Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

88 Small Animal Radiolo g y and Ultrasono graphy
character of the lesion (i.e., cystic or solid). Observing movement of the mass with respiration,
which identifies the mass as intrapulmonary, and identifying the position of the
lung lobes when the mass is pleural are useful in discriminating among diaphragmatic hernias
and pleural and pulmonary masses. Once a lesion is imaged, the sonogram can be used
to guide a needle for biopsy or aspiration. 26
P L E U R A L F LU I D
The radiographic appearance of pleural fluid, or hydrothorax, depends upon the nature
and amount of the fluid and the presence or absence of coexisting thoracic disease. Pleural
fluid will obliterate normal fluid-dense structures and will highlight air-containing structures
(Fig. 2-62). In the lateral radiograph, the cardiac apex and diaphragmatic outline may
be obliterated. Because of their elastic nature, the lung lobes will retain their normal shapes
and fluid will accumulate in the interlobar fissures dorsal and ventral to the lung lobes. This
outlines the lung lobes, accentuating their margins, and produces a “scalloping” or “leafing”
of the lobes. Fluid collecting between lung lobes produces linear or triangle-shaped
densities at the anatomical sites of the interlobar fissures. If fluid has been present for a significant
length of time (weeks to months), fibrin deposition or inflammation of the visceral
pleura may result in rounding of the lung lobe margins, which may be referred to as restrictive
pleuritis. The volume of fluid will determine the extent to which the heart and
diaphragm are obliterated, the width of the interlobar fissures, and the amount of
separation between the lung and the dorsal and ventral thoracic wall. In animals with freely
moveable pleural fluid, right and left lateral recumbent radiographs differ, because the
mediastinum will not prevent fluid movement from one side to the other, and the dependent
lung lobes will collapse to a greater extent than the nondependent lobes. Failure of the
fluid to shift position in opposite lateral recumbent radiographs indicates that the fluid is
trapped, the mediastinum is abnormally thickened, the fluid is fibrinous, or a pleural mass
is present. 124-126
The dorsoventral and ventrodorsal radiographs appear different when free pleural
fluid is present due to gravity’s effect on the heart, lung, and fluid. 127 In the ventrodorsal
view, the pleural fluid accumulates dorsally on either side of the vertebral column. The
cardiac silhouette usually will be visible, surrounded by and to some degree floating on
aerated lung. The costodiaphragmatic recesses or costophrenic angles will become
rounded or blunted. If only a small amount of fluid is present it may be localized to the
“paraspinal gutters” and may be only minimally apparent on the ventrodorsal view. More
often, fluid will be evident (i.e., separating the lungs from the lateral thoracic wall, separating
individual lung lobes) on both views. This is dependent on the amount of fluid and
is usually more obvious in the ventrodorsal than the dorsoventral view. Pleural fissure
lines will be evident, outlining the lung lobes in both views. The pattern and distribution
of these lines will change with the view. Part of the diaphragmatic outline will be obliterated
in either view; however, the cupula or dome will be less obvious in the dorsoventral
view and the crura will be obscured in the ventrodorsal view. In the dorsoventral radiograph,
the pleural fluid will gravitate to the sternum, the cardiac silhouette will be obliterated,
and the mediastinum will appear widened. The differences between ventrodorsal
and dorsoventral views may be used to determine the nature of the pleural fluid and to
detect masses or other lesions that might be masked by the fluid. A change in the fluid distribution
with changes in the animal’s position indicates that adhesions and large
amounts of fibrin are not present. Areas of the thorax masked by fluid in one view may be
evaluated in another view.
Horizontal-beam radiographs are also useful in detecting and evaluating pleural fluid.
Because the fluid should move with gravity, the region of interest should be up and the
fluid should move to the dependent side, unless it is fibrinous in nature, trapped by adhesions,
or a fluid-dense mass is present.
Inflation of the lung lobes should displace the pleural fluid evenly. If fluid accumulates
in the area of a lung lobe and does not move away with normal respiration, an abnormality
or disease within that lobe should be suspected. Positional maneuvers can be performed
to evaluate the lung lobe, although the lesion often will not be apparent until the pleural
fluid is removed (Figs. 2-63 and 2-64).