Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

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Chapter Two The Thorax 39 The sternebrae and their intersternebral cartilages should be evaluated. The amount of mineralization of the chondral and intersternebral cartilages usually increases with age and is greater in larger dogs. Malalignment of sternebrae is observed frequently and is insignificant unless accompanied by intrathoracic or extrathoracic soft-tissue swelling, pleural effusion, or clinical signs. Sternebral malformations usually are insignificant. Variations in the size and number of sternebrae and in the shape of the manubrium and xiphoid are common. Thoracic conformation varies with breed and each variation influences the appearance of the thoracic viscera. It is important to note the animal’s thoracic shape and evaluate the thoracic viscera accordingly. Three major categories may be observed: 1. Deep and narrow—such as Doberman Pinschers, Afghan Hounds, Collies, and Whippets (Fig. 2-11). 2. Intermediate—such as German Shepherd dogs, Labrador Retrievers, and Dalmatians (Fig. 2-12). 3. Shallow and wide—such as English Bulldogs, Basset Hounds, and Dachshunds (Fig. 2-13). Some variation occurs even within the same breed; therefore categorization should be based on each individual’s conformation. D I A P H R AG M The diaphragm defines the caudal margin of the thoracic cavity. It is visible radiographically, because the air-filled lung contacts its smooth cranial surface. With deep inspiratory efforts, the diaphragmatic attachments to the thoracic wall may appear on the ventrodorsal or dorsoventral views as peaks or scalloped margins along the diaphragm margin. This has been referred to as tenting of the diaphragm. The caudal margin of the diaphragm blends with the shadows of the liver and stomach. The diaphragm is composed of a right and left crus in its dorsal aspect and a central cupula, or dome, in the ventral portion. There are three separate openings through the diaphragm for the aorta, esophagus, and caudal vena cava. Because of its curved shape, the diaphragmatic profile changes with the animal’s position and the x-ray beam angle. 10 The diaphragm’s shape is influenced also by respiratory phase, pressure from intrathoracic structures (e.g., normal, hypoinflated, or hyperinflated lungs, pulmonary masses) or abdominal contents, breed, obesity, and age. Because of the diaphragm’s complex shape, aerated lung will be superimposed over the liver and pulmonary pathology may be more readily identified in those areas, depending upon the radiographic technique that was used. In the ventrodorsal and dorsoventral radiographs, the cranial peak of the cupula frequently lies to the right of the midline. The caudal vena cava interrupts the diaphragmatic outline at about this point. The crura may be identified as separate structures overlapping the cupula laterally, curving caudally toward the midline, and blending with the vertebral column at about T10. The crura are more often evident on a ventrodorsal view, because the angle of the x-ray beam divergence will allow some portions of the beam to be tangential to the individual portions of the diaphragm. However, there is considerable variation among individuals, so the appearance is variable and one, both, or neither crura may be seen. On the lateral radiograph, all three portions of the diaphragm are usually evident, with the dependent crus usually cranial to the opposite crus. 10 The margins of the caudal vena cava interrupt the outline of the right crus. When the animal is in right lateral recumbency, the air-filled gastric fundus may be identified caudal to the left crus. Fat within the falciform ligament ventral to the liver may outline the diaphragmatic cupula on the abdominal side. This may be a helpful finding in patients with pleural fluid in which diaphragmatic hernia is being considered as a possible diagnosis. Sonographically, the diaphragm is not identifiable specifically. The interface of the diaphragm and the air-filled lung creates a hyperechoic line that has the shape of the normal diaphragm. This interface frequently is misidentified as the diaphragm. The actual diaphragmatic muscle is obscured by the reverberation that occurs at the level of the diaphragm-lung interface. Diaphragmatic muscle is clearly visible only when there is simultaneous pleural and peritoneal effusion. The caudal vena cava usually can be identified traversing the diaphragm. Identification of the esophagus at the esophageal hiatus is more difficult due to air in the stomach and lung.
40 Small Animal Radiolo g y and Ultrasono graphy Fig. 2-11 A 2-year-old female Doberman Pinscher brought in for routine ovariohysterectomy. Thoracic radiographs were obtained at the owner’s request. The thorax is normal. A, On the lateral radiograph the cardiac size and shape are normal for a dog of narrow thoracic conformation. B, On the ventrodorsal radiograph a soft tissue–dense triangular structure is present in the cranial left thorax. This represents soft tissue and fat within the cranial mediastinum (arrows). In young dogs, the thymus may be identified in this location. Diagnosis: Normal thorax. A B
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A B C Fig. 2-63 A 10-year-old male
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Fig. 2-101 A 7-year-old male mixed
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Fig. 3-234 A 7-year-old female Brit
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Fig. 4-59 A 9-year-old female Russi
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Fig. 4-73 A 2-year-old female Saint
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Fig. 4-97 A 10-year-old male mixed
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Fig. 4-107 A 4-year-old male Chow C
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I N D E X A Abdomen, abnormal prese
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Index 729 Bone, cysts of, 525-527,
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Index 731 Ear, abscess of—cont’
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Index 733 Hydrocephalus—cont’d
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Index 735 Lungs. See also Pulmonary
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Index 737 Pregnancy, determination
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Index 739 S-shaped aortic deformity
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Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

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