Overview

The first area to be evaluated using the thoracic interpretation paradigm is the extrathoracic structures. These include the soft tissue and osseous structures of the thoracic limbs, cervical and thoracic spine, sternum and ribs, diaphragm, the organs of the cranial abdomen and the peritoneal space. This chapter reviews the normal radiographic appearance and abnormalities associated with each extrathoracic structure.

Thoracic Limbs

Depending on the size of the dog, there will be a variable amount of the thoracic limb present on the radiographic images. In smaller dogs and cats, the proximal antebrachium through the scapula will be seen and are superimposed over each other on lateral projections. In larger dogs, the scapula and glenohumeral joint may be the only portion of the thoracic limb visualized. Common abnormalities associated with the thoracic limbs include traumatic injury and degenerative joint disease of the glenohumeral or elbow joints. The degenerative change is seen as osteophyte formation of the periarticular structures. For the glenohumeral joint, this would include the caudal aspect of the glenoid cavity and the caudal aspect of the humeral head (Figure 14.1). Other common abnormalities would be fractures related to trauma (Figure 14.2).

Aggressive changes of the osseous structures of the thoracic limb should not be missed. Primary bone tumors are commonly seen in the proximal humeral metaphysis and can have aggressive areas of osteolysis and osteoproliferation (Figure 14.3). Primary tumors of the scapula are less common but should not be missed when evaluating the scapula on lateral images even though they are superimposed. Confirmation of laterality can be done using the ventrodorsal (VD) or dorsoventral (DV) radiograph (Figure 14.4). Palisading periosteal reaction along the diaphyseal cortices of the humeri can be seen in dogs with hypertrophic osteopathy (see Chapter 10).

Thoracic Wall, Vertebral Column, Ribs, and Sternum

A radiolucent space is noted between all thoracic vertebrae and all sternebrae except between S7 and the xiphoid process. This radiolucency represents the intervertebral disc and intersternebral cartilage, respectively. In the thoracic vertebral location, the intervertebral disc is made up of an outer annulus fibrosis and an inner nucleus pulposus, being a fibrocartilaginous joint. A cartilaginous center is noted to the synchondrosis joint of the sternebral discs. Degenerative changes of the intervertebral disc and intersternebral spaces are the most common changes seen involving the vertebrae and the sternum. These changes include intervertebral disc/intersternebral space narrowing, adjacent endplate sclerosis and ventral spondylosis deformans (intervertebral disc space) or ventral/dorsal osseous proliferation (sternal cartilaginous degenerative change). These areas of osteoproliferation are smoothly marginated, are located at the edge of the vertebral body and sternal segment, and do not appear aggressive (Figure 14.5).

The vertebral bodies of the thoracic spine articulate with the ribs on both the right and left sides. The ribs are uniform in mineral opacity and articulate ventrally with the costal cartilage at the costochondral junction (Figure 14.6). The costal cartilage will mineralize early on in the animal’s life so that a uniform mineral opaque structure is present that articulates with the sternum or with other costal cartilages caudally. The costochondral junctions can undergo enlargement and degenerative changes, creating an extrapleural sign (Box 14.1, Figure 14.7). Bassett hounds routinely have an extrapleural sign on lateral, ventrodorsal or dorsoventral radiographs (Figure 14.8).

Aggressive lesions of the vertebral bodies, disc spaces, ribs, and sternum are characterized by osteolysis and cortical disruption. Vertebral body tumors can be lytic and result in vertebral body collapse and a compression fracture (Figure 14.9). Discospondylitis is an infection of the intervertebral disc space. The radiographic features of discospondylitis include intervertebral disc space narrowing/collapse, endplate lysis of the caudal and cranial aspects of the vertebral bodies at the intervertebral disc space, and osseous proliferation associated with the vertebral bodies in an attempt to wall off the infectious agent (Figure 14.10) [1]. Infections of the intersternebral disc spaces have similar radiographic abnormalities. Nocardia and Actinomyces sp. infections have been described involving the sternum, vertebrae, pleural space, and the different lung lobes, particularly the accessory lung lobe (Figure 14.11). Primary tumors or secondary soft tissue tumors with sternebral involvement also occur (Figure 14.12).

Common rib abnormalities include fractures and tumors. Acute rib fractures can be seen as breaks in the cortices of the rib with displacement of one margin of the fracture relative to the other (Figure 14.13). Chronic rib fractures are characterized by varying degrees of callus formation and healing (Figure 14.13). Chronic rib fractures can also be seen in dogs and cats with thoracic bellows secondary to chronic pulmonary or pleural space abnormalities (Figure 14.14).

Rib tumors can occur anywhere along the osseous portion of the rib and the most common rib tumor is an osteosarcoma. Radiographic features of rib tumors are summarized in Box 14.2. The lesion is aggressive with an expansile appearance that results in cortical lysis (Figures 14.15 and 14.16). Varying degrees of osseous proliferation may be present with an aggressive amorphous appearance. A pleural effusion may or may not be present (Figure 14.17). Metastasis of the primary rib tumor to the lungs can occur. Secondary or metastatic neoplasia of the vertebrae and ribs is common in adenocarcinomas that have osseous metastatic changes (Figure 14.18). A differential for this pattern of metastasis is multiple myeloma (Figure 14.19).