (right, patient in dorsal recumbency). Note on the DV view the fluid surrounds the heart and therefore silhouettes with it; on the VD image, the lungs "cradle" the heart and therefore more of the heart can be seen on the radiograph.