Normal radiographic anatomy

The thoracic wall is composed of bones, soft tissues, and fat. The bony thorax includes the ribs, vertebrae, and sternebrae. Soft‐tissue structures are skin, muscle, blood vessels, nerves, and pleura.

The average number of ribs in both dogs and cats is 26 or (13 pair). The head of each rib articulates with the cranial aspect of the same numbered thoracic vertebra (Figure 3.6). For example, the third rib attaches to the cranial end of the third thoracic (T3) vertebra. The ribs in chondrodystrophic dogs (e.g., Bassett Hound, Dachshund) normally curve inward and indent the lungs at the costochondral junctions (Figure 3.12). Inward curving ribs may be mistaken for pleural fluid.

The gaps between ribs are the intercostal spaces. Comparing the right and left sides, the intercostal spaces (ICS) are relatively symmetrical; however, the symmetry may temporarily be lost due to oblique positioning or muscular contractions. Each ICS is numbered according to its cranial rib (e.g., the third ICS is caudal to the third rib).

The sternum contains 8–9 sternebrae, which are similar in appearance to the vertebrae. The first sternebra is the manubrium, which is the longest. The last sternebra is the xyphoid, which can vary significantly in size and shape. Located between the sternebrae are cartilaginous discs. These discs are similar in appearance and function to intervertebral discs. Two or more sternebrae may be fused or malaligned due to a congenital or developmental abnormality. Congenital abnormalities usually are asymptomatic and may be differentiated from pathology of the sternum by the lack of soft tissue swelling and absence of a periosteal response.

Costal cartilages extend from the distal end of each rib to the sternum. Cartilages from the first 8 ribs join the intersternebral discs. Cartilages from the caudal ribs attach at the xyphoid or merge together. Mineralization of the costal cartilages normally begins at a few months of age. Mineralization has been described as heterogenous, mottled or granular and frequently becomes more sclerotic and irregular over time (Figure 3.7). Mineralized costal cartilages can become quite large, especially in chondrodystrophic and large breed dogs. Large mineralizations may be mistaken for lesions in the lungs or thoracic wall. Costal cartilage mineralization sometimes occurs in short segments, appearing as transverse lines of increased and decreased opacity. These lines may be mistaken for fractures. Carefully inspect the costal cartilages when interpreting radiographic findings to avoid an erroneous diagnosis.

Thickened thoracic wall

The normal thickness (width) of the thoracic wall varies with the patient body condition. The wall is thicker in obese and heavily muscled patients and thinner in lean animals. Abnormal thickening usually is extracostal (outside the ribs) and may be unilateral or bilateral, focal or diffuse. Focal thickening is due to a mass (see thoracic wall mass below), whereas diffuse thickening may result from subcutaneous accumulation of fat, gas, or fluid (Figure 3.8).

Subcutaneous fat may represent obesity or a large lipoma. Obesity is bilateral and lipomas tend to be unilateral. The extracostal muscles may be outlined and displaced by fat, appearing as thin, curved, soft tissue opacity lines that resemble the shapes of the ribs.

Subcutaneous emphysema may be unilateral or bilateral, typically producing a mixed pattern of gas and soft tissue opacities. Emphysema may result from a penetrating wound or migration of gas from a ruptured trachea or pneumomediastinum. Superimposition of subcutaneous gas may compromise evaluation of the thoracic cavity and may mask or mimic free gas in the mediastinum or pleural space.

Subcutaneous fluid may be unilateral or bilateral and can obscure the margins of the extracostal muscles. Fluid may represent edema, inflammation, hemorrhage, or it may be iatrogenic (i.e., fluid therapy). Summation of subcutaneous fluid may increase the opacity of the thoracic cavity.

Thoracic wall mass

Masses in the thoracic wall can involve any osseous or soft tissue structure. They produce localized, defined areas of thickening. The margins of a mass may be well‐defined or indistinct, depending on the opacity interface. Masses that are walled‐off and surrounded by a different opacity may be well‐visualized, provided they are large enough to be distinguished. Adjacent inflammation or other fluid accumulation can obscure the margins of soft tissue opacity masses.

A mass in the thoracic wall may not be physically palpable or radiographically visible until quite large. Large masses can displace the adjacent ribs and widen the intercostal spaces. The ribs near a mass should be carefully examined for evidence of a periosteal response, osteolysis, and/or expansile remodeling.

A thoracic wall mass that grows inward, toward the thoracic cavity, can produce a bulge that displaces the adjacent lung. The bulge creates an extra‐pleural sign. The extra‐pleural sign helps differentiate masses that originate outside the pleura from those that originate in the lung or pleural space (Figure 3.9). The edges of an extra‐pleural mass gradually taper toward the inner chest wall, forming angles greater than 90°. Pulmonary and pleural masses form acute angles (less than 90°) with the chest wall. Pleural effusion is absent with extra‐pleural masses (unless the pleura is damaged), but fluid is common with masses that originate in the pleural space.

The opacity of a thoracic wall mass provides clues to its etiology. Gas opacity may result from a penetrating wound or infection with a gas‐producing bacteria. Herniation of a gas‐filled GI structure can produce a defined area of gas opacity. A fat opacity mass may represent a lipoma (Figure 3.10) or contents in a hernia. Lipomas can become very large, often with well‐defined margins. Indistinct margins suggest fluid adjacent to the lipoma (e.g., inflammation, edema) or a dissecting lipoma (or liposarcoma). Soft tissue opacity masses include tumors, abscesses, granulomas, seromas, and hematomas. Mineral opacity in a mass may be foreign material or dystrophic mineralization, the latter occurring in long‐standing abscesses, granulomas, or tumors.

A paracostal hernia can create an extra‐pleural mass that is mixed in opacity due to the presence of displaced viscera, gas‐filled bowel, and fat (Figure 3.26).

Abnormal bony thorax

Congenital skeletal abnormalities are common in dogs and cats. Most are benign and asymptomatic. It is important to recognize benign conditions because they can alter the shape of the thorax, distort the positions of intra‐thoracic structures, and vary the locations of landmarks used during surgical and advanced imaging procedures. Benign conditions often can be differentiated from pathology by a lack of soft tissue swelling and an absence of active bone remodeling. Poor patient positioning also can alter the appearance of the bony thorax, such as making the curvature of the spine appear abnormal and distorting the sizes and shapes of the ribs.

Normal radiographic anatomy

The diaphragm physically separates the abdominal and thoracic cavities and provides nearly 50% of the force needed for respiration. It consists of a cupula and two crura (Figure 3.22).

The cupula is the convex, dome‐shaped part of the diaphragm that bulges centrally and ventrally into the thoracic cavity. It is anchored to the sternum and ribs by fibrous tissue and connected to the spine by the crura.

The crura are the two muscular “legs” of the diaphragm (singular crus). The word crus is used to describe an anatomical structure that resembles a leg. Crus is the name given to the distal part of a pelvic limb, the part between the stifle and the tarsus. Each diaphragmatic crus extends dorsally and caudally from the cupula to an attachment on the L3‐4 lumbar vertebrae. The ventral margins of L3 and L4 frequently are less distinct than the adjacent vertebral bodies, a normal finding that may be mistaken for a periosteal response.

There are three natural openings in the diaphragm. Each opening, or hiatus, allows certain structures to normally pass through, but also serves as a potential site for a diaphragmatic hernia. Ventrally and to the right of midline is the caval hiatus, through which the caudal vena cava and lymphatic vessels pass. The lymphatics carry fluid in one direction, from the abdominal cavity to the sternal lymph node in the thoracic cavity. Centrally and left of midline is the esophageal hiatus, through which the esophagus and vagal nerves pass. Dorsally and near midline is the aortic hiatus, through which the aorta, azygos vein, and thoracic duct pass.

The costophrenic recesses are the well‐defined, wedge‐shaped junctions between the diaphragmatic crura and the thoracic wall (Figure 3.22). The appearance of these recesses may be helpful to assess the degree of lung inflation and to detect pleural fluid. Fluid that collects in a costophrenic recess makes that recess appear more rounded, more opaque, and less distinct.

The cranial or “thoracic” margin of the diaphragm tends to be sharply‐defined due to the opacity interface with lung air. The caudal or “peritoneal” margin often is only partially visible because it blends with the liver and stomach. Ventrally, the caudal margin of the diaphragm may be visible when adjacent to fat in the falciform ligament.

The position of the diaphragm is determined by the degree of lung inflation and the forces exerted by the abdominal contents, both of which are affected by patient position and gravity. When the patient is recumbent, the dependent (down) lung partially collapses and the weight of the abdominal viscera push the down side of the diaphragm cranially. The greater the abdominal contents, the greater the push against the diaphragm. In left lateral recumbency (left lateral view), the left crus commonly is seen cranial to the right crus. In right lateral recumbency, the right crus often is cranial to the left (Figure 3.23). The left crus often can be identified by the nearby gas in the fundus of the stomach (the stomach is located just caudal to the left crus). Note: a full stomach can push the left crus cranial to the right, even with the patient in right lateral recumbency. The right crus can be identified by the caudal vena cava, which emerges from the right side of the diaphragm.

Another difference between right and left lateral views is that the diaphragmatic crura tend to cross or overlap when in left lateral recumbency and they tend to be more parallel in right lateral recumbency (Figure 3.23).

In dorsal recumbency (VD view), both crura are dependent (down) and both move cranially. The diaphragm typically appears as three humps in a VD view: right crus, cupula, and left crus. In ventral recumbency (DV view), the cupula is down and moves cranially. The diaphragm typically appears as a single hump in a DV view because the cupula obscures both crura (Figure 3.24). In some DV views, the cupula moves so far cranially that it contacts the heart. The dome of the diaphragm may appear indented by the cardiac silhouette and its margin may be indistinct. The effects of patient orientation and gravity on the position of the diaphragm tend to be more evident in large dogs than in cats and small dogs.

Abnormal position of the diaphragm

A diaphragm may not appear as expected due to an error in radiographic technique. Errors that can distort the apparent position of the diaphragm inlcude rotation of the patient and off‐centering of the x‐ray beam. If the appearance of the diaphragm is not as you expect, first look for a normal cause of the discrepancy before diagnosing disease. When in doubt, repeat the radiographs paying special attention to proper imaging techniques.

Pathologic displacement of part or all of the diaphragm may be due to an increase or decrease in lung volume, abnormal abdominal content, or a defect in the diaphragm.

Non‐visualization of the diaphragm may be due to lack of an opacity interface or loss of diaphragmatic integrity. Any soft tissue opacity material between the diaphragm and air‐filled lung will obscure the margin of the diaphragm (e.g., pleural fluid, pulmonary disease, mass or fluid in the caudal mediastinum). Loss of diaphragm integrity may be due to a rupture or hernia (discussed on the next page).

Cranial diaphragm displacement most often is due to partial or complete lung collapse. Displacement may be unilateral or bilateral. Unilateral cranial displacement often is accompanied by a mediastinal shift. A physiologic or pathologic increase in the size of the abdominal contents can displace the diaphragm cranially (e.g., full stomach, advanced pregnancy, organomegaly, large tumor, severe effusion). Defects in the diaphragm that allow cranial displacement include hernias, ruptures, and eventration.

Eventration of the diaphragm produces a cranial bulge, usually involving only part or one side of the diaphragm (Figure 3.25). It most often is due to a congenitally thin and weak diaphragm, but may be caused by an injury to the phrenic nerve. Many patients with diaphragm eventration have no recent history or radiographic evidence of trauma and most are asymptomatic. In very severe cases, breathing can become compromised. The cranially protruding part of the diaphragm typically appears even, well‐defined, and relatively unchanged in serial radiographs. Differential diagnoses for an apparent eventration include diaphragmatic hernia, diaphragmatic mass, and a mass in the adjacent lung or caudal mediastinum.

Caudal displacement of the diaphragm may be due to an expanded thoracic cavity or absence of abdominal content. Expansion may be caused by hyperinflated lungs (e.g., emphysema, inspiratory dyspnea), severe pleural effusion, severe pneumothorax, or a large intra‐thoracic mass. Absence of abdominal content may be due to emaciation or loss of body wall integrity (e.g., hernia, rupture). Severe caudal displacement of the diaphragm can reveal its costal attachments, particularly in a VD/DV view (Figure 3.26). The attachments appear as sharp projections extending cranially from the cupula, sometimes described as “tenting” of the diaphragm.

Diaphragmatic hernia

Protrusion of viscera through a natural opening in the diaphragm is a hernia and protrusion through an acquired opening is a rupture. Because the two are difficult to differentiate in radiographs, we will use the term “hernia” for both.

Typical radiographic findings of a diaphragmatic hernia include a mixed opacity mass in the thoracic cavity and an indistinct or absent diaphragm margin (Figure 3.27). The most commonly displaced structures are the stomach, small intestine, liver, and fat. It may be difficult to identify the displaced viscera unless structures are visibly absent from their normal locations or there is a distinctive shape or pattern associated with the herniated content. Intestines may be recognized as gas or fluid‐filled tubes that extend beyond the limits of the abdominal cavity. Visualization of rugal folds helps identify the stomach.

Large protrusions into the thoracic cavity can displace and/or obscure the lungs and the cardiac silhouette. Pleural effusion is a frequent finding with diaphragmatic hernias. Fluid may be unilateral or bilateral and can obscure the displaced viscera and compromise diagnosis. Follow‐up radiographs made after thoracocentesis or may improve visualization of the displaced structures and diaphragm margins. Horizontal beam radiography also may be useful (Figure 2.15). The abdominal cavity may appear “empty” due to less viscera. Structures that remain in the abdomen often are cranially displaced or otherwise malpositioned.

Radiographic diagnosis of a diaphragmatic hernia is difficult when the protrusion of viscera is transient or intermittent. Some structures may only be partially displaced. Rarely, a hernia may extend between the soft tissue layers of the thoracic wall instead of into the thoracic cavity. These paracostal hernias can present with an extra‐thoracic swelling that may be difficult to differentiate from other causes of a thoracic wall mass. The opacity of the hernia mass varies depending on the amount of fat, soft tissue organs, and gas‐filled GI structures it contains (Figure 3.28).

A hiatal hernia occurs when abdominal contents protrude through a hiatus in the diaphragm. The esophageal hiatus most often is involved and the gastroesophageal junction most often protrudes. Other types of esophageal hiatal hernias include para‐esophageal, in which part of the stomach is displaced alongside the esophagus without involving the gastroesophageal junction, and gastroesophageal intussusception, in which part of the stomach enters the esophagus (see gastroesophageal intussusception). Protrusion through the aortic hiatus is called a para‐aortic hiatal hernia and protrusion through the vena caval hiatus is a para‐venous hiatal hernia. Fat or part of the stomach usually is involved in each of these hiatal hernias.

Diagnosis of hiatal hernia frequently is difficult because, in many cases, the abdominal contents are only intermittently displaced, which is called a sliding or dynamic hiatal hernia. Positioning the patient with the head tilted down may aid in detection.

The classic radiographic appearance of a hiatal hernia is an oval or semicircular mass near the left crus of the diaphragm (Figure 3.29). The mass generally is located near midline, between the caudal vena cava and the aorta. The size, shape, and opacity of the mass vary with the type and quantity of displaced viscera. Most hiatal hernias are soft tissue or fat opacity. The intra‐abdominal part of the stomach may be abnormal in shape and the expected gas bubble in the fundus may be absent. The stomach may appear continuous with the hiatal hernia mass if there is only partial gastric displacement. The caudal esophagus may or may not be dilated. A strangulated hernia that traps the stomach can lead to a tension viscerothorax (Figure 3.48).

A peritoneal‐pericardial diaphragmatic hernia (PPDH) is a congenital condition in which there is an abnormal opening between the peritoneal cavity and the pericardial sac that. The opening allows abdominal viscera to move into the pericardial sac (Figure 3.30). Structures commonly displaced include falciform fat, liver, gall bladder, stomach, and small intestine. The characteristic radiographic finding is a large, rounded cardiac silhouette composed of various displaced opacities, such as fat, soft tissues, and gas‐filled GI structures. The border of the cardiac silhouette may be irregular or even, but usually it is sharply defined because there is less cardiac motion blur. The cardiac and diaphragm borders frequently merge together without the normal overlap or edge distinction. The dorsal border of a PPDH may be visible in a lateral view as a thin horizontal line between the cardiac silhouette and the diaphragm, located just ventral to the caudal vena cava. This line is seen more often in cats with PPDH than in dogs.

PPDH differs from other diaphragm hernias in that the displaced viscera are confined to the cardiac silhouette. Pleural effusion is uncommon unless secondary right heart failure occurs. If needed, an upper GI study may be useful to identify the contents of the pericardial sac and help differentiate PPDH from pericardial effusion or cardiomegaly.

PPDH is always congenital, never acquired. A sternal deformity such as pectus excavatum or sternal dysraphism may be concurrent with a PPDH. Many affected animals are asymptomatic. When clinical signs are present, they tend to be more GI related in dogs and respiratory related in cats. PPDH most often is reported in long‐haired cats and Weimaraner dogs.

Diaphragm mass

Masses on the caudal side of the diaphragm usually blend with abdominal structures such as the liver and stomach and therefore are not identified in radiographs. A mass on the cranial side, however, often is outlined against the lung making it visible as a rounded or semi‐circular mass. The mass can appear semi‐circular because its caudal part is obscurred by the abdominal contents. Possible causes of a diaphragm mass include neoplasia, abscess, granuloma, hernia, and eventration.

Normal radiographic anatomy

The pleura are thin membranes that cover the lungs, line the thoracic cavity, and help form the mediastinum. The visceral or pulmonary pleura covers the lungs, blood vessels, and bronchi. It does not contain any sensory nerves and receives blood supply from the pulmonary circulation. The parietal pleura lines the walls of the thoracic cavity, the mediastinum, and the diaphragm. It contains many sensory nerves and receives blood supply from the systemic circulation. Between the pulmonary and parietal pleura is the pleural space.

The pleural space is essentially a “potential” space because it normally contains only a capillary thin layer of fluid. The fluid serves to moisten the pleural surfaces and reduce friction. Except for this fluid, the pulmonary and parietal pleura are in virtual contact with each other. The pressure in the pleural space is less than atmospheric pressure, which helps the lungs expand with the chest during inspiration.

Pleural fluid is produced continuously by the parietal pleura and resorbed by the visceral pleura. The turnover is about 75% every hour. Any condition that increases the volume of fluid entering the pleural space or prevents its absorption can increase the volume of pleural fluid. Excess fluid in the pleural space can interfere with lung expansion and convert the potential space into a true cavity. The same is true for gas that accumulates in the pleural space (i.e., pneumothorax).

Pleural fissure lines

Each lung is divided into individual lobes by the pulmonary pleura. The divisions between the lobes are created by inward extensions of the pleura, called pleural fissures. Normal pleural fissures rarely are identified in radiographs because the pleura is so thin. The fissures become visible when filled with gas or fluid and when the pleura is thickened.

The locations of the pleural fissures are known (Figures 3.31 and 3.32). When visible in radiographs, they appear as thin, curved, soft tissue opacity lines. Pleural fissure lines that are uniform in thickness with no tapering at either end, generally are caused by pleural thickening (lines that taper suggest pleural fluid as described on the next page). Pleural thickening may result from inflammation, edema, or fibrosis. Pleural fissure lines sometimes are seen in radiographs of older, clinically normal patients. In these patients, pleural thickening likely is a remnant of previous disease that is now inactive. Fissure lines in older animals often are considered to be incidental findings and have been referred to as an “aging change.”

Occasionally a normal pleural fissure is visible in a radiograph (Figure 3.33). This occurs when the x‐ray beam is perfectly aligned with the fissure (Figure 3.34). Unlike pleural lines caused by disease, normal pleural lines rarely repeat in radiographs because it is unlikely that the x‐ray beam will be aligned with the lung fissure every time.

Free fluid or gas in the pleural space is the most common cause of visible pleural fissures. Pleural fluid or gas outlines the lungs and separates the individual lung lobes (Figure 3.35). Separation of the lobes makes the interlobar fissures wider peripherally and narrower centrally. Pleural fluid produces wedge‐shaped fissure lines that are more opaque than air‐filled. Pleural gas produces wedge‐shaped fissure lines that are less opaque than lung (Figure 3.36).

A “reverse pleural fissure line” may be created by fat in the central thorax that extends into a lung fissure. Fat fissure lines are wider centrally and taper peripherally (Figures 3.36 and 3.42). They occur most often between the right cranial and middle lung lobes in overweight dogs.

Pleural fluid

Fluid in the pleural space is called pleural effusion. In general, the volume of fluid must exceed 10 ml/kg body weight to be detected in radiographs. This is about 50 ml in a cat or small dog and at least 100 ml in a medium‐size dog. As mentioned earlier, pleural effusion may result from excess fluid production, decreased absorption, or both. The type of fluid cannot be determined from radiographs. There are numerous causes of pleural effusion, many of which are associated with disease outside the pleural space (see Differential Diagnoses at the end this chapter).

Fluid in the pleural space usually is freely movable and bilateral in distribution, the latter due to mediastinal fenestrations that are present in most dogs and cats. Unilateral or asymmetric effusion may result from a congenital absence of fenestrations, disease that plugs the existing fenestrations, or fluid so viscous that it cannot pass through the fenestrations. Diseases that can obstruct the fenestrations include severe or chronic inflammation, fibrosis and neoplasia.

The earliest radiographic sign of pleural fluid is pleural fissure lines. Fissure lines created by small volumes of pleural fluid may be indistinguishable from pleural thickening. Small volume pleural effusions tend to be easier to see in a VD view than in a DV view. This is because in dorsal recumbency the pleural fluid is more likely to flow with gravity into the interlobar fissures and move laterally away from the mediastinal structures, where it often is easier to see (Figures 3.37 and 3.38). Horizontal beam radiography also may be useful to detect small volumes of pleural fluid (Figure 2.15).

As the volume of pleural fluid increases, the lungs are less able to expand. The lung edges become further separated from the thoracic wall with fluid opacity in between. This finding sometimes is described as “retraction” of the lung lobes. Vascular and bronchial markings do not extend all the way to the ribs. Compared to fully inflated lungs, retracted lungs are more rounded and more opaque. Summation of pleural fluid further increases lung opacity. Pulmonary vessels remain visible in patients with pleural effusion, which is an important finding to help differentiate fluid outside the lungs from fluid inside the lungs (latter may result from pneumonia, pulmonary edema, or hemorrhage). Fluid in the lungs will obscure the vessels because both are soft tissue opacity and there will be no opacity interface.

The central position of the heart within the thoracic cavity is maintained by fully inflated lungs. Less inflated lungs allow the heart to move to one side or the other. Pleural fluid limits lung inflation. When a patient with pleural effusion is placed in lateral recumbency, the heart “falls” toward the dependent side and slides dorsally along the curved thoracic wall (Figure 3.39). In a lateral radiograph, the cardiac shadow will be dorsally separated from the sternum.

Pleural fluid between the heart and sternum often outlines the ventral lung border to create a wavy or “scalloped” appearance (Figure 3.39). Fluid in the costophrenic recesses makes them more rounded and less distinct. Fluid that collects adjacent to the mediastinum often makes it appear widened.

A large volume of pleural fluid can lead to very small lungs and widely separated lung lobes. The individual lobes may resemble “leaves on a tree” (Figure 3.40). Large pleural effusions can displace the diaphragm caudally and make it appear flattened and stationary in serial radiographs. Large effusions also expand the thoracic cavity, making the ribs more perpendicular to the spine and the chest more rounded or “barrel‐shaped.” The overall opacity of the thoracic cavity may be increased to the degree that it resembles an underexposed radiograph; however, underexposure is not the cause if the vertebrae and other extra‐thoracic structures are properly exposed. Large volumes of pleural fluid may efface the cardiac and diaphragmatic borders and hide intra‐thoracic lesions. Follow‐up thoracic radiographs made after removing as much fluid as possible may yield additional diagnostic information.

Chronic pleural effusions and inflammatory pleural fluid should be removed with caution because pleural fibrosis may be present. Pleural fibrosis reduces lung elasticity, which means the lungs could rupture if they re‐expand too quickly. A typical radiographic finding of pleural fibrosis is multiple rounded lung lobes with irregular margins (Figure 3.40).

In patients with unilateral pleural effusion, lung not surrounded by fluid tends to be better inflated than the one that is surrounded by fluid. Unilateral effusions may not be evident in a lateral view alone because the edges of the inflated lung typically extend to the thoracic wall. The orthogonal VD/DV view often is needed to confirm the diagnosis.

Variations in body conformation may be mistaken for pleural effusion. As described, the normal inward curvature of the ribs in chondrodystrophic breeds can give the false impression of pleural fluid in a VD/DV view (Figure 3.12). Also, the normal hypaxial muscles in cats may resemble rounding of the costophrenic recess in a lateral view (Figure 3.41).

Thin, mineralized costal cartilages may be mistaken for pleural fissure lines (Figure 3.36). Cartilages, however, tend to be straighter or curve in the opposite direction than pleural fissure lines. Most costal cartilages can be traced to the end of a rib. Usually, there are multiple mineralized cartilages, most of which are not located in the area of a pleural fissure.

In overweight patients, the thoracic walls are thick and the edges of the lungs may not extend all the way to the ribs (Figure 3.42). In addition, summation of overlying fat increases the overall thoracic opacity and excess mediastinal fat causes widening of the mediastinum. These findings in overweight patients may be mistaken for pleural effusion.

Pleural gas

Free gas in the pleural space is a pneumothorax. A pneumothorax may result from a penetrating thoracic wound, a ruptured airway, or a ruptured lung. Free gas in the pleura space increases the pleural pressure, which leads to lung collapse. The severity of lung collapse depends on the volume of gas and the pressure in the pleural space.

As with pleural fluid, the earliest radiographic sign of pleural gas is pleural fissure lines. Gas fissure lines are more difficult to detect than fluid fissure lines because they produce less contrast with air‐filled lung. In most cases, lung collapse is the initial radiographic finding of pneumothorax. The lungs become separated from the chest wall and there is gas opacity in between. The vascular and bronchial markings do not extend to the ribs. The tiny vessels and bronchi in the peripheral lung may not be visible without brightening the radiograph (use a “hotlight” with film radiographs).

Pneumothorax tends to be easier to see in lateral and DV views. A DV view may be more diagnostic than a VD view because in ventral recumbency pleural gas rises to the dorsal thorax. The dorsal thorax is wider than the ventral thorax so the gas will be more spread out and less likely to be obscured by the midline structures (Figures 3.43 and 3.44). Horizontal beam radiography may be useful to detect a smaller volume of pleural gas (Figure 2.15).

As the volume of pleural gas increases, the degree of lung collapse increases. Smaller lungs contain less air and are more opaque than inflated lungs. Small lungs also cannot maintain the central position of the heart and other mediastinal structures, which means the mediastinum can shift laterally (see mediastinal shift in the next section of this chapter). In a lateral view, a mediastinal shift may appear as dorsal displacement of the cardiac silhouette. Dorsal separation of the cardiac shadow from the sternum sometimes is described as “elevation of the heart”. The term “elevation”, however, is a misnomer. Without the support of an inflated lung, the heart actually falls with gravity toward the dependent thoracic wall and slides dorsally along the normal curvature of the inner chest wall (Figure 3.45).

Free gas in the pleural space increases the opacity interface with the diaphragm, making its cranial margin appear sharper and more distinct. A large volume of pleural gas can caudally displace the diaphragm, making it appear flattened and stationary in serial radiographs.

Severe pneumothorax can reduce the overall opacity in the thoracic cavity to the degree that the thorax appears to be overexposed. However, overexposure is not the cause if the extra‐thoracic structures are properly exposed.

Conditions that can mimic pneumothorax include emaciation, hyperinflated lungs, hypovolemia, and superimposed skin folds (see Differential Diagnoses). With emaciated patients, there is relatively little tissue overlying the thorax and the lungs tend to be well‐inflated, both of which can lead to a thoracic cavity that is less opaque than expected. Hyperinflated lungs also can lead to a less opaque thoracic cavity (e.g., emphysema, manual inflation). Similarly, hypovolemia that results in small cardiovascular structures can diminish overall lung opacity. Hypovolemia also can lead to separation of the cardiac shadow from the sternum. A skin fold superimposed on the thoracic cavity may be mistaken for the edge of a retracted lung (Figure 3.46). The skin fold usually can be traced beyond the limits of the thoracic cavity. Each of these conditions of mimicry is differentiated from pneumothorax by the absence of free gas between the lungs and chest wall.

More common than missing a pneumothorax is an erroneous diagnosis of pneumothorax. This is especially dangerous when the clinician introduces gas into the thorax via chest tap after making an incorrect diagnosis, thus producing a pneumothorax when none existed. A favorite radiographic sign of pneumothorax is “elevation of the heart”. Rather than consider it an “elevation”, one should more accurately describe it as separation of the cardiac shadow from the sternum. The problem is that there are at least 3 causes for separation of the cardiac shadow from the sternum in addition to pneumothorax. (Figure 3.45). If it is a true pneumothorax, there will be other evidence for that diagnosis in addition to separation of the cardiac shadow.

Small volumes of pleural gas usually are resorbed within 48 hours, provided that gas does not continue to enter the pleural space.

A hydropneumothorax occurs when both fluid and gas are in the pleural space, The most common cause is severe thoracic trauma. Radiographic findings frequently include other signs of damage (e.g., fractured ribs, subcutaneous emphysema). The mixture of fluid and gas opacities in the pleural space can be challenging to interpret. Horizontal beam radiography may be helpful because fluid flows with gravity to the dependent side and gas rises to the “up” side.

A tension pneumothorax is present when pleural pressure exceeds atmospheric pressure. Pleural pressure this high can prevent the lungs from expanding, both during inspiration and expiration, which can quickly become an emergency situation. A tension pneumothorax occurs when gas enters the pleural space during inspiration (usually from a ruptured lung) but cannot exit during expiration because the thoracic wall is intact. The lung rupture acts like a one‐way valve, allowing gas to continue flowing in but not out. In radiographs, the lungs often are markedly collapsed with very small lobes (Figure 3.47). The lung lobes may be abnormal in shape. Cardiovascular structures usually are small due to hypovolemia and may appear even smaller due to an expanded thoracic cavity. The large volume of pleural gas displaces the diaphragm caudally, sometimes revealing it costal attachments.

A tension viscerothorax may develop following herniation and subsequent entrapment of a GI structure within the thoracic cavity. The trapped structure, usually the stomach, can become severely gas‐distended to resembles a large, cyst‐like mass with rounded, well‐defined margins (Figure 3.48). The mediastinum may be displaced to the opposite side. The part of the diaphragm through which the GI structure was displaced may be indistinct, which can help differentiate a viscerothorax from a unilateral pneumothorax. Gas in a pneumothorax outlines the lung, whereas gas in a viscerothorax is confined to the GI structure.

Pleural mass

Masses that originate in the pleura or pleural space usually are accompanied by pleural effusion. Pleural fluid can hide the lesion. A pleural mass may be visible in follow‐up radiographs made after removing the pleural fluid or by using horizontal beam radiography to reposition the fluid. Most pleural masses are difficult to see unless the x‐ray beam is tangential to the base of the mass.

Normal radiographic anatomy

The mediastinum is the central compartment in the thoracic cavity. It is located between the right and left lungs. The mediastinum extends along the midsagittal plane from the thoracic inlet to the diaphragm and from the sternum to the spine. For descriptive purposes, it commonly is divided into cranial, middle, caudal, and dorsal parts. The cranial mediastinum extends from the thoracic inlet to the cranial border of the cardiac shadow. The middle mediastinum contains the cardiac shadow. The caudal mediastinum extends from the caudal border of the cardiac shadow to the diaphragm. The dorsal mediastinum lies dorsal to the level of the trachea.

There are several openings in the mediastinum that allow communication with other parts of the body. Fluids, gases, and diseases can easily pass through these openings to enter or exit the mediastinum. Cranially, the mediastinum communicates with the cervical fascia via the thoracic inlet. Caudally, it opens to the retroperitoneal space via the aortic hiatus in the diaphragm. Centrally, it communicates with each lung through the pulmonary root or hilum. The lateral pleural walls that line the mediastinum are delicate and often fenestrated, providing an incomplete barrier between the right and left sides of the thoracic cavity.

Normally visible structures in the mediastinum include the cardiac silhouette, the trachea, the aorta, the caudal vena cava, and in young animals, the thymus (Figure 3.49). The mediastinum is a site for fat deposits, so variable amounts of fat usually are visible, too.

The other mediastinal structures either are too small to be seen or lack the opacity interface to be distinguished in survey radiographs. These include the esophagus, lymph nodes, vessels and nerves (e.g., cranial vena cava, left subclavian artery, brachiocephalic trunk, azygous vein, thoracic duct, phrenic nerves). The normal esophagus may be identified when it contains gas or fluid. In overweight patients, mediastinal structures not normally visible may be identified because excess fat provides the contrast needed to distinguish them. Structures may also be visible in emaciated patients because the absence of fat leads to a thinner mediastinum through which the soft tissue opacity structures may be discernable against air‐filled lungs.

The thymus often is visible in young dogs and cats. It is located in the ventral part of the cranial mediastinum. In dogs, the thymus may be visible up to about six months of age, rarely up to a year of age. It generally shrinks quickly after the deciduous teeth are lost. The canine thymus tends to be easiest to see in a VD/DV view, appearing as a triangular‐shaped, soft tissue opacity structure in the left cranial thorax (Figure 3.50). In cats, the thymus may be visible up to two years of age. It usually is easier to see in a lateral view where it appears as an indistinct soft tissue opacity structure in the cranioventral mediastinum.

Mediastinal lymph nodes may be visible when enlarged or mineralized. Knowledge of their normal locations aids in diagnosis (Figure 3.51). The cranial mediastinal lymph nodes are located ventral to the trachea and near the large cranial mediastinal blood vessels. They receive lymphatics from the neck, heart, esophagus, thymus, and thoracic wall. Enlargement of these nodes can widen the cranial mediastinum, increase mediastinal opacity, and displace the trachea dorsally and laterally. Caudal mediastinal lymph nodes frequently are absent in dogs and cats. The sternal lymph node is located immediately dorsal to the second or third sternebra. It usually is solitary but may be paired in some dogs. The sternal node receives lymphatics from the abdomen and enlargement generally is due to spread of intra‐abdominal disease or lymphoma. Tracheobronchial or hilar lymph nodes are located near the tracheal bifurcation. They receive lymphatics from the lungs and bronchi. Enlargement produces increased opacity in the hilar region and may lead to a mass effect that displaces the caudal trachea, usually ventrally.

The radiographic appearance of the mediastinum varies with its thickness. In lateral views, summation of the cranial mediastinal structures produces a homogenous soft tissue opacity ventral to the trachea (Figure 3.49). The opacity quickly fades toward the sternum because there are fewer structures in the ventral part of the cranial mediastinum, and it usually is very thin. The caudal mediastinum also is thin and seldom visualized in a lateral view. In overweight animals, fat deposits often widen the mediastinum (Figure 3.42).

The thin ventral mediastinum frequently produces a curved, soft tissue opacity line that resembles a pleural fissure line (Figures 3.31, 3.32, 3.49). It is visible both cranial and caudal to the cardiac silhouette and varies significantly in width depending on the amount of fat present. It is less evident in chondrodystrophic dog breeds. The cranioventral mediastinum conforms to the shapes of the right and left cranial lung lobes. It begins at the thoracic inlet and curves caudally, ventrally and to the left of midline. The caudoventral mediastinum conforms to the shapes of the accessory and left caudal lung lobes, curving caudally from the left cardiac border to the middle of the left hemi‐diaphragm. The caudoventral mediastinum is only visible in the VD/DV view and may be mistaken for the cardiophrenic ligament, which is not visible in radiographs.

Mediastinal shift

The position of the mediastinum is maintained by normal lung inflation. Lungs that are not evenly inflated allow the mediastinum to move or shift toward the smaller lung. Uneven lung inflation also allows the diaphragm on the side with the smaller lung to move cranially.

The most common cause of a mediastinal shift is lung collapse. In cases of unilateral collapse, the opposite lung generally is well expanded due to compensatory hyperinflation. Collapse of part of a lung may or may not result in a mediastinal shift; it depends on the degree of compensatory inflation in the unaffected lobes.

Other causes of a mediastinal shift include a large intra‐thoracic mass and a large volume of unilateral fluid or gas in the pleural space.

In general, the position of the mediastinum is assessed in a VD/DV view based on the position of the cardiac shadow (Figure 3.52). Rotation of the patient during radiography may be mistaken for a mediastinal shift because the heart follows the sternum. It is important to assess patient positioning before diagnosing a mediastinal shift. The spine and sternum should be superimposed and the right and left ribs equal length in a properly positioned VD/DV view. The dorsal spinous processes should be centered on their vertebral bodies.

Abnormal width of the mediastinum

The width of the mediastinum is assessed in a VD/DV view. In dogs, the normal width of the cranial mediastinum is about twice the width of a thoracic vertebra. In cats, the cranial mediastinum is about as wide as the spine. A wider than expected mediastinum may be localized due to a mass or it may be diffuse, involving the entire mediastinum. A mediastinal mass may arise in the cranial, dorsal, middle, or caudal part of the mediastinum.

Cranial mediastinal masses that arise ventral to the trachea create increased opacity dorsal to the sternum. When large enough, the mass may displace the trachea dorsally and laterally, usually to the right (Figure 3.53). Very large masses can displace the cardiac shadow caudally and push the tracheal bifurcation caudal to the sixth intercostal space (Figure 3.54). Caudal cardiac displacement usually is accompanied by dorsal cardiac displacement. Possible sites of origin for a cranioventral mediastinal mass include thymus, lymph node, sternum, and ectopic thyroid or parathyroid tissue. A mass may represent a tumor, cyst, abscess, or hematoma (see Differential Diagnoses). A large thymus in a mature dog or cat may be due to neoplasia or sometimes hemorrhage. Thymic tumors can grow to a very large size, particularly thymomas (Figure 3.54). A large sternal lymph node creates a soft tissue opacity mass dorsal to the second or third sternebra (Figure 3.55). Sternal lymphadenomegaly produces an extra‐pleural sign, easiest to see in a lateral view. Rarely, a cranial mediastinal mass will involve one or more sternebrae and may cause osteolysis, a periosteal response, or/and adjacent soft tissue swelling.

Cranial mediastinal cysts or branchial cysts are congenital, fluid‐filled cavities of embryologic origin. They usually are too small to be seen in radiographs of young animals but gradually enlarge over time. Branchial cysts are more common in cats than in dogs and typically produce a well‐defined soft tissue opacity mass just cranial to the cardiac shadow (Figure 3.56). An aspirate of the cyst generally yields clear, acellular fluid with low specific gravity.

Conditions that may mimic a cranial mediastinal mass include fat deposits and errors in patient positioning. Older, small breed dogs normally store fat in the cranial mediastinum, which may be mistaken for a mass. During positioning for radiography, many patients will tuck their heads and flex their necks which can produce a bend in the trachea that resembles a mass effect (Figure 3.115). Superimposition of the thoracic limb musculature also can mimic a cranial mediastinal mass. When in doubt, repeat the radiograph paying special attention to patient positioning.

Dorsal mediastinal masses occur dorsal to the trachea and most often are associated with the esophagus. Large masses can displace the trachea ventrally and laterally, usually to the right (Figure 3.57). A dilated esophagus can do the same and may be caused by abnormal retention of gas, fluid, food, or a combination of these. The type of esophageal content will affect the opacity of the dorsal mediastinum. An uncommon type of dorsal mediastinal mass is a large vertebral lesion (e.g., paraspinal tumor, severe lordosis, severe spondylosis).

Middle mediastinal masses or hilar masses most often represent enlarged tracheobronchial lymph nodes or a heart base tumor. Both of these produce soft tissue opacity along the distal trachea and both can become large enough to displace and compress the trachea. Tracheobronchial lymphadenomegaly occurs at the tracheal bifurcation (Figure 3.55). The enlarged lymph nodes can displace the trachea in any direction, but most often it is ventrally. Dorsal displacement of the trachea most often is due to left atrial dilation. Tracheobronchial lymphadenomegaly may be caused by lymphoma or a mycotic infection. Neoplasms other than lymphoma rarely cause hilar lymph node enlargement. Mineralization in a hilar lymph node typically is a healing response that most often is seen with histoplasmosis.

Heart base masses generally arise cranial to the tracheal bifurcation (Figure 3.58). The margins of a heart base mass often are indistinct due to effacement by pericardial fluid or the adjacent cardiovascular structures. Displacement of the trachea may be the only radiographic finding with a heart base mass. The trachea typically is pushed dorsally and laterally, usually to the right. Heart base tumors frequently are associated with the aorta (e.g., chemodectoma), but may involve the main pulmonary artery or the right atrium (e.g., hemangiosarcoma).

End‐on visualization of the right pulmonary artery in a lateral view may be mistaken for a hilar mass. This artery, whether normal size or enlarged, can appear as a round, soft tissue opacity “nodule” located ventral to the tracheal bifurcation (Figure 3.99).

Caudal mediastinal masses typically are associated with the esophagus or diaphragm (see Differential Diagnoses). They usually are located on midline or just left of midline (Figure 3.59). Large masses may efface the caudal cardiac and/or cranial diaphragmatic borders. A mass in the accessory lung lobe can be difficult to distinguish from a caudal mediastinal mass.

Diffuse widening of the mediastinum may be due to abundant fat in the mediastinum (i.e., obesity), dilation of the esophagus, or fluid in the mediastinum. Excess mediastinal fat is common and must be differentiated from the other disorders. In overweight animals without mediastinal disease, the width of the cranial mediastinum should not exceed the thickness of the thoracic wall, measured at the same level (Figure 3.42). Dilation of the esophagus is discussed in the next section under “Esophagus”. Fluid in the mediastinum may result from a ruptured esophagus, severe inflammation, or hemorrhage (see Differential Diagnoses). Keep in mind that abundant fat or fluid can obscure a mediastinal mass. Conditions that may mimic diffuse widening of the mediastinum include summation with pleural or pulmonary fluid and superimposition of a thoracic mass.

Abnormal opacity in the mediastinum

Orthogonal radiographs are necessary to avoid misinterpreting a structure located outside the mediastinum as something abnormal inside the mediastinum. Summation of dirt, debris, or medication on the skin or hair coat may be mistaken for mineral or metal in the mediastinum. The same is true with summation of a costal cartilage or a mineralized mass in the chest wall. Summation of subcutaneous gas not caused by pneumomediastinum may be mistaken for gas in the mediastinum.

Mineral opacity in the mediastinum may represent esophageal content or dystrophic mineralization (Figure 3.60). Dystrophic mineralization may be associated with a lymph node, a long‐standing mass, or a cardiovascular structure. Lymph node mineralization usually is a healing response, most often following a mycotic infection (e.g. histoplasmosis). It is rare with neoplasia. A chronic abscess, hematoma, or tumor may mineralize, either focally or diffusely. Cardiovascular mineralization may be incidental or caused by a disease, such as hypercalcemia. Heart valve mineralization usually is small, sharp, and linear. In blood vessels, mineralization tends to be curvilinear and heterogenous, most often in the wall of the aorta or a coronary artery.

Pneumomediastinum means there is free gas in the mediastinum. The gas provides an opacity interface to visualize structures that are not normally seen in survey radiographs. Radiographic signs vary with the amount of gas present. The signs tend to be easier to see in a lateral view where the spine and sternum are not superimposed. Small volumes of mediastinal gas may outline only the outer margin of the trachea and produce subtle, patchy areas of gas opacity (Figure 3.61). Larger volumes of gas may outline the cranial mediastinal blood vessels, the esophagus, and sharpen the margins of the aorta and cardiac silhouette (Figure 3.62). Gas can easily migrate in and out of the mediastinum through the normal openings at the thoracic inlet and aortic hiatus. Through these openings, gas can enter the cervical region and retroperitoneal space, respectively. Cervical gas readily spreads into the subcutaneous tissues along the thorax and abdomen (Figure 3.63). Retroperitoneal gas can outline the kidneys, abdominal aorta, and abdominal vena cava. Mediastinal gas also can diffuse through openings in the mediastinal pleura to enter the pleural space. Pneumomediastinum can progress to pneumothorax via the mediastinal fenestrations or a tear in the mediastinum.

A pneumomediastinum may result from a ruptured trachea or ruptured esophagus. It may also be caused by migration of gas from the cervical region or pleural space. Cervical gas may result from a penetrating wound in the neck. Pleural gas may result from a penetrating wound in the thorax or from a tear in a bronchus or lung. Dyspnea is uncommon with pneumomediastinum unless a secondary pneumothorax develops. Iatrogenic causes of pneumomediastinum include faulty intubation, over‐distention of a tracheal tube cuff (especially in cats), and faulty venipuncture. An uncomplicated pneumomediastinum often is self‐limiting and gas typically is resorbed in 2–10 days. Serial radiographs are useful to monitor the patient’s progress and response to therapy.

Conditions that can mimic a pneumomediastinum include both an abundance of mediastinal fat and a lack of mediastinal fat. Abundant fat can separate and outline the mediastinal soft tissues. A lack of fat (e.g., emaciated patient) may allow the mediastinal soft tissues to become more visible against air‐filled lungs.

Normal radiographic anatomy

The esophagus is relatively fixed in position at the caudal pharynx and at the diaphragm. The portion in between is freely moveable and quite distensible. The esophagus seldom is identified in survey radiographs because it usually is empty and collapsed. Small volumes of swallowed gas or fluid sometimes are visible, but these tend to be transient and variable in appearance in serial radiographs.

The cervical part of the esophagus runs from the cricopharynx (cranial esophageal sphincter) to the thoracic inlet. It lies dorsal and left lateral to the trachea. At the level of the first and second ribs, there is a slight ventral curvature or redundancy in the esophagus (Figure 3.64). The redundancy adds length to the esophagus to allow for movements of the head and neck. During flexion, the ventral curvature is more pronounced and may be mistaken for an esophageal diverticulum, (i.e., a pseudodiverticulum). Esophageal redundancy is accentuated in brachycephalic breeds where the length of the esophagus is confined to a shorter body.

The thoracic part of the esophagus extends from the thoracic inlet to the diaphragm. It runs caudally in the median plane, dorsal and left of the trachea, right of the aortic arch, and between the descending aorta and the caudal vena cava. Occasionally, the normal esophagus is visible in a lateral view as a horizontal, soft tissue opacity band between the aorta and caudal vena cava (Figure 3.49). The vagal nerves run alongside the esophagus and follow it through the diaphragm into the abdominal cavity. The abdominal part of the esophagus runs left of midline from the diaphragm to the cardia of the stomach. The esophagus ends at the gastroesophageal junction.

It is important to make both thoracic and cervical radiographs when examining the esophagus. Abdominal radiographs may be indicated as well because disease in the upper GI tract can affect the esophagus.

Contrast radiography of the esophagus

Complete evaluation of the esophagus often requires an esophagram. The procedure for esophagography, as well as its indications and contraindications, is described in Chapter 2, Contrast Radiography. Esophagrams differ in appearance between dogs and cats. In dogs, all of the esophagus contains striated muscle and the mucosal folds are longitudinal (Figure 3.64). The folds produce multiple linear filling defects along the entire length of the esophagus. In cats, the cranial 2/3 of the esophagus contains striated muscle and longitudinal mucosal folds, but in the caudal 1/3 there is smooth muscle and the mucosal folds are transverse or oblique in orientation. From about the level of the tracheal bifurcation to the stomach, the feline mucosal folds produce a pattern of filling defects commonly is described as “herringbone” (Figure 3.65).

In both dogs and cats, contrast medium in the esophagus should be transient and not retained. In some normal dogs, a small amount of contrast may persist for a short period of time near the larynx or thoracic inlet.

Abnormal content or opacity in the esophagus

Persistence of gas, fluid, soft tissue opacity material, or mineral opacity structures in the esophagus needs to be investigated. Abnormal opacity may be localized or diffuse, heterogenous or homogenous. Possible causes of retained gas, fluid, or solid material include physical obstruction, motility disorders, lodged foreign material, and a mass in the esophagus.

Gas in the esophagus may or may not be due to pathology. A small volume of esophageal gas often is incidental and transient. Transient gas commonly is seen between the thoracic inlet and the tracheal bifurcation.

Esophageal gas that is dorsal to the trachea may produce a tracheal stripe sign. The tracheal stripe results from the soft tissue opacity wall of the esophagus lying against the soft tissue opacity wall of the trachea with no different opacity between them (Figure 3.66). The two walls blend together to mimic a thickened dorsal tracheal wall.

The tracheal stripe sign may be mistaken for a pneumomediastinum. However, free gas in the mediastinum generally outlines both the dorsal and ventral tracheal walls as well as other mediastinal structures.

Larger volumes of transient esophageal gas sometimes are seen in patients that are sedated, anesthetized, or have swallowed a lot of air (e.g., panting, struggling, coughing, vomiting). Aerophagia usually is accompanied by abundant gas in the GI tract.

Persistent gas in the esophagus may indicate a motility problem. A gas‐distended esophagus sometimes is difficult to recognize in radiographs because the esophageal lumen is the same opacity as the lungs (Figure 3.70). In addition, the esophageal walls tend to be thin, faint, and widely separated (Figure 3.71). The walls of a dilated esophagus may be easier to recognize in the caudal thorax where they converge toward the diaphragm. Cranially, the ventral border of the longus coli muscles tends to be more distinct against a gas‐distended esophagus.

Foreign materials can lodge in the esophagus of a dog or cat. Common sites include the cranial cervical region, the thoracic inlet, the base of the heart, and just cranial to the diaphragm (Figure 3.67). Metal and mineral opacity objects usually are readily identified in radiographs. Soft tissue opacity materials must be outlined by gas or large enough to be distinguished. Objects that obstruct the esophagus may lead to dilation of the esophagus cranial to the object. The dilated esophagus may fill with gas, fluid, food, or a combination of these. Dilation may not be evident if the patient regurgitated and emptied the esophagus prior to radiography.

Masses associated with the esophagus may be intraluminal, mural, or extraluminal. A large esophageal mass may be difficult to distinguish from a mass in the adjacent lung. Many esophageal masses are soft tissue opacity (e.g., tumor, abscess, granuloma, hematoma). A soft tissue opacity mass in the esophagus often is difficult to detect unless surrounded by gas or a positive contrast medium. In an esophagram, an esophageal mass may appear as a localized thickening of the esophageal wall and/or a filling defect in the esophageal lumen (Figure 3.73). The mucosal margin of the thickened wall may be smooth or irregular, depending on the amount of mucosal damage. Ulcers sometimes are present, appearing as contrast‐filled outpouchings that extend away from the esophageal lumen. If the esophagus is obstructed by a mass, contrast fills the dilated lumen cranial to the obstruction.

Spirocercosis is a parasitic infection caused by Spirocerca lupi, a nematode with worldwide distribution. It is endemic in warm climates where the incidence can be as high as 85%. After being ingested, the parasites penetrate the gastric mucosa and travel along arteries to the thoracic aorta. They migrate through the aorta and end up in the caudal esophagus where they can produce granulomas along the dorsal esophageal wall. Parasite migration can produce small aneurysms in the wall of the aorta. Spirocera granulomas can grow quite large and may undergo malignant transformation to osteosarcoma. Osteosarcomas can mineralize and they can metastasize to the lungs. Large intra‐thoracic granulomas also can cause hypertrophic osteopathy.

The pathognomonic sign of spirocercosis is new bone production along the ventral margins of the thoracic vertebrae (Figure 3.68). New bone occurs in about 25% of affected dogs, most often caudal to the T5 thoracic vertebra. One or more vertebrae may be involved. The border of the new bone may be solid, lamellar, or brush‐like and may be well‐defined or ill‐defined. Diagnosis of spirocercosis is confirmed by endoscopy or finding parasite eggs in the feces.

Abnormal size and margination of the esophagus

An abnormally narrowed or dilated esophagus seldom is detected in survey radiographs unless there is persistent retention of gas, fluid, or food. Usually, an esophagram is needed for diagnosis and to evaluate the mucosal margin.

Abnormal narrowing persists in multiple radiographs and may be focal or regional. Possible causes of a narrowed esophagus include stricture, mural thickening or mass, and external compression, the latter may be the result of a vascular ring anomaly or hiatal hernia (see Differential Diagnoses). Liquid contrast media may be used to evaluate esophageal width and to visualize the mucosal margin. Barium provides the best mucosal coating. An irregular mucosal margin may be due to severe inflammation or neoplasia (Figure 3.69). If the esophageal narrowing does not obstruct the passage of liquids, it may not be evident in a standard esophagram. Mixing the contrast medium with food may aid in diagnosis.

Abnormal dilation of the esophagus commonly is called megaesophagus. Although megaesophagus is used as a general term to simply describe esophageal dilation, it is also the name given to flaccid dilation of the esophagus due to one of several congenital or acquired causes of weak or absent peristalsis. Congenital megaesophagus is more common in dogs than in cats, particularly in German shepherds. Acquired megaesophagus may result from numerous conditions, including severe or chronic inflammation, neuromuscular disease, immune‐mediated disease, chronic obstruction, and others (see Differential Diagnoses). Severe or chronic esophagitis usually is due to gastric reflux and can affect esophageal motility in animals at any age. Neuromuscular and immune‐mediated motility disorders often are idiopathic. They tend to occur in older animals (7–15 years of age) and many are incurable. Any cause of acquired megaesophagus may begin as mild or intermittent dilation and progress in severity. Many esophageal obstructions initially present with localized or regional dilation, but any persistent narrowing can progress to general dilation. Certain strong sedatives and anesthetic agents can cause transient esophageal dilation that usually resolves during recovery.

Radiographic diagnosis of esophageal dilation depends on size and content. As mentioned earlier, a large, gas‐filled esophagus can easily be missed in survey radiographs because the lumen is the same opacity as the adjacent lungs and the esophageal walls are thin, faint, and widely separated (Figures 3.70 and 3.71). In the lateral view, look for the tracheal stripe sign and a more distinct ventral border to the longus coli muscles. Widening of the mediastinum may or may not be evident with a gas‐filled esophagus. A dilated esophagus may displace the trachea ventrally and laterally, usually to the right. The base of the cardiac silhouette may also be displaced ventrally, making the cardiac shadow appear shortened (Figure 3.71). Many patients with a dilated esophagus have concurrent aerophagia and aspiration pneumonia. A gastric outflow problem should always be investigated as a possible etiology of a dilated esophagus.

Performing an esophagram in a patient with megaesophagus carries increased risk of aspiration, but sometimes the procedure is needed to help determine the etiology and full extent of the dilatation. A severely dilated esophagus may require a larger than expected volume of contrast medium for complete opacification. Inadequate dosing can result in an erroneous diagnosis of local esophageal dilation in a patient with general dilation.

Normal radiographic anatomy

What commonly is called the “heart” in a radiograph is actually an image of the pericardial sac and its contents. The contents include pericardial fluid, the origins of major blood vessels, the heart, and blood, all of which are soft tissue opacity and not individually distinguished. Because radiographs reveal only the outer margin of the pericardial sac, the terms cardiac silhouette or cardiac shadow are more accurate. The word “heart” will be used to describe the actual heart itself. Normal cardiovascular anatomy and circulation are depicted in Figures 3.88 and 3.89.

The pericardial sac consists of two layers. The inner visceral layer is called the epicardium and blends with the surface of the heart. The outer parietal layer is tough and fibrous. Between the visceral and parietal layers is a tiny volume of pericardial fluid for lubrication. The pericardiophrenic ligament forms a strong attachment to the diaphragm. It is not visible in radiographs, but the caudal mediastinum sometimes is mistaken for the pericardiophrenic ligament (VD/DV view).

The central position of the cardiac silhouette is maintained by normally inflated lungs. In most dogs and cats, the cardiac shadow is located between the T3 and T8 thoracic vertebrae, with its base at about the fifth or sixth intercostal space (base refers to the dorsum or “top” of the cardiac shadow). The cardiac base is formed by the atria, proximal aorta, main pulmonary artery, and cranial vena cava. Enlargement of any of these can alter the size or shape of the cardiac base. Immediately dorsal to the base lies the tracheal bifurcation and carina. The position of the carina relative to the spine is used to assess cardiac size. The ventrum or “bottom” of the cardiac shadow is called the apex. The cardiac apex is formed by the interventricular septum.

In a lateral view, the cardiac silhouette sits dorsal to the sternum with its long axis tilted cranially about 45° off perpendicular. The long axis is the apex‐to‐base dimension or length of the cardiac shadow. The short axis is the width of the cardiac silhouette; the widest dimension perpendicular to the long axis, often at about the level of the caudal vena cava (Figure 3.78). In a lateral view of most healthy dogs and cats, the cardiac base is situated at about 2/3 the height (dorsoventral dimension) of the thoracic cavity (Box 3.1). In most dogs, the cardiac shadow occupies about 2.5–3.5 intercostal spaces. In most cats, it occupies about 2.0–2.5 spaces.

In a VD/DV view, the normal position of the cardiac silhouette is on midline with its long axis tilted about 30° off parallel with the spine (Figure 3.78). In most dogs, the width of the cardiac shadow is about 2/3 the width of the thoracic cavity. The cardiac apex generally points left of midline. The canine cardiac silhouette may touch or overlap the diaphragm (Figure 3.79). In most cats, the width of the cardiac shadow is about 1/2 the width of the thoracic. The feline cardiac apex is variable in position (may be right or left of midline) and usually separated from the diaphragm by one or two intercostal spaces of lung (Figure 3.82).

The shape and apparent size of the cardiac shadow are related to the shape of the thorax and the size of the thoracic cavity. In dogs with a narrow, deep‐chested conformation (e.g., Doberman Pinscher, Irish Setter), the cardiac silhouette is more narrow and upright with relatively little sternal contact and a smaller cardiac‐to‐thoracic ratio (Figure 3.80). The cardiothoracic ratio is a subjective assessment of the size of the cardiac silhouette in relation to the size of the thoracic cavity. In dogs with a wide, shallow thorax or a “barrel‐chested” conformation (e.g., Bulldog, Pug), the cardiac shadow is wider and more rounded with greater sternal contact and a larger cardiothoracic ratio (Figure 3.81). Athletic dogs (e.g., Greyhound) also tend to have a relatively large cardiothoracic ratio (the heart appears relatively large in relation to the size of the thoracic cavity).

In cats, thoracic conformation is more uniform and the size and shape of the cardiac shadow are more consistent among different cat breeds. The feline cardiac silhouette is thinner and more elongated than in dogs with a relatively smaller cardiothoracic ratio (Figure 3.82).

In lateral radiographs of older animals, the cardiac shadow may be tilted further cranially and more parallel with the sternum. This is particularly true in older cats. Also in aged cats, the aortic arch tends to become more vertical and elongated and often bulges to the left (Figure 3.83). In a VD/DV view, the bulge in the proximal aorta cat may be mistaken for a mass in the cranial mediastinum or in the left lung.