AJR 2001; 176:707-711
© American Roentgen Ray Society
Focal Abnormalities of the Trachea and Main Bronchi
Edith M. Marom1,
Philip C. Goodman and
H. Page McAdams
1
All authors: Department of Radiology, Duke University Medical Center, Box
3808, Durham, NC 27710.
Received June 29, 2000;
accepted after revision August 14, 2000.
Address correspondence to E. M. Marom.
Introduction
Focal or diffuse lesions of the central airways are produced by a variety
of diseases. The etiology includes infection, malignancy, trauma, aspiration,
collagen vascular disease, and idiopathic entities such as sarcoidosis,
amyloidosis, or tracheopathia osteochondroplastica. Even though patients may
present with significant symptomatology, these airway abnormalities are
frequently not apparent or are overlooked on chest radiographs. Thus,
diagnosis is often delayed. If there is a clinical or radiographic suspicion
of tracheobronchial abnormality, further evaluation on CT is warranted. In
particular, helical CT permits excellent multiplanar reconstructions, which
are especially useful for surgical planning.
This pictorial essay concentrates on CT techniques useful in central airway
evaluation and on the diseases that cause focal airway abnormalities. We
recognize that the distinction of focal and diffuse airway abnormalities is
somewhat arbitrary and is complicated by the fact that many diseases can cause
either focal or diffuse airway narrowing. For purposes of this review,
however, we will discuss the diseases that most commonly affect the airway in
a focal manner, and we will discuss the diseases that often result in
long-segment or multifocal disease in a separate review.
Imaging Techniques
CT is the imaging modality of choice for radiologic evaluation of the
central airways [1,
2]. Helical CT is preferred
because it eliminates slice misregistration and respiratory motion artifacts,
thus producing excellent axial images of tracheobronchial anatomy. Thin slices
(2-3 mm) are preferred because weblike stenosis can easily be missed or
underestimated with thicker slices. With single-detector row CT, a collimation
of 2- to 3-mm with a pitch of 1.5-2.0 is used, whereas with multidetector row
CT, a 2.5-mm collimation with an effective pitch of up to 6.0 is used with at
least 30% overlap in reconstruction. Because a continuous volume data set is
obtained during a single breath-hold, nonaxial two-dimensional and
three-dimensional reconstructions can be produced. The reconstruction models
(e.g., multiplanar reformat or multiplanar volume reformats a well as external
and internal three-dimensional renderings) vary significantly in computational
complexity Figs.
(1A,1B,2A,2B,2C,3).
The diagnostic value of these techniques, which are most useful in improving
diagnostic accuracy or confidence, particularly in the evaluation of stenoses
in obliquely oriented bronchi, has previously been reviewed
[2,
3]. In addition, two- or
three-dimensional reconstructions present anatomic information in an
orientation that is more familiar to referring clinicians. Nevertheless, it
should be emphasized that axial CT usually provides all the information needed
to make the correct diagnosis.
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Fig. 1A. —Histoplasmosis in 39-year-old man with recurrent pneumonia.
CT scan (5-mm collimation) shows smooth left main bronchus thickening and
stenosis (arrow) and adjacent coarse calcifications. a = ascending
aorta.
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Fig. 1B. —Histoplasmosis in 39-year-old man with recurrent pneumonia.
Coronal reconstruction better reveals focal narrowing of left main bronchus
(curved arrow). Calcified lymph nodes can be seen surrounding
stricture, in subcarinal region, and in both hila (straight arrows).
L = left main bronchus, R = right main bronchus, S = calcified subcarinal
lymph nodes.
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Fig. 2B. —Foreign-body aspiration in 55-year-old man. Internal
volumetric rendering of CT scan (3-mm collimation) reveals well-circumscribed
soft-tissue nodule (arrows) along anterior (nondependent) surface of
bronchus intermedius.
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Fig. 3. —Stenosis at left bronchial anastomosis in 27-year-old woman 9
months after bilateral lung transplantation for cystic fibrosis. Coronal
oblique shaded-surface display image from CT scan (3-mm collimation) shows
focal stenosis at left bronchial anastomosis (arrows).
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Focal Airway Narrowing
Mucus is perhaps the most commonly encountered airway
"abnormality" on CT. Differentiation of mucus from a true airway
lesion is usually not problematic because mucus tends to be of low attenuation
on CT, has a "bubbly" appearance from mixing with air, and occurs
along dependent portions of the airway. Occasionally, however, mucus is thick,
tenacious, and adheres to nondependent portions of the airway; in these cases,
differentiation from a tumor can be difficult. Repeating the CT
examination—after having the patient cough vigorously—can be
useful for differentiating mucous from tumor in such cases
[4] (Fig.
4A,4B).
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Fig. 4B. —Adherent mucus in 66-year-old man being evaluated for
pulmonary embolism. Repeated CT scan that was obtained after patient
vigorously coughed shows that nodule is no longer present and, thus, likely
represented adherent mucus.
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Most tracheal stenoses are a complication of long-term tracheal intubation.
Narrowing may occur at the stoma after tracheostomy, at the level of the
inflatable cuff, or, less commonly, where the tip of the tube has impinged on
the tracheal mucosa [4].
Narrowing is often concentric, and multiple stenoses may occur
(Fig. 5). Tracheal or bronchial
stenosis due to trauma, although uncommon, is associated with high mortality.
These injuries typically occur within 2 cm of the carina after rupture of the
airway and may result in collapse of the subtended lung
[4]. Stenosis or dehiscence of
the main bronchi after lung transplantation is now less common than previously
noted but, when present, can be imaged effectively with CT
(Fig. 3). Stent placement in
these patients can also be monitored
[5]. Foreign-body aspiration,
although more common in children, may be encountered in adults (Figs.
2A,2B,2C
and 6). Long-term sequelae
include focal airway stenosis with adjacent lung atelectasis, recurrent
pneumonia, or bronchiectasis. Occasionally, CT reveals the location of the
endobronchial foreign material
[4].
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Fig. 5. —Tracheal stenosis caused by prolonged intubation in
57-year-old woman. CT scan (10-mm collimation) shows concentric soft-tissue
thickening endotracheally (open arrow) and in surrounding tissues
(solid arrows). Multiple biopsies and 5-year follow-up proved this
finding to be inflammatory changes in region of endotracheal balloon cuff.
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Fig. 6. —Tooth aspiration in 27-year-old man with multiple mandibular
fractures after motor vehicle crash. Anteroposterior chest radiograph after
line placement reveals tooth extending into left upper lobe bronchus
(arrow).
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Infection may cause focal (or diffuse) airway lesions. Specific pathogens
include Mycobacterium tuberculosis (Fig.
7A,7B),
Coccidioides immitis, Histoplasma capsulatum
(Fig.1A,1B),
Aspergillus species, mucormycosis, and Klebsiella
rhinoscleromatis [6].
Pathologically, endoluminal masses of granulation initially produce irregular
areas of stenosis. If untreated, smooth fibrotic stenoses with associated
distal pulmonary collapse or pneumonia may be seen. Prompt diagnosis and
treatment can reverse or ameliorate the condition. Fibrosing mediastinitis,
usually caused by H. capsulatum, can create focal and diffuse airway
narrowing by extrinsic compression. The diagnosis is suggested by the presence
of a calcified infiltrating mediastinal mass on CT
[7].
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Fig. 7A. —Left main bronchus stenosis from tuberculosis in 32-year-old
woman. (Courtesy of Choi YW, Seoul, Korea) CT scan (3-mm collimation) shows
normal caliber of right main bronchus and marked concentric narrowing of left
main bronchus (arrow). R = right main bronchus, e = esophagus.
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Fig. 7B. —Left main bronchus stenosis from tuberculosis in 32-year-old
woman. (Courtesy of Choi YW, Seoul, Korea) Coronal reconstruction shows left
main bronchus stenosis involves short focal segment (white arrow).
Incidental note is made of mediastinal lymph node calcification (black
arrow). r = right main bronchus.
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Benign neoplasms such as papilloma
(Fig. 8), true mucinous
adenoma, hamartoma (Fig. 9),
fibroma, chondroma, leiomyoma, and granular cell myoblastoma account for less
than 10% of adult primary tracheobronchial tumors
[6]. These tumors are typically
solitary, less than 2 cm in diameter, and usually appear as a smooth round
nodule on CT. Endobronchial hamartoma and lipoma contain fat, which readily
distinguishes them from other neoplasms when imaged on CT.
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Fig. 9. —Endobronchial hamartoma in 42-year-old man with persistent
right middle lobe atelectasis on chest radiographs (not shown). CT scan (5-mm
collimation) shows mass (black arrows) of mixed fat (white
arrow) and soft-tissue attenuation involving right middle lobe bronchus,
resulting in right middle lobe atelectasis. Lobectomy confirmed endobronchial
hamartoma.
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Malignant neoplasms account for 90% of adult primary tracheal tumors. Most
common among these are squamous cell and adenoid cystic carcinoma (55% and
18-40% of tracheal malignancies, respectively)
[1,
6]. Squamous cell carcinoma
typically occurs in older patients with the risk factors of cigarette or
alcohol abuse. These tumors are aggressive, manifest as large irregular
tracheal masses (Fig. 10), and
are associated with a poor prognosis. Adenoid cystic carcinomas are less
aggressive malignancies that are associated with a better prognosis than that
associated with squamous cell carcinomas. Adenoid cystic carcinomas affect men
more commonly than women and usually occur during the third to fifth decades
of life. These tumors typically manifest as focal polypoid endoluminal masses
involving the posterolateral wall of the middle to lower third of the trachea
(Fig. 11). Less common primary
tracheal malignancies include adenocarcinoma (9%), bronchial carcinoid,
mucoepidermoid carcinoma, chondrosarcoma, and leiomyosarcoma
[1].
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Fig. 10. —Squamous cell carcinoma of trachea in 73-year-old man with
hemoptysis. CT scan (10-mm collimation) shows mass centered around anterior
and left lateral tracheal wall (curved arrows); mass is disrupting
calcified tracheal ring (straight arrow).
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Fig. 11. —Adenoid cystic carcinoma of trachea in 39-year-old man with
increasing shortness of breath. CT scan (7-mm collimation) shows polypoid mass
(arrows) protruding into trachea, involving its posterolateral
wall.
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The most common primary malignancy of the major bronchi is non-small cell
lung carcinoma [1]
(Fig. 12). Airway tumors
previously termed bronchial "adenomas" are, in fact, lowgrade
malignancies that occur in the central airways of young adults and are not
related to smoking. These lesions are to be differentiated from true mucinous
adenomas, which are rare benign tumors of the central airways. The most common
of the so-called bronchial "adenomas" is the bronchial carcinoid.
There are two histologic subtypes of bronchial carcinoid, typical and
atypical, which are differentiated histologically by the higher mitotic rate
in the latter. Typical carcinoids constitute from 75% to 90% of this type of
lesion. These lesions are usually smaller (mean diameter, 2.3 cm) than
atypical carcinoids, do not metastasize to regional nodes, and are associated
with an excellent prognosis (Fig.
13). Atypical carcinoids account for 10-25% of this type of
lesion, are larger (mean diameter, 3.6 cm), frequently metastasize to regional
nodes, and are associated with a poor prognosis
[8]. Paraneoplastic syndromes,
such as the carcinoid syndrome or ectopic adrenocorticotropic hormone
secretion, are rare. Carcinoid tumors most frequently (85% of lesions) occur
in the central (main or lobar) bronchi and appear as hilar masses on chest
radiographs or cause obstructive atelectasis or pneumonia. Up to 26% of
central carcinoids display calcification on CT, and 15% of carcinoids occur
peripherally as solitary well-circumscribed pulmonary nodules.
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Fig. 12. —Squamous cell carcinoma of left upper lobe bronchus in
71-year-old man with hemoptysis. CT scan (10-mm collimation) shows
circumferential narrowing of left upper lobe bronchus (arrows).
Endobronchial biopsy revealed squamous cell carcinoma. e = effusion.
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Fig. 13. —Bronchial carcinoid tumor in 27-year-old woman with
expiratory wheezing. CT scan (7-mm collimation) reveals left upper lobe
atelectasis with compensatory hyperaeration of left lower lobe. Note
well-circumscribed mass in left main bronchus (arrow). Biopsy showed
typical carcinoid tumor. a = left upper lobe atelectasis.
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They are typically bright on T2-weighted MR images. Octreotide radionuclide
imaging has proved more sensitive than 123I-metaiodobenzylguanidine
scanning for carcinoid detection (86% versus 40-60% of tumors, respectively).
Carcinoids are usually not metabolically active on
18F-fluorodeoxyglucose positron emission tomography
[8].
The central airways may also be involved secondarily by malignant processes
as a result of either hematogenous metastases or direct invasion from
esophageal, thyroid, mediastinal, or pulmonary malignancies. Primary
malignancies with a tendency to metastasize to the airways include renal cell
carcinoma (Fig. 14), melanoma,
adenocarcinoma (Fig. 15), and
sarcoma [1,
6]. Radiographically focal
airway lesions are most common, although if direct invasion from an adjacent
source is present, then more diffuse disease may be observed.
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Fig. 14. —Endotracheal metastasis from renal cell carcinoma in
75-year-old man with stridor. Cone-down view of posteroanterior chest
radiograph reveals well-marginated mass (arrows) in extrathoracic
trachea. Biopsy results were consistent with renal cell carcinoma
metastasis.
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Fig. 15. —Endobronchial metastasis from colon carcinoma in 56-year-old
man with hemoptysis. CT scan (8-mm collimation) reveals 8-mm soft-tissue mass
in right main bronchus (arrow). Endobronchial biopsy results were
consistent with colon carcinoma metastasis.
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Summary
The differential diagnosis of focal diseases of the central airways is
limited. Early recognition is crucial because timely intervention may, in some
patients, reverse complications. Correct CT image acquisition and display
facilitate surgical planning by the clinicians.
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Introduction
Imaging Techniques
