Page 409 - Atlas of Small Animal CT and MRI

Pleural Space 399

typically range from 0–30 HU. Large fluid volumes cause CT lymphangiography
partial or complete atelectasis with the greatest effect on CT lymphangiography is performed to visualize tho-
the right middle lobe, followed by the two cranial lobes. 3 racic duct anatomy, to define location and character of
As normal lung collapses, its attenuation increases in chyle leakage, and to preoperatively plan for thoracic
proportion with the degree of volume loss. Completely duct ligation. Iodinated contrast medium is injected
atelectatic lung has attenuation of approximately either directly into a popliteal lymph node or into a
50–60 HU on unenhanced CT images. Large volumes of mesenteric lymph node using ultrasound guidance, and
effusion also cause significant displacement of the lungs thoracic CT is performed after the thoracic duct is fully
from their normal location (Figure 4.2.2). Pleural fluid opacified. The normal lymphangiogram reveals one or
tends to distribute to the dependent part of the pleural more thoracic duct branches coursing next to the
space, and aerated lung, although tethered at the hilus, is thoracic aorta and entering the cranial vena cava
buoyed toward the nondependent regions. Moderate to (Figure 4.2.5). Near this junction, a variable number of
large volumes of effusion, regardless of fluid characteris- smaller lymphatic branches are seen that connect with
tics, cause lung lobes to “float”, altering the course of the cranial mediastinal lymph nodes. 4,5
affected airways and distortion of lung contours. In patients with thoracic duct injury or obstruction,
Effusion also leads to lung lobe torsion (Figure 4.2.3) extravasated contrast medium may be seen dispersing
(see Chapter 4.6). 3 within the mediastinum (Figure 4.2.6). In other patients,
a proliferation of many small lymphatic vessels in the
Hemorrhagic effusion and hemothorax cranial mediastinum is indicative of lymphangiectasia
Hemorrhagic effusion may be caused by trauma, bleed- from lymphatic flow obstruction. The transverse view of
the thoracic duct on CT images provides a means of
ing masses, anticoagulant poisoning and other bleed- accurately determining the number of parallel branches
ing diatheses, or increased vascular permeability of and their location relative to the aorta in anticipation of
compromised tissue. Frank blood has attenuation of surgical ligation. 4,5
40–50 HU on CT images, although mixed hemorrhagic
effusions may be less dense than this. Depending on
the initiating cause, hemorrhagic effusions and hemo- Pleuritis/pyothorax
thorax may be asymmetrical or compartmentalized Infectious pleuritis or pyothorax may be due to direct
(Figure 4.2.3). Active hemorrhage may result in hema- penetrating injury or be a sequela of systemic disease.
toma formation, and cellular elements may settle to the For the sake of this discussion, pleuritis is a general
dependent regions, resulting in a denser dependent term defined as any inflammatory condition of the
effusion layer. pleural membranes, whereas pyothorax describes an
effusive, infectious, suppurative inflammatory condi-
Chylous effusion tion of the pleural space and pleura. In addition to the
Chylous effusions are usually caused by thoracic duct CT features descriptive of other effusions described
trauma or a disruption of the hydrostatic gradient above, infectious pleuritis/pyothorax sometimes has a
between the thoracic duct and cranial vena cava, lead- characteristically sedimentary component of rela-
ing to lymphangiectasia and increased lymphatic per- tively high attenuation due to the settling of solids,
meability. Mediastinal masses may also occasionally exudate inspissation, and inflammatory pleural pro-
result in a chylous effusion by obstructing lymphatic liferation. Pleural membranes are often markedly
6
return through the duct. While the fluid volume is thickened and may be highly contrast enhancing
often marked in patients with chylous effusion, clinical (Figures 4.2.7, 4.2.8). Small volumes of free gas may
progression of the disease may be relatively slow and be seen because of the presence of gas‐forming organ-
insidious. The composition of the effusion and its isms or penetrating injury. Rarely, foreign bodies
6,7
chronic contact with pleural surfaces typically results initiating a pyothorax can be seen within the effusion
in a low‐grade sterile pleuritis that in turn leads to (Figure 4.2.9).
pleural thickening that is easily detected on CT images
(Figure 4.2.4). Pleural thickening and loss of compli- Pleural masses
ance result in lung volume reduction and rounding of
the lobar margins. In severe cases, removal of effusion Most clinically significant pleural masses are malig-
may result in incomplete reinflation of the lungs and nant and include primary pleural tumors, such as mes-
the presence of an ex vacuo pneumothorax because of othelioma, or other neoplasms that arise from the
the restrictive pleuritis. thoracic wall, mediastinum, or diaphragm and invade

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