Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

70 Small Animal Radiolo g y and Ultrasono graphy
abscesses have similar internal structure with hypoechoic or anechoic cavities, while most
tumors have a more uniform, mixed echogenic appearance. Soft-tissue mineralization, or
hyperechoic foci with shadowing, may be identified. The echogenicity of the mass always
should be compared with the adjacent normal tissue. This can be useful in identifying a
lipoma that is uniformly hyperechoic compared with the surrounding tissue.
Ultrasonography is rarely tissue- or cell-specific. It is very useful in guiding a needle into a
mass or fluid-containing structure or cavity for aspiration or biopsy.
BONY ABNORMALITIES
Abnormalities of the bony thorax are discussed in conjunction with evaluation of the specific
portion of the axial skeleton (i.e., ribs, spine, sternum). However, those bony abnormalities
that may be associated with intrathoracic pathology are discussed briefly.
V E RT E B R A L L E S I O N S
Vertebral body fractures or dislocation may accompany thoracic trauma (Fig. 2-44).
Malalignment of vertebral segments and collapse or shortening of a vertebral body may be
observed. In order to detect pathologic fractures, the density of the vertebral bodies should
be evaluated carefully.
Primary or metastatic neoplasms may affect the ribs or vertebral bodies. These areas
should be examined for productive, destructive, or mixed lesions, especially in patients
radiographed for pulmonary metastasis (see Fig. 2-42). Mediastinal masses may invade
adjacent vertebral bodies, and the bony lesion can indicate the etiology of these masses.
Osteomyelitis or discospondylitis may be identified incidentally in a patient radiographed
for other reasons.
R I B F R AC T U R E S
Rib lesions may accompany thoracic abnormalities and orthopedic injuries. 31,32,77 Rib
fractures often result from chest wall trauma but may be from underlying diseases resulting
in pathologic fractures. The interruption of the cortical margins, a radiolucent fracture
line, malalignment of fragments, or increased density from overlapping fracture
ends may be observed when rib fractures occur. Pathologic fractures also may be associated
with lytic lesions as well as periosteal reaction of the rib. Chest wall asymmetry and
uneven spacing of the ribs suggests the possibility of rib fractures or mediastinal tears.
Medially displaced rib fragments can penetrate the lung. Thoracic wall instability, or flail
chest, can result from adjacent segmental rib fractures. Fracture of several adjacent ribs
at two or more sites, or a large tear completely through an intercostal muscle, can produce
a thoracic wall that moves inward on inspiration and outward at expiration, called
paradoxical chest wall motion. This may interfere markedly with pulmonary function. 78
Thoracic radiographs exposed at both inspiration and expiration may document the
extent of the instability.
Pathologic fractures should be suspected whenever a solitary rib fracture or fractures
in nonadjacent ribs are detected. Bony proliferation or lysis of the rib will be seen if the
rib is examined closely. A subcutaneous or pleural soft-tissue mass may be detected if
the rib area is examined carefully.
Tearing of the intercostal muscles will result in the space between two ribs being significantly
larger on one side than on the other side (Fig. 2-45). This commonly is seen in a
small animal that has been bitten by a large dog. If the tear is extensive, the lack of chest
wall continuity can result in paradoxical chest wall motion and severely compromised respiratory
function. Asymmetric rib spacing may also occur in association with soft-tissue
trauma such as bruising, previous thoracotomy, hemivertebra, and pulmonary or pleural
disease.
Healed rib fractures should be recognized and not mistaken for more significant
pathology such as a metastatic tumor. Cortical malalignment or expansion of the bony
margin will persist for years after the original injury. The bony trabecular pattern will be
smooth and evenly mineralized, and soft-tissue swelling will be absent or minimal. These
features are useful in discriminating between healed fractures and rib metastasis.