The mediastinum

image of The mediastinum
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The mediastinum is the space between the left and right pleural cavities and encloses all the midline structures of the thorax. Its lateral are the mediastinal pleurae, which are reflections of the parietal pleurae as they leave the thoracic inlet and diaphragm. The mediastinum is continuous cranially with the fascial planes of the neck via the thoracic inlet and caudally with the retroperitoneal space through the aortic hiatus. The chapter considers Radiographic anatomy; Interpretive principles; Diseases.

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Image of 8.1
8.1 Transverse diagram of the mediastinum at the level of the second thoracic vertebra, showing the close association between the various soft tissue opacity structures in the cranial mediastinum. B = Brachiocephalic artery; CA = Right and left carotid arteries; Cm = Cranial mediastinum; CrVC = Cranial vena cava; DCV = Right deep cranial vertebral vein and left deep cranial vertebral vein; Ls = Left subclavian artery; M = Longus colli muscle; O = Oesophagus; T = Trachea; VV = Right and left vertebral veins. (Adapted from with permission)
Image of 8.2
8.2 DV view of the thorax, showing the position of the cranial, middle and caudal parts of the mediastinum, including the position of the thymus. A = Accessory lobe of the right lung (seen as a dotted area); CdVC = Caudal vena cava; Cm = Cranial mediastinum; CrVC = Cranial vena cava; CVM = Caudoventral mediastinal reflection; Mr = Mediastinal recess, which accommodates the accessory lobe of the right lung; RCr = Right cranial lung lobe (seen as a dotted area); Th = Position of vestigial thymus. (Adapted from with permission)
Image of 8.3
8.3 Lateral and DV thoracic radiographs of a skeletally mature cat highlighting the structures normally visible in the mediastinum. Note that in the normal cat the cranial mediastinum is no wider than the superimposed spine at this level. a = Trachea; b = Carina; c = Cardiac silhouette; d = Aorta; e = Caudal vena cava; f = Position of oesophagus; g = Width of cranial mediastinum; h = Cardiac silhouette apex; i = Caudoventral mediastinal reflection.
Image of 8.4
8.4 DV radiograph of a mature dog showing the cranioventral part of the mediastinum (arrowed). The animal is slightly rotated, which accentuates this structure. Close-up of a right lateral radiograph of a 2-year-old dog showing the cranioventral mediastinum (arrowed) as a thin line of soft tissue opacity between the right cranial lobe (R) and the left cranial lobe (L). Transverse CT image obtained at the level of the first rib and viewed on a wide window (window width 2000 HU, window level −500 HU). The cranioventral mediastinum is clearly seen between the left and right cranial lobes (arrowed).
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8.5 Close-up of a VD radiograph of the caudal lung fields of a skeletally mature normal dog. The caudoventral mediastinal reflection is seen as a narrow band of soft tissue opacity (arrowed) separating the accessory lobe (A) from the left caudal lobe (L). Transverse CT image obtained at the level of the accessory lobe (window width 2000 HU, window level −500HU). The caudoventral mediastinal reflection is clearly delineated (arrowed). The extension of the accessory lobe across the midline can be appreciated (A).
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8.6 DV thoracic radiograph of an 8-year-old Bulldog bitch. The cranial mediastinum is within normal limits for this breed (arrowed).
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8.8 Dorsal view of the tracheal bifurcation in the dog. The location of the tracheobronchial lymph nodes is shown. Lateral radiograph of a spayed 8-year-old Labrador Retriever bitch with chronic systemic coccidioidomycosis. Many small pulmonary, hilar and cranial mediastinal lymph nodes (some arrowed) are faintly mineralized in this dog. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
Image of 8.10
8.10 There are several important differentials for increased perihilar opacity. The normal perihilar structures are seen on lateral and DV views. Note that the angle between the caudal mainstem bronchi can vary with breed and centring of the X-ray beam. The mass effect created by simultaneous enlargement of the left and right tracheobronchial lymph nodes is demonstrated. Both nodes can accentuate the ventral deflection of the trachea at the carina. The left and right nodes can be separated by looking for the adjacent bronchus. The left bronchus bifurcates immediately ventral to the carina. The mass effect created by an enlarged middle tracheobronchial lymph node. Note that the caudal lobar bronchi are separated on the DV view and pushed on the lateral view. The mass effect created by the cranial mediastinal lymph nodes. If these nodes are large enough they can elevate the trachea. Note that heart base tumours are often located in this region and can produce a similar effect. Enlarged pulmonary arteries (and sometimes veins) can sometimes be mistaken for enlarged lymph nodes. This is a diagram of enlargement of the main pulmonary artery in dirofilariasis. Left atrial enlargement can be easily distinguished from middle tracheobronchial node enlargement. Both separate the caudal mainstem bronchi on a VD/DV view, but on the lateral view the bronchi are depressed ventrally with node enlargement and dorsally (especially the left) with left atrial enlargement. Enlarged perihilar (right, left and middle tracheobronchial) lymph nodes in a 3-year-old male neutered crossbred dog with systemic fungal disease. Note the ventral depression of the trachea and caudal lobar bronchi. Transverse CT image (soft tissue window). The enlarged middle tracheobronchial lymph node (arrowed) is identified dorsal to the heart (H). A consolidated region of the caudal part of the left cranial lung lobe is identified adjacent to the nodes (*). Dorsal CT reconstruction (lung window). The middle tracheobronchial lymph node (M) is identified displacing the caudal lobar bronchi laterally. Close-up of a lateral thoracic radiograph of a dog with multicentric lymphoma. The middle (turquoise), right (red) and left (blue) tracheobronchial and cranial mediastinal lymph nodes (yellow) are moderately enlarged. Close-up of a lateral thoracic radiograph of a 5-year-old Cocker Spaniel with multicentric lymphoma, demonstrating ventral deviation and split of cranial (red) and caudal (blue) lobar bronchi. Ventral lobar bronchial deviation is an important feature in hilar opacities to differentiate left atrial enlargment (dorsal bronchial displacement) from lymphadenopathy. (a, Reproduced from with permission)
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8.11 Lateral and VD images of a 6-year-old Australian Cattle Dog with mildly enlarged sternal lymph nodes (arrowed in (a), red lines in (b)) secondary to abdominal neoplasia.
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8.12 Close-up of a VD radiograph of the cranial thorax in an 8-year-old Border Collie. The widening of the cranial mediastinum (arrowed) was due to enlarged cranial mediastinal lymph nodes.
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8.13 Lateral view of the thorax of a 5-month-old female Domestic Shorthair cat. The thymus is seen as a faint triangular soft tissue opacity (arrowed) cranial to the heart. It was not visible on the DV view.
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8.14 DV thoracic radiograph of a 1-year-old Boxer bitch, showing a small thymic sail (arrowed) just cranial to the heart and to the left of the midline.
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8.15 Sagittal plane T2-weighted MRI scan of the thorax of a 3-month-old Italian Spinone bitch, demonstrating the thymus as a hyperintense mass (T) cranial to the heart.
Image of 8.16
8.16 Transverse diagram of the mediastinum at the level of the eighth thoracic vertebra and demonstrating the location of the plica venae cavae and Sussdorf’s space. (A = Aorta; Az = Azygos vein; CdVC = Caudal vena cava; CPL = The true cardiophrenic ligament is located ventrally; CVM = Caudoventral mediastinum; H = Outline of position of the heart; Mr = Mediastinal recess; O = Oesophagus; P = Plica venae cavae; SS = Sussdorf’s space. (Reproduced from with permission)
Image of 8.17
8.17 VD view of a mature dog with moderate rotation of the thorax. Note that the cardiac silhouette has moved in the direction as the sternum. Mediastinal shift is unlikely to be present. A straight VD view. Note that the cardiac silhouette is now in a normal position and no mediastinal shift is present. The previous displacement of the cardiac silhouette was simply due to rotation. DV thoracic radiograph of an 8-year-old neutered male Oriental cat with a unilateral tension pneumothorax, showing contralateral mediastinal shift. Although the radiograph is poorly positioned with marked rotation, the apex of the heart has rotated in the direction to the sternum, allowing confirmation of the mediastinal shift.
Image of 8.18
8.18 VD thoracic radiograph of an anaesthetized neutered male 6-year-old mixed breed dog, showing ipsilateral mediastinal shift due to lung atelectasis. The cardiac silhouette has moved towards the left due to diminished left lung volume. DV thoracic radiograph of a puppy with congenital lobar emphysema (see also Chapter 12). The trachea and cardiac silhouette are markedly shifted into the right hemithorax due to the presence of the emphysematous lobe on the left side. DV thoracic radiograph of a dog with a left cranial lobar pulmonary mass. Note the mediastinal shift seen as a movement of the cardiac silhouette towards the right, away from the mass.
Image of 8.19
8.19 Lateral and DV diagrams of the thorax illustrating the five main locations of mediastinal masses. A = Descending thoracic aorta; C = Caudal vena cava; F = Fat in ventral mediastinum; M = Shadow of intrathoracic part of longus colli muscle; O = Oesophagus; T = Trachea; V = Cranioventral masses; W = Craniodorsal masses; X = Hilar and perihilar masses; Y = Caudodorsal masses; Z = Caudoventral masses. (Reproduced from with permission)
Image of 8.21
8.21 A 9-year-old neutered male Labrador Retriever with a mediastinal and pleural effusion. The arrows demarcate a triangular ‘reverse fissure’, likely to represent mediastinal fluid.
Image of 8.22
8.22 Tranverse CT images obtained from a normal dog with a soft tissue algorithm and viewed with a soft tissue window (window width 300 HU, window level 40 HU) at three different levels in the thorax. All images were taken after the administration of intravenous non-ionic iodinated contrast media. At the level of T2. The cranial vena cava (C) and trachea (T) are marked. At the level of the heart base. The aorta (A), oesophagus (O) and contrast within the left ventricle (L) and right ventricle (R) are seen. At the level of the ninth thoracic vertebra. The aorta (A), oesophagus (O) and caudal vena cava (C) are seen. The other hyperattenuating structures within the lung fields (seen as small round structures) are the pulmonary arteries and veins beginning to fill with contrast medium.
Image of 8.24
8.24 A 5-year-old mixed breed dog with a perforating oesophageal foreign body (a fish hook). A subtle pneumomediastinum is visible as small amounts of gas delineating the outer surface of the cranial intrathoracic trachea (arrowed). This is an extremely important radiographic finding. Lateral thoracic radiograph of an 8-year-old neutered female Domestic Shorthair cat with pneumomediastinum, pneumothorax, extensive subcutaneous emphysema and pulmonary contusions after a road traffic accident. A ruptured trachea was identified on endoscopic examination. Close-up of a lateral radiograph of an emaciated 12-year-old Domestic Shorthair cat with extension of pneumomediastinum to pneumoretroperitoneum. The thoracic and abdominal aorta and kidneys are highlighted by free gas. Minimal trauma to the aortic hilus allows gas tracking into the retroperitoneal space. This is an incidental finding but it should be differentiated from free abdominal gas originating from ruptured abdominal organs.
Image of 8.25
8.25 Transverse CT image (lung window) from a neutered male 10-year-old Domestic Shorthair cat at the level of the heart. A small volume pneumomediastinum (M) and pneumothorax are present. Gas is also tracking along the peribronchial and perivascular sheaths. The cat had a CT scan for nasal neoplasia but the pneumomediastinum arose due to a presumed bronchial tear from anaesthetic ventilation (barotrauma).
Image of 8.26
8.26 CT image at the level of the first cervical vertebra of a male 3-year-old Labrador Retriever, which presented with a pneumomediastinum. A hyperattenuating structure was identified in the left ventrolateral perilaryngeal tissues surrounded by soft tissue swelling (window width 300 HU, window level 50 HU). During exploratory surgery this structure was found to be a stick.
Image of 8.27
8.27 Mediastinal masses: sites, causes and main radiological signs.
Image of 8.28
8.28 Lateral and DV thoracic views of a 3-year-old Siamese cat with thymic lymphosarcoma. The hilar lymph nodes are enlarged and a small volume pleural effusion is also present. The origin of the mass is clearly mediastinal on the DV view as it is located in the midline and displaces both left and right cranial lung lobes caudally and abaxially. The carina is displaced to the seventh intercostal space, indicating caudal cardiac displacement. Lateral thoracic radiograph of a Himalayan cat with nodular fat necrosis in the retrosternal fat deposit in the caudoventral mediastinum. The eggshell-like mineralization is a typical feature of these incidental lesions that are more commonly seen in the peritoneal space. Lateral thoracic radiograph of a dog that was involved in a road traffic accident, causing mediastinal haematoma formation and pneumothorax. The mediastinal haematoma can be seen as an irregular soft tissue mass filling out the cranial and caudoventral mediastinum. The trachea is deviated dorsally by the haematoma.
Image of 8.29
8.29 The appearance of mediastinal fat on ultrasonography. Dorsal image of the cranial mediastinum of a 6-year-old Cocker Spaniel in obese body condition. The hypoechoic cardiac chambers are labelled (heart). Cranial to the heart and distal to the thickened, hyperechoic thoracic wall is a large amount of homogenous tissue with a coarse echotexture, representing mediastinal fat. Dorsal view of the cranial thorax of a 9-year-old male neutered Maltese. There is hyperechoic tissue (F) with a triangular shape present cranial to the heart, the tip is surrounded by a very small amount of pleural effusion (arrowed). Mediastinal fat was suspected and confirmed by fine-needle aspirates. (Courtesy of G. Seiler)
Image of 8.30
8.30 Dorsal image of the cranial thorax of a spayed female 10-year-old Domestic Shorthair cat. A large heterogenous mass (arrowed) is seen cranial to the heart in the cranial mediastinum. The mass is surrounded by bilateral pleural effusion (*). Carcinoma with haemorrhage was diagnosed on biopsy of this mass. Cranial mediastinum of a neutered 5-year-old Flat-Coated Retriever bitch with a round cell sarcoma. A lobulated mass is identified (*). Enlarged mediastinal lymph nodes are also present. (Courtesy of G. Seiler)
Image of 8.31
8.31 Dorsal view of the cranial thorax of a neutered male 5-year-old Domestic Shorthair cat. The cranial mediastinum is hyperechoic (fat) but several hypoechoic nodular structures are seen within it, representing enlarged cranial mediastinal lymph nodes (arrowed). There is a thin layer of pleural effusion present along the thoracic wall; small pockets of fluid may also be present within the mediastinum. (Courtesy of G. Seiler)
Image of 8.32
8.32 Transverse left intercostal view, dorsal is to the right. A large, hypoechoic, lobulated mass occupies the cranioventral thorax (M). The mediastinal vessels are seen as round hypoechoic structures in the dorsal mediastinum and are partially surrounded by the mass. Ultrasound-guided biopsy of the mass led to the diagnosis of thymoma. Dorsal view of the cranial mediastinum. The cranial mediastinum is filled by a hypoechoic, lobulated mass which was subsequently diagnosed as thymoma. (Courtesy of G. Seiler)
Image of 8.33
8.33 CT image at the level of the fifth intercostal space of a spayed 7-year-old Labrador Retriever bitch with a thymoma (T) (window width 300 HU, window level 50 HU). Post-contrast CT image at the level of the fifth intercostal space of a neutered male 5-year-old crossbreed with a thymoma (T) (window width 300 HU, window level 50 HU). Pleural effusion (P) is also present.
Image of 8.34
8.34 CT image at the level of the seventh intercostal space of a spayed 2-year-old Dogue de Bordeaux bitch with lymphoma, undergoing CT-guided biopsy of this lesion (window width 2000 HU, window level −500 HU). The needle (N) is shown within the skin and from this position is advanced further into the mass (see also Chapter 3).
Image of 8.35
8.35 Right lateral (shown on left) and ventral (shown on right) images of the head, neck and cranial thorax of a 12-year-old hyperthyroid Domestic Shorthair cat, obtained 20 minutes after intravenous injection of sodium Tc-pertechnetate. Thoracic radiographs obtained prior to the study revealed an ill defined cranial mediastinal mass. On the scintigraphic images there is a focus of uptake associated with the right thyroid lobe, much higher in intensity than the salivary glands. The left thyroid lobe is not visible. A second intense focus of uptake is seen in the thorax, corresponding to the lesion seen on radiographs. Final diagnosis was hyperthyroidism with ectopic hyperfunctioning thyroidal tissue in the cranial mediastinum. (Courtesy of F. Morandi)
Image of 8.36
8.36 Left lateral thoracic radiograph of a 12-year-old Siamese cat with two small cranial mediastinal cysts (arrowed). Dorsal view of the cranial thorax (cranial to the heart) of a spayed female 12-year-old Domestic Shorthair cat. On radiographs a round soft tissue opacity was seen cranial to the cardiac silhouette. Ultrasonographically an anechoic fluid-filled structure separated into two cavities by a thin septum was found and a mediastinal cyst was diagnosed. CT image of the caudal mediastinum of an 11-year-old St Bernard with two large fluid-filled cystic structures (C) in the caudal mediastinum wedged between the caudal vena cava (CV), oesophagus (*) and aorta (A). Final diagnosis was cystic adenocarcinoma arising from the accessory lung lobe. GB = Gallbladder. (Courtesy of G. Seiler)
Image of 8.37
8.37 Dorsal view of the ventral aspect of the cranial mediastinum of a 3-year-old cat with a moderate pleural effusion. The entire width of the cat is included in the image. Note the ribs (R) on the left and right. The cranial mediastinum is seen as a narrow echogenic band (between arrows) between the left and right pleural spaces which are filled with an anechoic effusion. Note the small blood vessels identified within the mediastinum with colour Doppler.

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