AJR 2000; 174:1315-1321
© American Roentgen Ray Society
Using CT to Diagnose Nonneoplastic Tracheal Abnormalities
Appearanceof the Tracheal Wall
Emily M. Webb1,
Brett M. Elicker and
W. Richard Webb
1
All authors: Department of Radiology, University of California, 505 Parnassus
Ave., M396, San Francisco, CA 94143-0628.
Received September 10, 1999;
accepted after revision October 19, 1999.
Address correspondence to W. R. Webb.
Introduction
The CT diagnosis of nonneoplastic tracheal disease is based on the
appearance of the tracheal wall on inspiratory scans, changes in the tracheal
wall with expiration, and the location and extent of tracheal abnormalities.
Recognizing specific tracheal wall abnormalities is of primary importance
because specific diseases tend to affect different components. By noting the
portion affected and its abnormal characteristics, a diagnosis may often be
suggested. In this pictorial essay, alterations in the appearance of the
tracheal wall in certain diseases will be emphasized.
CT Technique
CT performed with thin collimation is preferred for showing abnormalities
of the tracheal wall. Helical CT with 3-mm collimation, a pitch of 2:1, and
reconstruction at 2-mm intervals adequately shows these abnormalities and
allows volumetric imaging [1].
Two- or three-dimensional reconstructions of the trachea showing wall
abnormalities, lumenal morphology, and extent of disease may be useful in
selected patients. Expiratory or dynamic expiratory scans may be obtained
after or during forced exhalation to show tracheomalacia
[2]. Individual high-resolution
CT scans may also show tracheal wall abnormalities to best advantage.
Normal Trachea
From the inside out, the tracheal wall is composed of several layers:
mucosa, submucosa, cartilage or muscle, and adventitia (Fig.
1A,1B).
Horseshoe-shaped bands of hyaline cartilage support the anterior and lateral
tracheal walls. The posterior tracheal wall lacks cartilage and is supported
by a thin band of smooth muscle (the trachealis muscle); this portion of the
trachea is the posterior tracheal membrane.
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Fig. 1A. —Components of normal tracheal wall. Drawing shows horseshoe-shaped
tracheal cartilage supporting anterior and lateral tracheal walls. Posterior
tracheal membrane is thinner. Mucosa and submucosa internal to cartilage are
thin and difficult to see on CT scan.
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Fig. 1B. —Components of normal tracheal wall. Histopathologic section shows
horseshoe-shaped cartilage (large arrow) as dark. Posterior tracheal
membrane (small arrows) is bowed anteriorly. (magnification,
x2) (Courtesy of Warnock M, San Francisco, CA)
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On CT, the tracheal wall is usually visible as a 1- to 3-mm soft-tissue
stripe, delineated internally by air in the tracheal lumen and externally by
mediastinal fat or lung (Figs.
2A,2B,2C
and 3). The posterior tracheal
wall appears thinner and more variable in contour because of its lack of
cartilage; it can appear convex, concave, or flat. Cartilage in the tracheal
wall may appear slightly denser than surrounding fat and soft tissue.
Calcification of cartilage is most common in older patients, particularly
women (Fig. 3). Internal to
cartilage, the mucosa and submucosa are thin and difficult to see.
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Fig. 2A. —76-year-old woman with normal trachea. High-resolution CT scan shows
anterior tracheal wall to be 1- to 2-mm thick. Note cartilaginous
calcification. In locations of cartilaginous calcification, no soft tissue is
seen internal to it. Posterior tracheal membrane, being thin, is difficult to
distinguish from adjacent esophagus. At this level, trachea appears round.
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Fig. 2B. —76-year-old woman with normal trachea. High-resolution CT scan at
level lower than A shows right posterolateral tracheal wall to be thin
and outlined by lung. Trachea appears more oval at this level.
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Fig. 2C. —76-year-old woman with normal trachea. Expiratory high-resolution CT
scan near level of B shows posterior tracheal membrane bowing forward
in normal fashion. Note scant inward movement of lateral tracheal walls with
expiration.
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Fig. 3. —82-year-old woman with dense calcification of tracheal cartilage.
High-resolution CT scan shows calcified cartilage that appears to constitute
entire tracheal wall. Posterior tracheal membrane, lacking cartilage, appears
thin and uncalcified. Its relatively flattened contour is normal.
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The trachea is divided into extrathoracic and intrathoracic portions at the
point where it passes posteriorly to the manubrium
[3]. During forced expiration,
CT typically shows significant anterior bulging of the posterior membrane of
the intrathoracic trachea (Fig.
2A,2B,2C);
the anterior and lateral tracheal walls change little. As shown on dynamic CT,
the mean anteroposterior diameter of the trachea decreases 32% during forced
expiration, whereas the transverse diameter decreases by only 13%
[2].
Saber-Sheath Trachea
Saber-sheath trachea is common and almost always associated with chronic
obstructive pulmonary disease
[3]. It is characterized by a
marked decrease in the coronal diameter of the intrathoracic trachea
associated with an increase in its sagittal diameter; in patients with this
condition, the extrathoracic trachea is normal. In its earliest stages,
narrowing is visible only at the thoracic inlet. On CT, inward bowing or
displacement of the lateral tracheal walls as a result of cartilage weakness
is seen (Fig.
4A,4B,4C,4D).
CT during forced expiration may show further inward bowing of the tracheal
walls (tracheomalacia), which usually have normal thickness.
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Fig. 4A. —85-year-old man with history of chronic obstructive pulmonary
disease and saber-sheath tracheal deformity. CT scan above thoracic inlet
shows trachea to have normal contour. Calcification of cartilage is
normal.
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Fig. 4B. —85-year-old man with history of chronic obstructive pulmonary
disease and saber-sheath tracheal deformity. CT scan at thoracic inlet shows
side-to-side narrowing of trachea. Tracheal cartilage is calcified. Tracheal
wall is normal in thickness.
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Fig. 4C. —85-year-old man with history of chronic obstructive pulmonary
disease and saber-sheath tracheal deformity. CT scan lower than B shows
more severe tracheal narrowing in its sagittal dimension. Side-to-side
narrowing of trachea at and below thoracic inlet, without thickening of
tracheal wall, is typical of saber-sheath trachea.
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Fig. 4D. —85-year-old man with history of chronic obstructive pulmonary
disease and saber-sheath tracheal deformity. Drawing shows typical appearance
of saber-sheath trachea compared with normal trachea. Cartilage is shown as
dark gray. Narrowing of trachea is caused by deformity of tracheal cartilage.
Mucosa and submucosa are normal.
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Tracheal Stenosis
Acquired tracheal stenosis is usually caused by intubation or tracheostomy
[3]. Inflammation and pressure
necrosis of the tracheal mucosa most commonly occur at either the tracheostomy
stoma or at the level of the tube balloon; the stenosis is typically 1.5-2.5
cm in length. Acute postintubation stenosis results from mucosal edema or
granulation tissue. On CT, this condition may be seen as eccentric or
concentric soft-tissue thickening internal to normal-appearing tracheal
cartilage (Fig.
5A,5B,5C).
The outer tracheal wall has a normal appearance without evidence of deformity
or narrowing. Expiratory CT shows little change in tracheal diameter. In
patients with chronic stricture, thickening of the mucosa and submucosa may be
absent or of mild degree, with tracheal narrowing resulting from deformity of
the tracheal cartilage or posterior membrane (Fig.
6A,6B).
In patients with chronic stricture, tracheomalacia may result from weakness of
tracheal cartilage and can be a cause of dyspnea. Most typically,
tracheomalacia results in a side-to-side narrowing of the intrathoracic
trachea on CT performed during or after forced expiration. Circumferential
narrowing of the trachea may also occur.
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Fig. 5A. —80-year-old man with acute tracheal stenosis representing
granulation tissue after intubation. Three-millimeter collimated CT scan shows
narrowing of tracheal lumen by increased thickness of soft tissue internal to
tracheal cartilage and thickening of posterior tracheal membrane. Tracheal
cartilage (arrows) is faintly calcified and appears normal in shape.
Outer tracheal wall has normal configuration and trachea has normal oval
shape.
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Fig. 5B. —80-year-old man with acute tracheal stenosis representing
granulation tissue after intubation. On parasagittal reconstruction from 3-mm
collimated CT scans, narrowing appears focal and hour-glass-shaped.
Granulation tissue (arrow) is visible internal to calcified cartilage
of anterior tracheal wall.
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Fig. 5C. —80-year-old man with acute tracheal stenosis representing
granulation tissue after intubation. Drawing shows appearance of tracheal
stenosis caused by granulation tissue compared with normal trachea. Soft
tissue internal to tracheal cartilage is thickened.
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Fig. 6A. —66-year-old man with chronic tracheal stenosis after intubation.
Three-millimeter collimated CT scan shows tracheal lumen narrowed primarily
because of collapse and inward displacement of calcified tracheal cartilage.
Lateral tracheal walls are displaced inward, similar to saber-sheath trachea
in appearance. Slight thickening of mucosa and submucosa caused by granulation
tissue or fibrosis is also visible internal to cartilage. This finding would
not be expected in saber-sheath trachea. Expiratory images showed no further
tracheal collapse.
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Fig. 6B. —66-year-old man with chronic tracheal stenosis after intubation.
Drawing shows chronic tracheal stenosis, characterized by collapse of tracheal
cartilage. Mucosa and submucosa may be normal or thickened.
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Wegener's Granulomatosis
Wegener's granulomatosis, resulting in mucosal and submucosal inflammation
and ulceration, involves the trachea in 15-25% of patients. Subglottic
involvement is most typical with variable involvement of the distal trachea
and proximal mainstem bronchi
[4]. Destruction of tracheal
cartilage may occur, but malacia is not typical. CT findings include
circumferential wall thickening and narrowing of the tracheal lumen (Fig.
7A,7B,7C).
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Fig. 7A. —55-year-old woman with Wegener's granulomatosis, lung nodules, and
tracheal and bronchial involvement. CT scan with 7-mm collimation shows
circumferential thickening of tracheal wall, with involvement of both
cartilaginous (anterior and lateral) and membranous (posterior) portions.
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Fig. 7B. —55-year-old woman with Wegener's granulomatosis, lung nodules, and
tracheal and bronchial involvement. CT scan with 7-mm collimation shows
circumferential thickening of both right and left bronchial walls.
Irregularity of mucosal surface suggests ulceration.
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Fig. 7C. —55-year-old woman with Wegener's granulomatosis, lung nodules, and
tracheal and bronchial involvement. Drawing shows typical appearance of
Wegener's granulomatosis. Mucosal and submucosal inflammation results in
concentric thickening of tracheal wall and mucosal ulceration in some
patients.
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Amyloidosis
Tracheobronchial amyloidosis is a rare condition. Primary tracheobronchial
amyloidosis is generally not associated with parenchymal abnormalities
[5]. Deposits are submucosal
and most commonly involve the entire trachea. Infrequently, a solitary
submucosal nodule is present. On CT, diffuse amyloidosis leads to concentric,
smooth, or nodular thickening of the submucosal tracheal wall (Fig.
8A,8B,8C).
Cartilage is normal but concentric calcification or ossification may occur
without malacia.
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Fig. 8A. —47-year-old man with tracheobronchial amyloidosis. CT scan shows
smooth concentric tracheal wall thickening that involves posterior wall to
lesser degree. Concentric submucosal calcification is seen along inner
tracheal wall.
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Fig. 8B. —47-year-old man with tracheobronchial amyloidosis. CT scan shows
concentric thickening and calcification of both main bronchi, typical of
tracheobronchial amyloidosis. (Courtesy of Im JG, Seoul, Korea)
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Fig. 8C. —47-year-old man with tracheobronchial amyloidosis. Drawing shows
typical appearance of tracheal amyloidosis. Submucosal deposits of amyloid
result in concentric, smooth, or nodular thickening of tracheal wall.
Calcification (shown as black) is common and may also be concentric.
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Tracheobronchopathia Osteochondroplastica
Tracheobronchopathia osteochondroplastica, characterized by development of
osseous or cartilaginous nodules or both in the submucosa of the trachea and
bronchial walls, is rare. Nodules are associated with tracheal cartilage,
sparing the posterior membrane
[6]. CT findings include
thickened tracheal cartilage with small 3- to 8-mm calcific nodules along its
inner aspect, protruding into the tracheal lumen (Fig.
9A,9B).
The appearance is much more irregular than that seen with normal cartilage
calcification. Malacia is not present.
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Fig. 9A. —64-year-old man with tracheobronchopathia osteochondroplastica. CT
scan shows typical findings of this entity: irregular nodular areas of
calcification (arrow) involving anterior and lateral (cartilaginous)
portions of tracheal wall. Calcifications are much more irregular than those
seen in healthy patients and result in irregular narrowing of tracheal lumen.
(Courtesy of Müller NL, Vancouver, B. C.)
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Fig. 9B. —64-year-old man with tracheobronchopathia osteochondroplastica.
Drawing shows typical appearance of tracheobronchopathia osteochondroplastica.
Submucosal nodules of calcified cartilage occurring in relation to cartilage
(shown in black) involve anterior and lateral tracheal walls. These nodules
result in irregular inner tracheal wall.
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Relapsing Polychondritis
Relapsing polychondritis is characterized by recurrent episodes of
cartilage inflammation most commonly affecting the ear, nose, joints, and the
laryngeal and tracheal cartilage. The upper airways are affected in greater
than 50% of patients. Diffuse tracheal inflammation is limited to the
cartilage and perichondrium and does not affect the mucosa or submucosa
[7]. CT usually shows wall
thickening limited to the anterior and lateral tracheal walls, sparing of the
posterior membrane, collapse of the cartilage, and narrowing of the lumen
(Fig.
10A,10B,10C,10D).
If cartilage calcification is present, the cartilage may appear thicker than
normal (Fig.
10A,10B,10C,10D).
The inner and outer borders of the thickened tracheal wall are smooth. Both
the extrathoracic and intrathoracic trachea are involved
(Fig. 11). The luminal
narrowing may be fixed or tracheomalacia may be present (Fig.
10A,10B,10C,10D).
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Fig. 10A. —44-year-old man with polychondritis and progressive shortness of
breath on exertion. Inspiratory CT scan obtained with 3-mm collimation shows
thickening of anterior and lateral tracheal walls caused by thickening of
tracheal cartilage. Cartilage is partially calcified. Posterior tracheal wall
is normal in thickness.
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Fig. 10B. —44-year-old man with polychondritis and progressive shortness of
breath on exertion. CT scan through proximal main bronchi shows anterior
bronchial wall thickening similar to that seen in A.
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Fig. 10D. —44-year-old man with polychondritis and progressive shortness of
breath on exertion. Drawing shows typical appearance of polychondritis.
Anterior and lateral tracheal walls are thickened because of thickening of
tracheal cartilage. Posterior membrane appears normal.
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Fig. 11. —50-year-old woman with polychondritis. Coronal reconstruction of CT
scans shows involvement of both extrathoracic and intrathoracic trachea,
characteristic of this disease. Note thickening of lateral tracheal walls
(arrows).
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Tracheal Diverticulum
Tracheal diverticulum is commonly associated with chronic obstructive
pulmonary disease [8].
Diverticula almost always occur along the right posterolateral tracheal wall
near the thoracic inlet between the cartilaginous and muscular portions of the
tracheal wall (Fig. 12). They
can appear as isolated paratracheal air cysts or may be seen communicating
with the tracheal lumen.
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Fig. 12. —88-year-old man with tracheal diverticulum, detected incidentally.
CT scan shows it to arise from posterolateral right tracheal wall
(arrow) near thoracic inlet, posterior to tracheal cartilage.
Communication with tracheal lumen is easily seen.
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Mounier-Kuhn Syndrome (Tracheobronchomegaly)
Tracheobronchomegaly is a rare condition characterized by marked dilatation
of the trachea and mainstem bronchi. It is associated with atrophy of
cartilaginous, muscular, and elastic components of the tracheal wall and may
be seen in a variety of systemic diseases resulting in connective tissue
abnormalities. CT findings include a thinning of the tracheal wall and a
tracheal diameter of more than 3 cm at 2 cm above the aortic arch
[9]
(Fig. 13). Tracheal scalloping
or frank diverticulosis can also be seen but is difficult to appreciate in
cross section. The trachea tends to collapse with forced expiration
(tracheomalacia).
Miscellaneous Inflammatory Lesions
Tracheal involvement occurs in a small percentage of patients with
sarcoidosis. In addition, submucosal granulomas result in tracheal wall
thickening. Ulceration of tracheal mucosa, submucosal fibrosis, and irregular
thickening of the tracheal wall are rare manifestations of ulcerative colitis.
On CT, these appearances cannot be distinguished from other causes of diffuse
tracheal narrowing. Various infectious organisms including bacteria, viruses,
and fungi can also affect the trachea. Active tuberculosis involving the
tracheal wall may result in its irregular thickening.
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Introduction
CT Technique
