Thoracic Imaging 2003 - Society of Thoracic Radiology

cular diseases, drug reactions, radiation, and following bone
marrow and lung transplantation. HRCT commonly reveals foci
of lower lobe peripheral (at least 50% of cases) consolidations
and peribronchiolar consolidation (3), often recurrent and
migratory. Small nodules and ground-glass opacity may also be
seen (2, 6).
HRCT Classification of Small Airway Diseases
For the radiologist, an approach to small airway diseases that
utilizes HRCT is far more useful than approaching such patients
using a pathologic classification system. Major manifestations
of small airway diseases on HRCT include centrilobular nodules
with or without tree-in-bud, bronchial wall thickening,
bronchial dilation, mosaic perfusion, and air trapping (3). The
combination of these findings (as well as other findings, such as
consolidation), their distribution, and the clinical and laboratory
findings may allow a specific diagnosis. When these findings
are insufficiently specific, HRCT provides an accurate map to
direct tissue-sampling procedures.
Centrilobular Nodules
Centrilobular nodules represent opacities within the center of
the secondary pulmonary lobules. These nodules may reflect
infected material, bronchial impaction, or alveolitis. Nodules are
identified as centrilobular on HRCT by noting their relationship
to the secondary lobule. Functionally, such nodules are seen
adjacent to, but not touching, the costal and fissural pleura. The
distribution is often patchy, but may be diffuse (particularly in
the case of hypersensitivity pneumonitis). Once nodules are
identified as centrilobular in distribution, the presence or
absence of tree-in-bud opacities must be ascertained. Tree-inbud
represents impaction of small bronchioles, almost always
due to pyogenic infections or mucous.
The differential diagnosis of centrilobular nodules with treein-bud
opacity includes (1):
1. bacterial pneumonia
2. tuberculous and non-tuberculous mycobacteria
3. Aspergillus infection (especially airway-invasive
Aspergillus)
4. cystic fibrosis
5. bronchioloalveolar carcinoma (rare)
Once centrilobular nodules with tree-in-bud are identified,
the differential diagnosis is sufficiently narrowed to particular
infections such that attempts to recover respiratory pathogens
for microbiologic diagnosis are indicated.
The differential diagnosis of centrilobular nodules without
tree-in-bud is more extensive (1):
everything that causes centrilobular nodules with tree-in-bud
hypersensitivity pneumonitis
respiratory bronchiolitis
Langerhans cell histiocytosis
pneumoconioses
LIP
pulmonary edema
BOOP
Associated findings (for example, bizarre shaped cysts with
Langerhans cell histiocytosis) and the distribution of the findings
are important for narrowing the differential diagnosis.
Inhomogeneous Lung Opacity
The appearance of areas of increased and decreased lung
attenuation on inspiratory HRCT imaging is termed inhomogeneous
lung opacity (1). When inhomogeneous opacity is due to
increased pulmonary parenchymal attenuation, ground-glass
opacity is present. When the inhomogeneous opacity is due to
areas of decreased lung attenuation, mosaic perfusion is present.
When inhomogeneous lung opacity is due to the presence of
both increased and decreased lung attenuation, the “headcheese”
sign is present (1).
Mosaic Perfusion
When decreased opacity on inspiratory images is the predominant
finding (as opposed to small nodules), mosaic perfusion
may be present. Mosaic perfusion is caused by alterations
in perfusion of the lung parenchyma, resulting in areas of relatively
increased attenuation (hyperperfused) and areas of relatively
decreased attenuation (hypoperfused) (1). Mosaic perfusion
may be differentiated from ground-glass opacity by the
observation that vessels within the areas of relatively decreased
lung attenuation are abnormally small, whereas in the cases of
ground-glass opacity, vessels are equal in size throughout all
areas of inhomogeneous lung opacity. The small size of the vessels
within areas of mosaic perfusion reflects the diminished
blood flow within these areas of lung.
There are two major categories of causes of mosaic perfusion:
small airway diseases and vascular occlusion (1). Vascular
occlusion is commonly the results of chronic thromboembolic
disease or capillaritis from vasculitis. The major categories of
small airway diseases resulting in mosaic perfusion include
bronchiolitis obliterans (constrictive bronchiolitis), asthma,
chronic bronchitis, and hypersensitivity pneumonitis. Vascular
and small airway causes of mosaic perfusion may be differentiated
with expiratory imaging (7). When the cause of mosaic perfusion
is vascular, the inhomogeneous opacity seen on the inspiratory
image remains roughly similar on the expiratory image.
When the cause of the mosaic perfusion on the inspiratory
image is related to small airway disease, however, the appearance
of the inhomogeneous lung opacity is accentuated (7). This
occurs because lung parenchymal attenuation increases with
expiratory imaging as air within the lung is exhaled. With vascular
causes of mosaic perfusion, air trapping is not present and
all areas of lung increase in attenuation in a similar fashion
(there are some normal regional differences, however). With airway
causes of inhomogeneous opacity, air trapping impedes the
expulsion of air from some areas of lung, whereas other areas
decompress normally. This results in an accentuation of the
inhomogeneous opacity with expiratory imaging. Overall, small
airway causes of mosaic perfusion are far more common than
vascular etiologies.
Normal Inspiratory Scans with Air Trapping on Expiratory
Imaging
Most patients with air trapping seen on expiratory scans have
inspiratory scan abnormalities, such as bronchiectasis, mosaic
perfusion, airway thickening, nodules, and tree-in-bud, which
may suggest the proper diagnosis. Occasionally, air trapping
may be the sole abnormal finding on an HRCT study; the inspiratory
scan is normal (8). In this situation, expiratory HRCT
techniques are valuable for demonstrating the presence of an
underlying airway abnormality (8, 9). This situation may reflect
less extensive physiologic derangements than conditions in
which abnormalities are visible on the inspiratory images. The
differential diagnosis of this situation includes constrictive bronchiolitis
(bronchiolitis obliterans), asthma, chronic bronchitis,
and hypersensitivity pneumonitis (8). When air-trapping conforming
to the configuration of individual pulmonary lobules is
detected on expiratory HRCT (“lobular low attenuation”), large
or small airway diseases may be the cause. In one investigation,
bronchiectasis was the most common cause of such a pattern
(10). In this case, the bronchiectasis is not always seen exactly
in the areas of lobular low attenuation, but the latter reflects the
presence of a more widespread abnormality.
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