MY9700691
CH4-CHEST
CT of Pleural Abnormalities which the soft-tissue opacity parietal pleura
and innermost intercostal muscle are separated
by a layer of low attenuation fat
W. Richard Webb
3) In the paravertebral regions, a distinct stripe of
Dept of Radiology, University of California of San density, 1 or 2 mm in thickness, indicates the
Francisco, San Francisco presence of pleural thickening.
The pleural surface of the lung and adjacent chest CT Technique for Examining the Pleura or Pleuro-
wall are made up of a number of tissue layers. On CT Pulmonary Disease
in normal patients, a 1 to 2 mm thick stripe of soft Usually contiguous 1 cm collimation, or spiral CT
tissue density (the "intercostal stripe") is often visible with 7 mm collimation is sufficient for imaging
in the anterolateral and posterolateral intercostal pleural abnormalities. Contrast enhancement at an
spaces, at the point of contact of lung with chest injection rate of 1-2 ml/sec is essential; following
wall.1 This stripe primarily represents the innermost contrast opacification thickened pleura, fluid collec-
intercostal muscle. Although the parietal pleura, a tions, and collapsed or consolidated lung can be
thin layer of extrapleural fat, and the endothoracic distinguished [4]. This is not always possible without
fascia lie internal to the innermost intercostal muscle contrast infusion.
and rib segments, they are not normally visible on CT
orHRCT. Diagnosis of Pleural Fluid Collections
Is it an exudate or a transudate?
In the paravertebral regions, the innermost intercostal
muscle is anatomically absent. In this location, a very Distinguishing an exudate from a transudate can be
thin line (the "paravertebral line") is sometimes vis- important in differential diagnosis and clinical
ible on CT at the lung-chest wall interface; this line management. Exudative effusions can have a variety
represents the combined thicknesses of the visceral of causes, but often reflect the presence of a pleural
and parietal pleura and the endothoracic fascia.1 abnormality associated with increased permeability
of pleural capillaries.56 Common causes of an
CT Diagnosis of Pleural Thickening exudate include pneumonia, empyema, and neo-
plasm. Transudative effusions are unassociated with
Asbestos exposure is known to result in thickening of pleural disease, and are usually the result of systemic
the parietal pleura.1'3 Thus, patients with asbestos abnormalities which cause an imbalance in the
exposure serve as an excellent model of parietal hydrostatic and osmotic forces leading to pleural
pleural thickening for the purpose of understanding fluid formation. Common causes of a transudative
the CT appearances of pleural disease. In patients effusion include congestive heart failure, cirrhosis,
with asbestos exposure, parietal pleural thickening overhydration, and nephrotic syndrome.
can result in 3 findings:
Most effusions appear to be near water in attenuation
on CT. CT numbers cannot be used to reliably predict
1) A stripe of soft tissue density, 1 mm or more in
the protein content or specific gravity of the fluid, and
thickness, can be seen internal to rib segments.
whether it is a transudate or an exudate. Acute or
Since parietal pleural thickening is often subacute hemothorax can sometimes appear
associated with thickening of the normal inhomogeneous in attenuation with some regions,
extrapleural fat layer, the thickened pleura particulary dependent regions, having a CT attenua-
may be separated from the rib by several mm. tion value greater than that of water.
2) Thickening of the extrapleural fat layer can
also allow the thickened parietal pleura to be The appearance of the parietal pleura on contrast
Seen as distinct from the innermost intercostal enhanced CT can be of value in predicting the nature
muscle. This results in the "sandwich sign", in of a pleural fluid collection [4]. The presence of
82
thickened parietal pleura on contrast enhanced CT, in Stage 1 (Exudative Stage)
association with a pleural effusion, indicates that the An exudative parapneumonic effusion probably results
fluid collection is an exudate, with only rare from increased permeability of the visceral pleura.
exceptions. Such an effusion should be tapped for Effusions in this stage are commonly exudates, as the
diagnosis. The absence of pleural thickening in a name implies, and are typically small, sterile, and
patient with pleural effusion indicates that the effusion have normal glucose (> 40 - 60 mg/dL) and pH values
is either a transudate or exudate.47 In a study we ha ve (> 7.2). They will usually resolve with appropriate
performed,4 pleural thickening is present in only 61 % antibiotic treatment. Stage 1 effusions may not show
of consecutive patients with an exudate; this finding pleural thickening on CT. As indicated above, about
had a specificity of 96% in diagnosing exudate and a 50% of exudative parapneumonic effusions will show
positive predictive value of 97%. pleural thickening; these are usually crescentic, with-
out evidence of loculation.
Pleural thickening on CT is least sensitive (27-48%)
in patients with an exudate associated w ith neoplasm.
Stage 2 (Fibropurulent or Acute Empyema Stage)
On the other hand, CT shows pleural thickening and
enhancement in nearly all patients with empyema. In The term "empyema" is generally used when a pleural
a study by Waite et al.,7 23 of 24 empyemas demon- effusion is infected, although its true definition
strated pleural thickening and enhancement on CT. In necessitates the presence of "pus" in the pleural space.
our study, all 10 empyemas showed pleural thicken- Although most empyemas occur in association with
ing; pleural thickening was less frequently (56%) pneumonia, approximately 10% are unassociated with
seen in association with uninfected parapneumonic obvious lung disease. The term "empyema" is
effusions. synonymous with "fibropurulent parapneumonic
effusion".
Thus, in a patient with symptoms of infection and a
pleural effusion, thepresenceof parietal pleural thick- The fibropurulent stage of a parapneumonic effusion
ening on CT indicates thepresenceof an exudate, and
is characterized by the presence of organisms in the
thoracentesis is necessary. If pleural thickening is
pleural fluid, increased effusion, increased WBC and
absent, empyema or complicated parapneumonic
PMN in the fluid, fibrin deposition along the pleural
effusion (one needing drainage for treatment) is highly
unlikely. In a patient with malignancy, CT does not surfaces, a tendency for loculation, decreased glucose
help in assessing the presence or absence of a malig- and pH levels, and increased LDH (>1000 IU/L).
nant effusion, and thoracentesis is usually required
regardless of what CT shows. In a patient who has a pneumonia, the presence of a
localized or loculated pleural effusion strongly
Sonography is also useful in diagnosing the presence suggests the presence of an empyema. On plain
of an exudate, with an accuracy nearly identical to radiographs, empyemas or loculated fluid collections
that of CT. Yang et al8 found that all effusions having will often have a somewhat lenticular configuration,
the sonographic appearances of septation, complex appearing much larger in one dimension (i.e. as seen
nonseptation, or homogeneous echogenicity were on the PA radiography) than the other (i.e. on the
exudative. Anechoic effusions, however could be lateral radiography). Furthermore, because the
either transudative or exudative. The sensitivity of collection may have a obtuse angles at the point it
sonography in this study was 66%, with a specificity contacts the chest wall, it may appear much more
of 100% and a positive predictive value was 100%. sharply defined in oneprojection than the other. In the
presence of a bronchopleural fistula (BPF), an
Diagnosis of Pleural Fluid Abnormalities in empyema cavity can contain air. Gas within an
Patients with Pneumonia empyema is presumptive evidence of a BPF, although
Pleural fluid accumulates in approximately 40% of the presence of agas-forming organism is occasionally
patients with pneumonia, and the term parapneumonic associated with this finding.
effusion is used to describe this occurrence.
Parapneumonic effusions are usually classified in 3 With conventional radiographic techniques, an
stages (these are also known as the 3 stages of an empyema contain ing air may be difficult or impossible
empyema). to differentiate from a peripheral lung abscess abutting
the chest wall. This distinction can be an important
83
one to make because empyemas are often treated by thoracentesis, 2) positive gram stain, 3) pH < 7.0,
tube thoracostomy in addition to systemic antibiotics, 4) glucose < 40 mg/dL. Although pleural thickening
whereas most lung abscesses require antibiotics only. seen on CT is common in an acute empyema, this
On CT, empyemas typically have a regular shape and does not constitute a "pleural peel", and surgery is
are round, elliptical, or lenticular in cross section, rarely required. Follow tube drainage, the pleural
although crescentic collections can also be seen;910 thickening usually resolves within a period of days or
they tend to be sharply demarcated from the adjacent weeks,12 although focal pleural abnormalities can
lung. Parietal pleural thickening is almost always remain. Tube thoracostomy is generally avoided in
seen on CT, while visceral pleural enhancement is patients with a tuberculous empyema.
somewhat less common. With contrast infusion, a
"split-pleura" sign is commonly visible, with the
Stage 3 (Organization Stage)
enhancing visceral and parietal pleural surfaces split
apart by the fluid collection, but this sign need not be In patients with chronic empyema, especially
present The pleural layers usually appear smooth and empyema which is tuberculous in origin, organiza-
of uniform thickness.7 Lung abscesses tend to be tion of the empyema, with ingrowth of fibroblasts,
rounded in shape, ill-defined, and have walls of can result in extensive pleural fibrosis and the deve-
irregular thickness. Empyemas also compress and lopment of an inelastic fibrotic "pleural peel". This
displace lung and vessels, acting like a space-occupy- can cause lung restriction and decreased lung volume
ing mass, while lung abscesses usually destroy lung ("trapped lung"). On CT, a thickened layer of
without displacing it. extrapleural fat is frequently visible, separating the
thickened parietal pleura from the intercostal muscle
In some patients with lung abscess and an air-contain- or rib. Calcification, which usually is focal in its early
ing empyema indicating the presence of a stages, may become extensive.13
bronchopleural fistula, the site of the BPF can be
demonstrated on CT. Absence or interruption of the Dense pleural thickening, even with calcification,
enhancing visceral pleura adjacent to an air-filled does not indicate that the pleural disease is inactive.
abscess cavity (the "interrupted pleura sign") implies Loculated fluid collections resulting from active
that this is the site of perforation. It should be noted, infection may be seen on CT within the thickened
however, that an interrupted pleura does not always pleura [14]. Similar pleural thickening can be the
indicate the presence of a bronchopleural fistula. result of chronic or prior infection, chronic inflamma-
tory disease, hemothorax occurring because of trauma,
In patients with uninfected parapneumonic effusion neoplasm, or radiation. Except in patients with neo-
or empyema, it is common (60-80 % of cases) to see plasm, calcification can occur.
extrapleural fat thickening when parietal pleural thick-
ening is present. Increased attenuation of extrapleural Pleural Neoplasms
fat representing edema is less common (30% of CT findings which are most helpful in distinguishing
cases).7 malignant and benign pleural diseases include 1)
circumferential pleural thickening, 2) nodular pleural
Several pulmonary infections can involve the chest thickening, 3) parietal pleural thickening greater than
wall by direct extension ("empyema necessitatis"). 1 cm, and 4) mediastinal pleural involvement.15 The
Tuberculosis accounts for about 70% of cases of specificities of these findings for diagnosing malig-
empyema necessitatis, but other organisms such as nant pleural disease were determined to be 100%,
actinomycosis, nocardiosis, and other bacteria can be 94%, 94%, and 88% respectively, while their sensi-
responsible. In this setting, extensive extrapleural fat tivities were 41%, 51%, 36%, and 56%.15 If one or
thickening, edema, and subcutaneous collections can more of these findings are considered to indicate
be seen on CT." malignancy, overall diagnostic accuracy is about
75%.
As tube thoracostomy is commonly used to treat
empyema, it is important to consider the indications Malignant (Diffuse) Mesothelioma
for tube placement in patients with a fibropurulent Diffuse mesothelioma is a highly malignant, progres-
effusion. These will vary depending on the clinician sive neoplasm with an extremely poor prognosis.16 In
involved and the clinical setting, but Light suggests most patients', malignant mesothelioma is related to
the following criteria: 1) thick pus present on asbestos exposure, and although it is rare in the
general population, the incidence in asbestos workers
84
is up to 5%. It is characterized morphologically by Pleural effusions are found in 15% of patients with
gross and nodular pleural thickening, which can Hodgkin's disease, and usually reflect lymphatic or
involve the fissures. Hemorrhagic pleural effusion venous destruction rather than pleural involvement,
often occurs. Malignant mesothelioma spreads most because they tend to resolve following local
commonly by local infiltration of the pleura. CT is mediastinal or hilar radiation. Pericardial effusions,
very helpful in diagnosis, and the frequency of vari- on the other hand, present in 5%, usually indicate
ous CT findings have been described." direct involvement of the pericardium.
Pleural thickening is common (90%), and is often Local Fibrous Tumor of the Pleura (Benign
irregular or nodular in contour-, focal pleural masses Mesothelioma)
can sometimes be seen. Pleural fluid collections are Local ized fibrous tumor of the pleura is the new name
visible on CT in 75%. Fluid can be difficult to of an uncommon tumor, formerly known as benign
distinguish from tumor, even on CT, since tumor mesothelioma. This lesion is usually detected
nodules can sometimes appear low in density. How- incidentally on chest radiographs. However, it can be
ever, CT scans with the patient prone or decubitus can associate with hypoglycemia and hypertrophic
help to distinguish underlying tumor from free fluid. pulmonary osteoarthropathy. It usually arises from
Also, enhancement of the pleura after contrast infusion the visceral pleura and therefore can be within a
can also help differentiate tumor from adjacent fluid fissure, but more commonly involves the costal
collections on CT.16-20 pleural surface. They appear as solitary, smooth,
sharply defined, often large lesions, contacting a
Although pleural mesothelioma is visible most pleural surface.
frequently along the lateral chest wall, mediastinal
pleural thickening or concentric pleural thickening is On CT, necrosis can result in a multicystic appear-
seen withextensivedisease. The abnormal hemithorax
ance with or without contrast infusion. Although it is
can appear contracted and fixed (40%), with little
generally believed that pleural abnormalities resultin
change in size during inspiration. Thickening of the
obtuse angles at the point of contact of the lesion and
fissures, particularly the lower part of the major
fissures, can reflect tumor infiltration of the fissures chest wall, benign mesothelionas typically show acute
or associated pleural effusion; involvement of the angles with slightly tapered pleural thickening adjacent
fissures is seen on CT in 85 %. Malignant mesothel ioma to the mass.21 This thickening may reflect a small
typically spreads by local invasion, involving the amount of fluid accumulating in the pleural space at
mediastinum and sometimes the chest wall (15%), the point where the visceral and parietal pleural
but hematogenous pulmonary metastases, and distant surfaces are separated by the mass. A similar "beak"
metastases do occur. or "thorn" sign is often visible on plain radiographs in
patients with a benign fissural meshotheolioma.
Metastases
Pleural metastases usually result in pleural effusion References
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