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The thoracic boundaries

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Abstract

The boundaries of the canine and feline thoracic cavity consist of the thoracic skeleton, the cranial and caudal thoracic apertures and the covering soft tissue structures. The rib cage is covered by thoracic, pectoral, spinal and other musculature, subcutaneous fat and skin, and serves as an attachment for the thoracic extremities. The chapter considers Radiographic anatomy; Interpretive principles; Diseases of the thoracic skeleton; Diseases of the thoracic inlet; Diseases of the diaphragm.

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Figures

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14.1 Lateral view of a canine thoracic spine (ribs removed) where two bony processes arise from the transverse process. The mamillary process (*) diverts craniodorsally and eventually merges with the cranial articular process of T11, whereas the accessory process (#) projects caudally.
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14.2 Lateral thoracic radiograph of a normal 8-year-old Domestic Shorthair cat. The diaphragm is visible in its ventral portion (arrowed), outlined by the aerated lungs and retrosternal fat cranially, and falciform fat caudally.
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14.3 The feline and canine rib cage in inspiration (left) and expiration (right). The feline thorax has a truncated cone shape with near-parallel oriented ribs, allowing maximal respiratory excursion but offering less stability. The canine thorax is barrel-shaped. The fan-shaped array of ribs allows less respiratory excursion but offers more stability. (Redrawn after with permission from ). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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14.4 Close-up of a DV thoracic radiograph of an 8-year-old Boxer with a primary osteosarcoma in the distal third of the left tenth rib. There is an expansile predominantly osteolytic rib lesion (arrowhead) and adjacent external and internal (extrapleural) soft tissue swelling (arrowed).
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14.5 Lateral thoracic spinal radiograph of a 13-year-old Jack Russell Terrier with chronic hyperadrenocorticism. All visible vertebrae and ribs are relatively osteolucent. The ribs are almost not visible and the vertebral margins stand out against the more demineralized medullary cavity. Obesity contributes to the low image contrast.
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14.6 Close-up of the left thoracic wall from a DV radiograph of a 9-year-old Burmese cat with a primary rib osteosarcoma. Notice the marked irregular new bone formation, primarily on the external thoracic wall. (Courtesy of the Animal Health Trust)
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14.7 Lateral thoracic radiograph of a 7-month-old Domestic Shorthair cat that presented with spontaneous bilateral patellar fractures. There is increased medullary opacity of all visible bones consistent with osteopetrosis. Smooth callus from an old healed rib fracture (arrowhead) can be seen. The exact cause of the disease could not be established, but the multiple pathological fractures and age of the cat suggest a genetic disorder.
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14.8 Lateral thoracic radiograph of a Shar Pei. The extremely folded skin can create a bizarre pattern of opaque and lucent stripes. Notice how the folds extend beyond the ventral and dorsal margins of the thoracic cavity. Close-up of a DV radiograph of the left caudal thorax of a dog. There is a very lucent area in the caudolateral thorax, separated from normal lung tissue by a sharp line. The line extends cranially beyond the thoracic cavity (arrowheads) indicating that this is a skin fold. A close inspection with an intense light source should be undertaken to rule out pneumothorax.
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14.9 Lateral thoracic radiograph of a 5-year-old mixed breed dog that sustained bite injuries from another dog. Notice the finely dispersed gas bubbles over the ventral thorax and subcutaneous gas pockets dorsally.
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14.11 Unilateral diaphragmatic displacement. A depressed right hemidiaphragm and a raised left hemidiaphragm, lead to an increased symmetry of the diaphragmatic shadow. A raised right hemidiaphragm or a depressed left hemidiaphragm, lead to an increased asymmetry of the diaphragmatic shadow. Note the different levels of the costodiaphragmatic angles between the right and the left side saw tooth-like diaphragmatic surface: tenting of the diaphragmatic rib cage insertion (forceful diaphragmatic contraction). (Adapted from with permission). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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14.12 DV thoracic radiograph of a 3-year-old Bengal cat with an old healed chest wall injury from a road traffic accident. Marked asymmetry of the diaphragm is present due to the abnormal thoracic wall conformation. This is a clinically non-significant finding as the costophrenic angles are at the same level and no respiratory signs were present.
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14.14 Lateral thoracic radiograph of a 3-year-old Domestic Shorthair cat with inspiratory hyperpnoea. The lungs are maximally extended, resulting in caudal deviation and flattening of the diaphragm, and extension of the cranial lung margins beyond the thoracic inlet. DV thoracic radiograph of a 2-year-old Domestic Shorthair cat with left pyothorax and right compensatory hyperpnoea. The diaphragmatic silhouette (arrowhead) is displaced and bent caudally by the increased intrathoracic pressure. Recognition of the diaphragmatic silhouette helps to rule out a diaphragmatic rupture with abdominal organ prolapse.
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14.15 Close-up of an inspiratory VD radiograph of the diaphragmatic silhouette of a 14-year-old hyperpnoeic Domestic Shorthair cat with asthma. Notice the subtle tent-shaped margins of the diaphragm (arrowhead) due to maximal phrenic contraction and pulling at the costal arch insertion. VD thoracic radiograph of an 8-year-old Cairn Terrier with severe inspiratory dyspnoea. For an inspiratory radiograph the diaphragmatic dome is not flat enough, yet its irregular contour, tenting and rib spacing suggest increased respiratory effort against some obstructive force. Lateral neck radiograph demonstrates a large laryngeal mass, diagnosed as a lymphoma.
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14.17 Close-up of a lateral radiograph of the thoracic inlet of a Soft-Coated Wheaten Terrier with a ventrally fused left first thoracic and supernumerous seventh cervical rib. This is an incidental finding.
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14.18 Close-up of a lateral thoracic radiograph of the cranial sternum of a 2-year-old Labrador Retriever without trauma history. The manubrium is kinked, most likely due to an embryogenic fusion anomaly. This is an incidental finding.
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14.19 Hemivertebra formation. Normally, paired mesodermal segments, called somites, merge and form a cartilaginous precursor of the vertebral body (upper and lower pair). If a mismatch in the fusion pattern occurs (hemimetametric displacement), unmatched somites will form wedge-shaped hemivertebrae, resulting in a spinal curvature deformity.
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14.20 Lateral thoracic spinal radiograph of an 8-year-old Bulldog with hemivertebra formation of the eighth thoracic vertebra, resulting in marked kyphosis. The corresponding VD radiograph shows the additional scoliosis and associated abnormal rib spacing. Hemivertebra formation in the mid-thoracic spine is almost ubiquitous in Bulldog-type breeds and is usually of no clinical significance. Severe cases may lead to spinal cord compression.
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14.21 Lateral thoracic radiograph of a 12-year-old Domestic Shorthair cat with a thymoma. There is uneven spacing of the mid-thoracic ribs. 3D reconstruction from a CT scan demonstrates scoliosis and fusion of the origin of the right fifth and sixth ribs and missing left sixth rib due to hemivertebra formation.
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14.22 Lateral thoracolumbar spinal radiograph of a skeletally mature Boxer with bony spur formation on the endplates (spondylosis deformans) and exuberant new bone formation along multiple vertebral bodies (disseminated skeletal hyperostosis). Despite the sometimes dramatic appearance, neither condition usually has any clinical significance.
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14.23 Lateral thoracic myelogram of a 12-year-old German Shepherd Dog with chronic aggravating hindlimb weakness. There is marked dorsal deviation and thinning of the subarachnoid contrast medium column at the T6–7 intervertebral disc space. Sagittal gross image at postmortem. The T6–7 intervertebral disc was degenerated and protruded. The spinal cord (removed) was markedly compressed and atrophied by the disc and dorsal longitudinal ligament proliferation. The mid-thoracic spine should not be discounted if spinal cord abnormalities are clinically confined to the T3–L3 spinal cord segment. (Courtesy of L. Jarrett)
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14.24 VD thoracic radiograph of a normal 9-year-old Basset Hound. A soft tissue opaque rim separates the right lateral lung margin from the ribs. This is due to the S-shaped rib anatomy in this chondrodystrophic breed.
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14.25 Lateral thoracic spinal radiograph of a skeletally mature dog with congenital hypothyroidism. The vertebrae are stunted and have irregular epiphyses.
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14.26 Close up of a lateral thoracic radiograph of a 4-month-old Golden Retriever with mild pectus excavatum and an atrial septal defect. Notice the dorsal deviation of the caudal sternebrae, causing an extrapleural sign. Rib cage anomalies are commonly associated with other congenital defects. Lateral thoracic radiograph of a 7-year-old Pekingese with severe pectus excavatum. VD view showing the associated left-sided deviation of the heart.
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14.27 Close-up of a DV thoracic radiograph of a 7-year-old Golden Retriever with multiple segmental rib fractures as a result of a road traffic accident. There is also adjacent lung consolidation consistent with contusion. Flail chest causes paradoxical respiratory movement and requires surgical stabilization.
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14.28 Lateral inspiratory thoracic radiograph of an 11-year-old Domestic Shorthair cat with dyspnoea and a suspected trauma history. There is a marked dorsal deviation of the sternum, resembling a pectus excavatum malformation. Repeat expiratory radiograph reveals a normal sternal position. This is consistent with a flail sternum caused by multiple bilateral fractures of the ventral rib cartilages. Due to the incomplete mineralization of those cartilages, the fractures are not radiographically visible. (Courtesy of the University of Pennsylvania)
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14.29 Close-up of a lateral thoracic radiograph of a 7-year-old German Shepherd Dog that was involved in a road traffic accident. There is dorsocaudal lung opacification, a small pneumothorax and several rib fractures which are difficult to see. The corresponding DV radiograph demonstrates more clearly a series of six rib fractures. Orthogonal views can be very helpful in identifying rib fractures.
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14.30 Close-up of a lateral thoracic radiograph of a 10-year-old Domestic Shorthair cat with chronic inspiratory dyspnoea due to a nasal adenocarcinoma. Notice the serial fractures in the proximal aspect of three ribs with round widened margins (non-union). The history and fracture location (serratus dorsalis cranialis muscle insertion) are most consistent with stress fractures, secondary to chronic hyperinflation.
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14.31 Close-up of a lateral thoracic radiograph of a 10-year-old Labrador Retriever with an old untreated dislocation of the fourth intersternebral joint. The sternum is more likely to dislocate than to fracture on blunt force trauma. This is usually of no clinical significance, although some instability might be palpable.
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14.32 Close-up of a thoracic spinal radiograph of a 4-month-old English Springer Spaniel, which was involved in a road traffic accident. There is a minimally displaced physeal fracture of the caudal endplate of the eleventh thoracic vertebra, which is barely recognizable (arrowhead). Postmortem specimen radiograph clearly demonstrates the fracture and displacement. A diligent fracture search is indicated if spinal trauma is suspected. The muscular bracing often prevents large displacement of fracture elements. DV postmortem specimen thoracic spinal radiograph of a 2-year-old paraplegic Irish Terrier, which was involved in a road traffic accident. There is complete luxation of the T5–6 intervertebral and right vertebrocostal joint, a fracture of T6 and complete shearing of the spinal cord. Thoracic spinal dislocation is unusual in dogs and cats, and most likely to be related to strong bending forces associated with a blunt force trauma.
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14.33 CT image of the twelfth thoracic vertebra of a 1-year-old Pug, which was involved in a road traffic accident. There is a comminuted compression fracture involving the ventral, middle and dorsal compartment of this vertebra with compression of the spinal cord.
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14.34 The different facets of osteochondromatosis. Most commonly encountered as a polyostotic small irregular mineralization at the costochondral junction, which may give rise to an extrapleural sign (arrowhead), as in this 12-year-old Golden Retriever. Occasionally, a single costochondral osteochondroma is present, as in this 4-year-old Shetland Sheepdog. Other areas of the ribs can be affected, as in this dog with multiple cartilaginous exostoses in the ribs and appendicular skeleton. Other bones, such as the sternum, can give rise to osteochondromatosis, as in this 3-year-old Domestic Shorthair cat. If osteochondromas occur in the thoracic spine, such as seen on this CT image of the sixth thoracic vertebra in a 2-year-old Great Dane, they can cause significant spinal cord compression. Osteochondromas are osteoproliferative lesions with variable margination. In contrast to most malignant bone tumours they are not osteolytic, but may cause pressure atrophy of adjacent bones.
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14.35 Cranial thoracic close-up of a DV radiograph of an 11-year-old Golden Retriever bitch with a chondroblastic osteosarcoma, originating from the right second rib. This tumour is osteoproductive, expands into the axilla and impinges on the brachial plexus. Lesion-oriented oblique radiograph of the left tenth rib of a 8-year-old Boxer with a primary rib osteosarcoma (see also Figure 14.4 ). The rib osteolysis (arrowhead) and internal soft tissue swelling are highlighted by this view. Left lateral thoracic radiograph of a 6-year-old Bull Mastiff with a primary osteosarcoma of the right sixth rib and haemothorax. This view highlights the extrapleural sign, owing to the inward-growing mass surrounded by aerated lung. Right lateral view of the dog in highlights the lysis and amorphous bone production of the mass.
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14.36 Different appearances of soft tissue chest wall masses. Close-up of a DV radiograph of the right shoulder of a skeletally mature cat with a large axillary soft tissue mass, which displaces the scapula laterally. No bony erosion is evident. Extrathoracic masses can create bizarre radiographic features. Lateral thoracic radiograph of a cat with a left chest wall fibrosarcoma. The mass shadow merges with the cardiac silhouette, creating the appearance of a partially ectopic heart. VD view of the cat in (b) clearly shows the large soft tissue mass on the external thoracic wall. Subcutaneous lipomas are frequently seen in dogs and can create size-dependent indistinct opacifications. Occasionally, they arise from the internal thoracic wall and compress the thoracic organs, as shown in this DV thoracic radiograph of a skeletally mature dog. Due to its fat opacity, this lipoma does not cause border obliteration with the heart and diaphragm.
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14.37 Ultrasonographic image of the chest wall of a dog with cutaneous haemangiosarcoma. Notice the mixed echoic appearance of the mass, containing cystic hypoechoic content consistent with central necrosis or haemorrhage. In comparison, this ultrasonographic chest wall image of a dog with a lipoma has a much more homogenous hyperechoic appearance.
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14.38 Thoracic CT image at the level caudal to the carina of a 21-month-old Rottweiler with a large primary chondrosarcoma of the left sixth rib and haemothorax.
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14.39 Lateral scintigram of the cranial thoracic skeleton of a 16-year-old mixed breed dog with a primary osteosarcoma of one of its caudal most ribs, obtained approximately 2 hours after intravenous injection of a bone-binding diphosphonate compound. The primary tumour is not included in the scan but numerous focal areas of intense bone tracer localization, consistent with costal, sternal and spinal metastases can be seen. (Courtesy of F. Morandi)
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14.40 Lateral myelogram of a 7-year-old Border Collie with a primary bone tumour of the second thoracic vertebra (T2). There is very subtle osteolysis in the vertebral body and the ventral myelographic contrast medium column is elevated. Radiography has a poor sensitivity to detect bony lysis. The corresponding CT image demonstrates the extensive lysis and spinal cord compression. Lateral radiograph of the caudal thoracic spine of a 6-year-old mixed breed dog with vertebral neoplasia of the ninth thoracic vertebra. The vertebra is mildly osteosclerotic compared with its neighbours, an unusual manifestation of a spinal tumour. Lateral thoracic radiograph of a 10-year-old Labrador Retriever with multiple myeloma. Notice the numerous punched-out osteolucencies in multiple vertebral bodies, a classic radiographic feature of this neoplasm.
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14.41 Lateral thoracic radiograph of a 3-year-old Labrador Retriever with a caudodorsal mediastinal mass that was diagnosed as a paraganglioma. The corresponding post-contrast CT image at the level of the seventh thoracic vertebra shows the mass infiltrating the spinal canal. Note the compression of the spinal cord (*) and in the dorsal mediastinum where it completely surrounds the aorta (O). Neuroendocrine tumours are rare but the dorsal mediastinum is a common location. They are often extremely invasive.
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14.42 Close-up of a DV thoracic radiograph of a 6-year-old Domestic Longhair cat with chronic osteomyelitis of the right tenth rib (arrowhead) as a result of cat bite injuries. There is marked smoothly marginated bony proliferation along this rib, consistent with chronic or previous osteomyelitis. Adjacent ribs are also mildly affected.
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14.43 Lateral thoracic radiograph of a 4-month-old kitten with nutritional hyperparathyroidism. Notice the marked generalized osteopenia, spinal curvature deformity and sternal dislocation.
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14.44 Ultrasonographic image of a dog with a migrating grass awn (between white callipers) within the intermuscular layers of the chest wall. Another partially visible grass awn is ventral to it. The spindle-shape appearance is characteristic for this type of plant material. Ultrasonographic image of a 2-year-old Dobermann with a wooden skewer foreign body in the pleural cavity. A linear hyperechoic line surrounded by hypoechoic material can be seen. Ultrasonographic image of the chest wall of a 5-year-old German Shepherd Dog with a chest wall abscess. There is a hypoechoic area surrounded by a thick hyperechoic capsule, consistent with abscessation.
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14.45 Possible features of sternal osteomyelitis. An 8-year-old German Shepherd Dog with an irregular periosteal reaction along several sternebrae, and soft tissue swelling. Dog with a smooth periosteal reaction, sternebral fusion and adjacent soft tissue swelling with sinus tract. This is an ongoing chronic infection. A 3-year-old Weimaraner 5 months after sterniotomy, showing predominantly osteolytic changes along several vertebrae, lucencies around the cerclage wires and soft tissue swelling.
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14.46 Close-up of a lateral thoracic spinal radiograph of a 7-year-old Irish Setter with discospondylitis in the T7–8 intervertebral disc space. The disc space has collapsed and there is an irregular osteolucency in the ventral aspect of the endplates of both adjacent vertebrae (arrowhead). This is a relatively early stage of the disease.
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14.47 Lateral and DV thoracic radiographs of an 18-month-old dyspnoeic Jack Russell Terrier that had been involved in a road traffic accident. A large lucent cavitary structure is present in the left hemithorax, consistent with a bloated displaced stomach. Note the absence of a gastric shadow in the cranial abdomen. A distended displaced stomach is a surgical emergency. Lateral thoracic radiograph of a 7-year-old Domestic Shorthair cat. There is loss of the ventral diaphragmatic outline, dorsal displacement of the cardiac silhouette and trachea, increased opacity of the ventral thoracic cavity and cranial displacement of the abdominal organs with loss of visibility of the falciform fat, consistent with a diaphragmatic rupture and abdominal organ prolapse.
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14.48 Lateral barium study radiograph of a 9-month-old Domestic Shorthair cat with a diaphragmatic rupture. The barium-filled pyloric antrum and liver are prolapsed into the thorax.
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14.49 Lateral and VD radiographs of pneumoperitoneum in a 9-year-old German Shepherd Dog following a laparotomy. The normal diaphragmatic silhouette is outlined by gas on both sides. Pneumo- or capnoperitoneography can be used to outline interruptions in the diaphragmatic silhouette. Spontaneously occurring free peritoneal gas tends to accumulate between the liver and the diaphragm, and should be recognized as abnormal and raise suspicion for an abdominal hollow viscus organ rupture.
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14.50 Slightly oblique VD radiograph (lesion-oriented view) of a 3-year-old Siamese cat with a right paracostal body wall hernia. Food and gas-filled loops of intestine can be identified within the hernial sac. Local soft tissue swellings of the chest wall warrant diagnostic imaging procedures.
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14.51 Different types of hiatal hernias. Sliding hiatal hernia. Paraoesophageal hiatal hernia into the pleural space. Paraoesophageal hiatal hernia into the Sussdorf’s space (). LES = Lower oesophageal sphincter; ▪▪▪▪▪ = With or without hernial sac. (Adapted from with permission). Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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14.52 Lateral thoracic radiograph of a 4-year-old Domestic Shorthair cat with a sliding intramediastinal hiatal hernia. There is an ovoid soft tissue opacity in the caudodorsal thorax between the aorta and the CdVC, which varied in appearance between different radiographs, consistent with a hiatal hernia. Hiatal hernias may develop secondary to chronic respiratory disease with increased inspiratory effort.
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14.53 Lateral barium study of a 4-month-old Cornish Rex cat with a hiatal hernia. The rugal folds are clearly outlined with barium, and the gastric fundus is located in the caudodorsal thorax. Contrast studies can be helpful to confirm the presence of a hiatal hernia.
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14.54 Lateral and VD radiographs of a 16-weeks-old Persian cat with a PPDH. The cardiac silhouette is grossly enlarged and merges with the diaphragmatic outline ventrally (lateral view) and centrally (VD view). These features distinguish PPDHs from diaphragmatic ruptures.
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14.55 Lateral barium study of a 3-year-old Domestic Shorthair cat with a PPDH. The stomach axis is cranioventrally displaced, and the pylorus and proximal duodenum are contained within the pericardial sac.
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14.56 Ultrasonographic image of the liver of a 10-year-old British Shorthair cat with an incidental PPDH. The liver (L) lies in direct contact with the left ventricle (LV) without intersecting the diaphragm. Mitral valves are arrowed.
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14.57 Lateral thoracic radiograph of a 14-year-old Domestic Shorthair cat with hyperthyroidism. The right kidney is partially herniated into the very dorsocaudal mediastinum and deviates the aorta. This is consistent with an aortic hiatal hernia, which was an incidental finding in this cat.
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14.58 Lateral thoracic radiograph of a Labrador Retriever with muscular dystrophy. Notice the scalloped irregular protrusions of the diaphragm caused by the hypertrophied diaphragmatic musculature. (Courtesy of N. Lester)
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14.59 DV thoracic radiograph of a 3-month-old Boxer with right-sided diaphragmatic paralysis. The diaphragm has an increased asymmetry due to the elevated right hemidiaphragm; the right costophrenic angle is positioned more cranially than the left. Note the cranial direction of the ribs of the right thoracic wall.
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14.60 Inspiratory lateral thoracic radiographs of a 9-year-old Cocker Spaniel following a pericardectomy and iatrogenic damage to both phrenic nerves. The position of the diaphragm is unchanged between the two stages of the respiratory cycle, but the lungs are more inflated on the inspiratory radiograph. Expiratory lateral thoracic radiographs of a 9-year-old Cocker Spaniel following a pericardectomy and iatrogenic damage to both phrenic nerves. The position of the diaphragm is unchanged between the two stages of the respiratory cycle, but the lungs are more inflated on the inspiratory radiograph.

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