The Thorax

CHAPTER three The Thorax


Some structures, the larynx and the trachea, directly associated with the respiratory tract lie outside or partially outside the thorax.



THE PHARYNX, LARYNX, AND HYOID APPARATUS




Normal Appearance


The pharynx and larynx are readily identifiable on properly exposed lateral views of the neck (Figure 3-1, A and B). On ventrodorsal projections, the larynx overlies the cervical vertebrae, and most of its detail is lost. Good radiographs will show the soft palate, hyoid apparatus, epiglottis, and cricoid cartilage. The diameter of the larynx is somewhat wider than that of the trachea (Figure 3-1, A and B).


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Figure 3-1 A and B, The larynx. Normal larynx in a dog. There is faint mineralization of the epiglottis and the thyroid cartilage. The cranial tip of the epiglottis is ventral to the soft palate. The hyoid apparatus is normal. A, Soft palate; B, epiglottis; C, stylohyoid bones; D, epihyoid bones; E, keratohyoid bones; F, basihyoid bone; G, thyrohyoid bones; H, cranial cornu of the thyroid cartilage; I, corniculate process of the arytenoid cartilage; J, cricoid cartilage; K, muscular process of the thyroid cartilage; L, lateral ventricle; M, vocal cord. C, This Greyhound was bleeding from the mouth with marked swelling of the tongue and pharyngeal region. On the lateral study of the larynx, the hyoid is fractured through both epihyoid bones, and there is considerable displacement of the fracture ends. Diagnosis: fractured epihyoid bones. D, A radiopaque pharyngeal foreign body in a cat. The pharynx is distended with air. The foreign body is in the caudal pharynx and cranial esophagus. Air is seen in the proximal esophagus caudal to the foreign body, which was part of a chicken wishbone. E and F, Brachycephalic airway syndrome. E, This is a young Bulldog. The larynx is at an unusual angle and is lying in an almost vertical position compared with the normal (A). The nasopharynx is occluded by the soft palate. There is mineralization of the laryngeal cartilages. F, This is a young Pug. The larynx is lying in an abnormal caudoventral position that is consistent with severe brachycephalic airway syndrome.




Abnormalities


Abnormalities of the larynx are usually diagnosed by methods other than radiologic. Displacement of the larynx, compression, or calcification may result in visible radiographic changes. Fractures of the hyoid bones occur (Figure 3-1, C). Foreign bodies or mass lesions in the pharynx or larynx are usually visible because of surrounding contrasting air (Figure 3-1, D).



Pharyngeal Dysphagia and Cricopharyngeal Achalasia


A number of abnormalities may affect the oropharynx, and they are difficult to distinguish from one another. Both the oropharynx and nasopharynx should be examined. Plain radiographs are seldom diagnostic in cases of dysphagia. These are functional disorders and require fluoroscopy to make a definitive diagnosis.


In pharyngeal dysphagia resulting from structural or neurologic disorders, inefficiency of pharyngeal contractions results in the retention of food material within the pharynx. Affected animals make repeated efforts to swallow food. Coughing may occur. Food material may come down the nose. Aspiration pneumonia is common. With fluoroscopy, a barium examination will show retention of barium in the pharynx despite repeated efforts to swallow it. Barium may enter the trachea, and air is often seen within the esophagus.


Cricopharyngeal achalasia results from failure of the cricopharyngeus muscle to relax during swallowing or from lack of coordination of the mechanisms involved in swallowing. The clinical signs are similar to those seen in pharyngeal dysphagia. A barium study shows retention of barium in the pharynx and cervical esophagus. On fluoroscopy, pharyngeal contractions are seen to force the barium against the caudal wall of the pharynx, with only a small amount of barium entering the esophagus, where it tends to remain. Barium may come down the nose or enter the larynx or trachea. Failure of the cricopharyngeus muscle to relax can be treated surgically. However, cricopharyngeal myotomy is contraindicated in other disorders of this region. Hence, accurate diagnosis is important.


Pharyngeal dysphagia and cricopharyngeal achalasia should be considered in the differential diagnosis in young animals that have difficulty swallowing.






THE TRACHEA




Radiography


Lateral and ventrodorsal views of the neck and thorax are necessary for routine examination of the trachea. Oblique views are helpful in demonstrating the trachea without superimposition of the vertebrae and sternum, as occurs on the ventrodorsal view. Care must be taken that no rotation of the thorax is present on lateral views; this will cause an apparent displacement of the trachea. The neck should be comfortably extended. Overextension results in a pseudonarrowing at the thoracic inlet, whereas flexion of the head or neck or elevation from the table top will cause tracheal deviation in the cranial thorax (Figure 3-2, A and B).



Contrast studies may be carried out as described for bronchography, but the contrast medium is deposited further cranially. Such studies are now rarely performed because endoscopy is more informative.



Normal Appearance


The trachea is visualized more clearly on lateral views. The air within it acts as a contrast medium, contrasting with the soft tissue opacity of the neck muscles and structures within the mediastinum. On ventrodorsal or dorsoventral views, the trachea is more difficult to see because of the superimposed vertebrae and sternum. The trachea, in the cranial mediastinum, lies to the right of the midline, becoming centrally placed at its bifurcation. On the lateral view, it forms an acute angle with the line of the thoracic vertebrae. The angle is greater in dogs with a deep, narrow thorax and more acute in dogs with a shallow thorax. A rounded lucency over the base of the heart marks the point of bifurcation. It represents the origin of the right cranial lobe bronchus seen end-on. A second rounded lucency may be seen that represents the origin of the left cranial lobe bronchus. The trachea curves somewhat ventrally toward its bifurcation between the fifth and sixth ribs. Only the primary bronchi near the bifurcation are recognizable on normal radiographs. Smaller bronchi cannot be identified. The diameter of the tracheal lumen varies slightly during inspiration and expiration. It is slightly smaller than the width of the larynx. It has been suggested that the width of the lumen should be three times the width of the proximal third of the third rib. Alternatively, the tracheal diameter can be expressed as a ratio to the thoracic inlet as measured on the lateral view. Normally the trachea is approximately one fifth the depth of the thoracic inlet (Figures 3-2, A, and 3-3, D).




Abnormalities



Displacement


The trachea may be displaced by cranial lung lobes, pleural, cervical, or mediastinal masses or by an enlarged heart. Adjacent masses tend to displace rather than compress it. Compression may occur at the thoracic inlet or over the base of the heart. The trachea may be compressed between a mass and the ribs, the spine, the aorta, or the heart, which are all relatively rigid structures.


A distended esophagus may displace the trachea ventrally. An enlarged heart displaces the trachea dorsally. Cranial mediastinal masses usually displace it dorsally and laterally, and they may displace the terminal trachea caudally. Intrathoracic masses may displace the tracheal bifurcation cranially. Enlarged tracheobronchial lymph nodes may depress, elevate, or compress the trachea and separate the mainstem bronchi.


Before making a diagnosis of tracheal displacement, one must be sure that the animal has been positioned correctly. Undue extension may result in the trachea appearing compressed at the thoracic inlet (Figure 3-3, A). Extreme flexion of the neck during radiography may result in a ventral displacement of the trachea in the cranial thorax (Figure 3-3, B and C). Dorsal displacement may be seen with lateral or ventral flexion of the neck. This results in artifactual displacement of the trachea in the cranial mediastinum, simulating a mass (Figure 3-3, C and D). Rotation of the thorax on the lateral view will cause an apparent elevation. Some deviation of the trachea to the right is often seen in normal dogs in the cranial thorax. It may be more pronounced in brachycephalic breeds (see Figure 3-29, G). A ventrodorsal or dorsoventral view is required to demonstrate deviation in the lateral plane (Figure 3-3, E).


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Figure 3-3 A, Pseudonarrowing of the trachea at the thoracic inlet from hyperextension of the neck. B, A lateral view of the thorax with the neck flexed shows a slight bend in the trachea at the thoracic inlet from hyperflexion. C and D, Artifactual tracheal displacement in a dog. C, When the initial radiograph was obtained, the patient’s neck was partially flexed. There is dorsal displacement of the thoracic trachea, just cranial to the aortic arch. This displacement mimics dorsal tracheal deviation associated with a cranial mediastinal mass. D, A second radiograph was obtained with the dog’s head and neck in a neutral position. The trachea is now seen to be in a normal position, with no evidence of displacement. E, The trachea is displaced to the left by a mass in the neck. This was a thyroid mass. F and G, Tracheal collapse. A 7-year-old Yorkshire Terrier had been coughing in violent spasms for several months. Lateral studies of the trachea in expiration (F) and inspiration (G). The intrathoracic tracheal lumen is markedly narrowed at expiration, and the difference between the two phases of respiration is well demonstrated. H and I, A 9-year-old Boston Terrier was depressed, cyanotic, and tachypneic. H, A lateral radiograph shows severe narrowing of the intrathoracic trachea. There is widespread infi ltration of the lungs from hemorrhage. There is some air in the cervical esophagus. This was submucosal hemorrhage in the trachea caused by anticoagulant poisoning. I, Four days later there is almost complete resolution of the condition. (This is the same case as Figure 3-25, I and J.) J, This 1-year-old Pug had a history since birth of repetitive collapse after exercise. The tracheal lumen is grossly narrowed throughout its length. Diagnosis: tracheal hypoplasia.



Collapse


Tracheal collapse affects the smaller breeds of dogs in middle to old age. It may be acquired or congenital. The congenital form manifests itself in later life. The clinical signs comprise varying degrees of respiratory distress and a paroxysmal, chronic, dry “goose honk” cough. Because the usual type of collapse is in the dorsoventral plane, lateral radiographs are the most informative. Both inspiratory and expiratory radiographs of the full length of the trachea should be made with the forelimbs at right angles to the spine. A skyline or tangential view of the thoracic inlet with the dog in sternal recumbency and the head and neck extended dorsally is occasionally useful (see Figure 3-2, B). Extreme care should be taken because such positioning may exacerbate the clinical signs.



Radiologic Signs







Fluoroscopy and/or endoscopy is useful if the clinical signs suggest the diagnosis and may demonstrate collapse even when the inspiratory and expiratory radiographs are negative. Fluoroscopy is particularly useful in diagnosing mainstem bronchial collapse.


Some obese animals may have an apparent narrowing of the trachea because of superimposition of fat or flaccidity of the trachealis muscle, causing it to project into the tracheal lumen. In fat animals, the trachea is less clearly visualized than in thinner ones. The esophagus may overlie the trachea in such a way as to give the appearance of collapse. Careful examination may show the true tracheal outline.


Hepatomegaly has been reported in association with tracheal collapse, as has an enlarged left atrium, causing pressure on the left stem bronchus. Reasonable variations in the position of the neck will not affect the lumen of a normal trachea. Hyperextension may cause a pseudonarrowing. Tracheal collapse must be distinguished from congenital hypoplasia.













THE THORACIC CAVITY



THE SKIN


The skin forms part of the background opacity on radiographs of the thorax. Foreign substances on the skin may cause radiographic opacities within the thoracic shadow, thus simulating abnormalities. The skin should be examined visually and manually in cases of doubt. Prominent skinfolds often cause well-defined lines that traverse the thorax in a craniocaudal direction on ventrodorsal views. These lines may seem to represent lung edges and so lead to a false diagnosis of pneumothorax and collapsed lungs. Such skinfolds can usually be traced out beyond the confines of the thoracic cavity. On the lateral study, they may also be seen on the ventral third of the thorax associated with the forelimbs (see Figure 3-23, I and J). Teat shadows or skin masses superimposed on lung shadows should not be mistaken for intrapulmonary opacities (see Figure 3-6, P to S). Subcutaneous emphysema makes the thoracic cavity appear more radiolucent and can cause linear streaks or a honeycomb effect. The air-filled lungs provide good contrast for the demonstration of intrathoracic structures.


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Figure 3-6 Normal thorax. Right lateral recumbent (A) and dorsoventral (B) studies of the thorax. C, Dorsoventral thorax of a brachycephalic dog. In this breed type the mediastinum is wider than usual. D, Lateral radiograph of the thorax of a cat showing that the caudal lung fields do not fill the diaphragmaticolumbar recess (arrows) as do those of the dog. E and F, A 4-year-old Cavalier King Charles Spaniel with extensive intrathoracic fat deposits. E, The cardiac silhouette is displaced away from the sternum by a large fat deposit lying on the sternum. F, On the dorsoventral view the width of the mediastinum is increased because of the presence of fat. G and H, This 9-year-old German Shepherd had a malignant bone tumor. These are right (G) and left (H) lateral recumbent views of the thorax.G, A soft tissue opacity (arrows) can barely be distinguished overlying the cardiac shadow. H, Two soft tissue opacities superimposed on the cranial half of the cardiac silhouette are clearly shown. This also indicates that the opacities are in the right (uppermost) lung and emphasizes the importance of opposing recumbent views to demonstrate lesions in the chest. I and J, Normal feline thorax. On both projections, the heart has an ovoid or leaflike shape. The normal pulmonary vessels form a fine herringbone pattern in the caudal lung lobes. On the lateral view the caudodorsal lung margin curves away from the thoracic spine and is separated from it by a band of soft tissue. K and L, Lateral (K) and dorsoventral (L) views showing a prominent aortic arch (arrow) in an old cat. M, A transverse computed tomographic (CT) image of the cranial thorax, displayed in a lung window, at the level of the fourth rib. The major vessels, including the cranial vena cava and brachycephalic trunk, are visible in the mediastinum ventral to the trachea. The ventral mediastinum is a thin linear structure (arrows) running dorsoventrally between the left and right cranial lung lobes (the right side of the thorax is on the left side of the image). N, A transverse CT image of the thorax of a normal dog just caudal to the level of the tracheal bifurcation, displayed in a lung window. The esophagus and aorta are seen within the dorsal mediastinum, dorsal to the left and the right caudal lobar bronchi. The heart is the slightly irregularly shaped soft tissue structure ventral to the bronchi. In this window, internal cardiac structure is not visible. O, A transverse CT image of the thorax of a normal dog, at the level of the cardiac apex, displayed in a lung window. The cardiac apex is the soft tissue structure visible in the ventral thorax (star). A fold of the mediastinum, the plica vena cava, extends from the cardiac apex dorsally and toward the right and wraps around the caudal vena cava. The accessory lung lobe is positioned within the pocket formed by the caudal mediastinum on the left (arrows) and the plica vena cava and the caudal vena cava on the right. The thoracic aorta is visible just ventral to the thoracic spine within the mediastinum. Immediately ventral to this the esophagus (asterisk) is a fusiform soft tissue structure within the mediastinum. Multiple bronchi and paired pulmonary arteries and veins are seen within the left and right caudal lung lobes and the accessory lung lobe. P and Q, Pulmonary pseudonodule created by nipple in a dog. P, A close-up ventrodorsal view of the right caudal thorax shows an indistinct soft tissue opacity, a nodulelike lesion (arrows) superimposed on a rib, just medial to the costochondral junction. To exclude the possibility of a pulmonary nodule, the patient was examined. An enlarged nipple/teat was identified in the area, and barium paste was applied. Q, The second radiograph shows that the suspected nodule is now more evident as a result of the barium coating. R and S, Cutaneous nodule mimicking a pulmonary lesion in a dog. R, An ovoid soft tissue nodule (arrows) is seen superimposed on the cranial aspect of the heart at the level of the costochondral junctions on the initial lateral radiograph. Physical examination of the patient revealed a small cutaneous mass in this area. S, Barium paste was applied, and the radiograph was repeated. The second radiograph shows barium paste on the suspected pulmonary lesion, demonstrating that it is cutaneous in origin.



Radiography


For routine examination at least two views are necessary: a lateral view and a dorsoventral or ventrodorsal view. A comprehensive study should include two opposing lateral views and either a dorsoventral or a ventrodorsal view. Radiographs should be made during the inspiratory pause because when the lungs are filled with air, maximal contrast is achieved between the different structures within the thorax. The beam should be collimated to include the entire thorax from a point 2 cm cranial (just cranial to the manubrium) to the first rib to a point caudal to the first lumbar vertebra (to the midbody of the second lumbar vertebra). A grid should be used if the thorax measures 15 cm or more in thickness.


A machine with a capability of at least image1⁄30—or better, image1⁄60—of a second is desirable. Rare-earth screens reduce exposure times. At slower speeds, motion is not effectively excluded unless the animal is anesthetized and artificially ventilated. Motion causes blurring of intrathoracic structures and makes viewing of finer details impossible. Underexposure gives the impression of increased lung opacity. Films of obese patients are relatively underexposed, which may mimic pulmonary change. Overexposure blackens out normal vascular patterns and may mask pathologic changes. The use of a high-kilovoltage combined with low-milliamperage per-second technique gives a wider range of contrast than one of low kilovoltage. A good technique will barely outline the spinous processes of the cranial thoracic vertebrae on the lateral view.


It is essential that a technique be elaborated that will make possible the production of technically repeatable films. Unless radiographs of comparable technical quality can be produced on a day-to-day basis, it will not be possible to monitor progressive changes in pulmonary structures. The use of a technique chart and accurate measurement of the thickness of the thorax with calipers are essential aids to the production of good-quality radiographs.


On films made at expiration, the lung fields appear more opaque, and much of the pulmonary vasculature detail is lost. But expiratory films may be of value in pulmonary emphysema, in which air cannot be expelled from the lungs. In addition, expiratory films are useful for the identification of small volumes of pleural air or fluid and bronchial or tracheal collapse (see Figure 3-3, F).



Lateral View


The animal is placed in lateral recumbency. The forelimbs are drawn cranially and positioned parallel to one another. This position prevents superimposition of the triceps muscles on the apical portions of the cranial lung lobes. The sternum should be supported and should be on the same level and parallel to the thoracic vertebrae so that there is no rotation of the thorax relative to the incident x-ray beam. The neck is extended, or at least not flexed. The beam is centered at the fifth intercostal space, usually at the caudal edge of the scapula. There is little difference between radiographs made in left lateral recumbency and those made in right lateral recumbency. Right lateral recumbency may be preferred because in this position the phrenicopericardial ligament restricts movement of the cardiac apex toward the dependent side.


If pathology is suspected in one lung, that lung should be uppermost because the dependent lung does not inflate fully, and contrast in it is therefore diminished. For this reason, two opposing lateral studies are needed to fully evaluate both lungs. Standing lateral views are occasionally useful if pleural fluid or air is suspected. For this view, the cassette is positioned along the thoracic wall with the animal in the standing position. A horizontal beam is used, centered on the fifth intercostal space. A lateral view, using a horizontal beam and with the animal in sternal recumbency, is an alternative. Dorsal recumbency using a horizontal beam is occasionally used to examine the ventral thoracic region, but this position requires care in animals with compromised respiration.





Normal Appearance


On a satisfactory inspiratory radiograph, there should be good contrast between the pulmonary vessels, cardiac silhouette, and air-filled lungs. On the lateral view, the caudal vena cava is almost parallel to the long axis of the body. Little or no contact exists between the diaphragm and the cardiac shadow, nor should there be any superimposition of one on the other, though whether this is the case depends to some extent on the conformation of the animal. Animals with a deep, narrow thorax tend to have greater separation between the cardiac outline and the diaphragm than do animals with a more rounded thorax. The diaphragmaticolumbar recess is at approximately the level of the twelfth thoracic vertebra, and the diaphragm appears more flattened than rounded in deep-chested dogs. In left lateral recumbency on full inspiration, the ventral cardiac border may be displaced away from the sternum by the tip of the right middle lung lobe. This should not be mistaken for evidence of pneumothorax. It may also be displaced from the sternum in obese animals.


In the cat there is less variation in conformation than in dogs. The cardiac silhouette is somewhat oval in shape, and its long axis lies more obliquely to the sternum. The sternebrae are often not in direct alignment with one another. The diaphragmaticolumbar recess is not occupied by lung tissue. This should not be mistaken for fluid.


On the dorsoventral and ventrodorsal views, the angles between the diaphragm and the heart (cardiophrenic angles) and the diaphragm and the ribs (costophrenic angles) are well opened, and the cranial part of the diaphragm lies at approximately the level of the eighth to the tenth thoracic vertebrae. On the lateral view, projection of the dorsal curvature of the ribs unilaterally beyond the spine indicates that the animal was rotated axially when the radiograph was made. On dorsoventral and ventrodorsal views, the sternebrae and the spine should be superimposed on one another (Figure 3-6).


In the cat, the thorax appears more elongated and the cardiac outline is centrally located. In old cats, the aortic arch is often seen protruding cranial to the heart on the left side. The heart tilts more cranially in older cats (see Figure 3-6, D, I to L).



Ultrasonography


Routine ultrasonographic examination of the thoracic cavity is restricted in the main to examination of the heart. The ribs and lungs usually prevent imaging of thoracic structures. If the pathology is adjacent to the chest wall or the lungs have been displaced by fluid, then ultrasonographic examination is very useful. The transmission of sound waves is inhibited by the high acoustic impedance between the subcutaneous tissues and bone and between the subcutaneous tissues and the air-filled lung. Therefore acoustic windows must be found that optimize visualization of the heart. The acoustic windows lie on either side of the cranioventral thorax, where the heart is in contact with the ribs and consequently can be imaged directly through the intercostal spaces. The usual location is over the site of apex beat. This imaging location is termed the parasternal position.


The preferred technique for cardiac assessment is imaging the animal in the lateral recumbent position using a cutout table or elevated cutout platform on a table. The transducer is placed on the dependent side of the thorax. Recumbency ensures that the heart is as close as possible to the rib cage, displacing the adjacent lung. This position may also be useful in the general examination of other thoracic structures.


Sometimes an abdominal approach is used, called the subcostal approach. The transducer is located just caudal to the xiphisternum and angled cranially to locate the heart, using the liver as an acoustic window. This window is used to obtain a better Doppler angle of the aorta. This location is also useful for examining the diaphragm and caudal thorax. Rarely, the thoracic inlet may be used to image the cranial mediastinum and heart base—the suprasternal position.


Dyspneic animals are often more comfortable if they are examined while they are standing or lying in the sternal position. Sedation may be required with intractable dogs and cats. The intercostal space width is a practical limiting factor in small dogs and cats. The acoustic window is on the cranioventral thorax, where the intercostal spaces become narrow. The transducer footprint must be small enough to be able to be placed on an intercostal space close to the sternum. It must be capable of being angled cranially and caudally around the ribs.


A sector-type transducer is required for ultrasound examination of the heart because the transducers or probes have small footprints that allow examination of the heart through an intercostal space. In small dogs and cats a probe with a frequency range of 7 to 10 MHz is recommended. For medium-sized dogs, a frequency range of 5 to 7 MHz is desirable. Large- and giant-breed dogs may require a probe with a frequency range of 2 to 3 MHz.


Linear transducers may be used because they can be placed longitudinally along the intercostal space. However, the information thus gained may be limited because of the inability to manipulate the transducer.


Many ultrasound machines now offer multiple-frequency transducers that can cover a range of frequencies and allow the same probe to be used to evaluate patients of different sizes. Some ultrasound probes may offer the option of using a lower frequency for color and pulsed-wave Doppler interrogation than is used for B-mode imaging. The use of a lower frequency for Doppler interrogation allows the measurement of high-velocity blood flow without the problem of aliasing. Harmonic imaging uses ultrasound waves at frequencies that are multiples of the fundamental frequency of the probe. Harmonic imaging results in improved border delineation and image contrast. It works more effectively at relatively low frequencies. Implementation of harmonic imaging technology varies considerably from one manufacturer to another. In general, it results in significant improvement in image quality in echocardiography when using lower frequencies such as those for medium-sized to large dogs.


Depending on the purpose of the examination, the appropriate area of the thorax should be close clipped, the skin cleaned with surgical spirit if it is greasy or dirty, and acoustic gel applied.



THE BRONCHI






May 27, 2016 | Posted by in ANIMAL RADIOLOGY | Comments Off on The Thorax

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