Ground glass opacity (GGO), a hazy, increased attenuation of lung with preservation of bronchial and vascular margins, is a nonspecific radiologic finding that has a lengthy differential diagnosis. This paper provides an abbreviated review of the physiologic correlates of the HRCT scan findings of GGO, focusing on infiltrative processes and their different GGO patterns of presentation.
Ground glass opacity (GGO) is described as a "hazy increased
attenuation of lung, with preservation of bronchial and vascular
margins; it is caused by partial filling of air spaces,
interstitial thickening, partial collapse of alveoli, normal
expiration, or increased capillary blood volume."1 GGO is a
nonspecific finding, and the differential diagnosis of the many
causes of GGO can be lengthy. An "ABCs" approach and a "pattern"
approach to the interpretation of GGO on HRCT scanning of the lungs
have previously been described.2,3 This paper provides an
abbreviated review of the physiologic correlates of the HRCT scan
findings of GGO, focusing on infiltrative processes and their
different GGO patterns of presentation.
GGO can be patchy, resulting in a mosaic pattern of lung
attenuation. Such a pattern can be seen in infiltrative lung
disease, airway abnormalities (e.g., asthma, bronchiolitis
obliterans), and chronic pulmonary vascular disease (e.g., chronic
thromboembolic disease).4 The distinction between these three
entities can be made by observing the size of the pulmonary vessels
in the area of increased lung attenuation (increased in both airway
disease and vascular disease, but not in infiltrative disease), and
by examining air trapping on expiratory scans (indicating airway
disease) (figure 1).
Pitfalls in the interpretation of GGO on CT
As recognition of GGO is based on a subjective assessment of
lung attenuation, it is important to understand the parameters that
can interfere with lung density and make attenuation measurements
unreliable.5,6 Window widths and levels that are too narrow can
erroneously create the appearance of GGO by artificially "blooming"
small structures. In evaluating for GGO, collimation ideally should
be 1.0 to 1.5 mm. True GGO can not always be visualized with a
thicker collimation because of volume averaging, and a thicker
collimation sometimes results in a pseudo-GGO pattern. GGO is
therefore best imaged with high-resolution CT (HRCT).
Lung attenuation normally increases homogeneously with
expiration. This increased attenuation can obscure underlying
pathologic GGO. Furthermore, if the expiratory nature of the
examination is not recognized, an erroneous interpretation of
pathologic GGO can be made.
Cardiac and respiratory motion also can create pseudo-GGO, which
can be distinguished from pathologic GGO by recognizing the
blurring and double images of vessels and fissures. GGO in the
gravity dependent portions of the lungs is often seen as a result
of microatelectasis, which can be differentiated from pathologic
GGO by re-scanning the area of question with the patient in the
Infiltrative processes resulting in GGO
Many patterns of distribution of ground glass opacity can be
seen on HRCT of the lungs. It is important to emphasize that most
such disease processes can and do result in more than one pattern,
often simultaneously; the patterns change depending upon the acuity
or chronicity of the disease process. We have categorized the
etiologies of GGO according to the most commonly seen patterns of
Diffuse pattern of GGO
-Disease processes commonly resulting in a diffuse pattern of GGO
on CT scanning are listed in table 1. Acute rejection is common
after lung transplantation. However, differentiating between
reperfusion edema, infection, and rejection can be difficult both
clinically and radiographically. HRCT is reported to be 65%
sensitive and 85% specific in making the diagnosis of acute
rejection in the lung transplant population.7 The only significant
HRCT finding in acute rejection (seen in 65% of these patients) is
GGO, which is patchy and localized in mild rejection and widespread
in severe rejection (figure 2). The main differential diagnosis in
this group of patients is cytomegalovirus pneumonia, which can have
an identical radio-graphic appearance.
Adult respiratory distress syndrome (ARDS) is a form of
nonhydrostatic pulmonary edema, characterized by leaky capillary
membranes. These leaks lead to extravasation of protein-rich fluid
into the interstitial and alveolar spaces of the lung. Among the
common causes of ARDS are aspiration, contusion, smoke inhalation,
and sepsis. CT scan findings of ARDS include bilateral and
gravity-dependent lung opacities.8 Early in the course of ARDS, all
patients demonstrate GGO on CT, which persists on follow-up CT in
50% of patients (figure 3).9
Both cardiogenic and non-cardiogenic edema occurs when the
capacity of the lung lymphatics to drain capillary transudate is
exceeded. Etiologies include venous and lymphatic obstruction,
increased capillary permeability, and hypoproteinemia.5 HRCT scan
findings in patients with hydrostatic pulmonary edema include areas
of GGO, interlobular septal thickening, peribronchovascular
interstitial thickening, increased vascular caliber, pleural
effusion, and thickening of fissures.10
Extrinsic allergic alveolitis, also called hypersensitivity
pneumonitis, is a complex immunologic reaction by the lung,
primarily to inhaled organic antigens. The clinical presentation
may be acute, subacute, or chronic. HRCT scan findings will vary
with the stage of disease. In the acute and subacute phases,
findings include GGO (in 82%), small nodules (55%), a reticular
pattern (36%), and air trapping.11 GGO correlates histologically
with mononuclear cell infiltration of the alveolar walls.11 The
distribution of GGO can be diffuse, patchy, or centrilobular
(figure 4) in this condition.
Pulmonary hemorrhage can be diffuse, patchy, or focal, depending
on the underlying cause, of which there are many.12 In the acute
phase, CT scans show consolidation or GGO (figure 5). In the
subacute phase, CT shows 1- to 3-mm nodules that are distributed in
a uniform fashion, commonly accompanied by GGO and interlobular
Infectious pneumonia of any cause (e.g., bacterial, viral,
mycobacterial, fungal, and parasitic) can cause GGO to appear on
HRCT scans. A diffuse pattern of GGO in the absence of associated
CT scan findings is a characteristic presentation for
cytomegalovirus pneumonia (CMV) and Pneumocystis carinii pneumonia
(PCP). CMV is the most common viral pathogen to cause substantial
morbidity and mortality in patients with AIDS,14 and is a
relatively common complication in organ transplant recipients. In
patients with AIDS and CMV pneumonia, CT scanning will show GGO,
dense consolidation, bronchial wall thickening or bronchiectasis,
and interstitial reticulation without air-space disease (although
GGO may occur in isolation).15 In organ transplant recipients with
CMV pneumonia, CT scanning shows small nodules, consolidation, GGO,
and irregular lines (figure 6). The presence of an isolated ground
glass infiltrate without additional findings in patients with AIDS
is highly suggestive of PCP (figure 7).16
Patchy GGO patterns
-Many of the causes of a patchy distribution of GGO on HRCT
scanning, listed in table 2, may also result in a diffuse pattern
of GGO. Pulmonary alveolar proteinosis is a disease of the lung
that results in filling in of the alveoli by a periodic
acid-Schiff-positive proteinaceous material that is rich in
lipid.17,18 HRCT scanning of this disorder shows GGO, with an
overlying branching pattern of white linear structures forming
geometric shapes and outlining polygonal, triangular, and square
forms.19,20 This pattern is often referred to as "crazy paving,"
and is characteristic, but not pathognomonic, of the diagnosis of
alveolar proteinosis (figure 8).21 Other processes that can show a
crazy paving pattern at HRCT scanning include ARDS, lipoid
pneumonia, and PCP.
Focal GGO patterns
-There is overlap between causes of diffuse, patchy, and focal
distributions of GGO (table 3) with chest CT scanning. When
pulmonary hemorrhage is due to focal neoplasm, trauma, or pulmonary
infarction, a focal pattern of opacity results. Certain infections,
such as lobar pneumonia, also may result in a focal pattern of
Bronchoalveolar lavage is a procedure used to diagnose pulmonary
diseases and to identify predictors of prognosis. The technique
involves injection of normal saline through a bronchoscope that is
generally wedged into the lingular or middle lobe bronchus. Most,
but not all of the fluid is aspirated back into the scope and
examined for inflammatory and immune mediator cells and specific
proteins.22 The residual fluid demonstrates a segmental or lobar
distribution of GGO on CT scanning, which should suggest the
possibility of recent bronchoalveolar lavage, especially if the GGO
is observed in the right middle lobe or lingula.5
The "halo" pattern of GGO
-A "halo" of GGO occasionally can be seen around a nodule or focal
area of lung consolidation. Table 4 lists the processes known to
produce the halo sign. It was first reported as a sign of early
invasive pulmonary aspergillosis in patients with leukemia.23 The
GGO represents a peripheral ring of hemorrhage or hemorrhagic
infarction surrounding target lesions of pulmonary aspergillosis
(figure 9). Several infectious and noninfectious causes of the CT
halo sign have since been reported.24 In most patients, hemorrhagic
nodules can be distinguished from nonhemorrhagic nodules by the
presence of a halo of GGO.
Another cause of focal GGO, or a nodule with a surrounding halo
of GGO, is the post-biopsy pseudo nodule. These pseudo nodules have
been described in patients who have undergone lung transplantation
and transbronchial lung biopsy,25 but they may be seen in any
patient after lung biopsy.
A peripheral pattern of GGO
-Processes that are known to result in a peripheral lung
distribution of GGO with HRCT scanning are listed in table 5. This
particular distribution pattern can be very helpful in narrowing
the differential diagnosis, especially when combined with other
clinical data and associated CT scan findings.
Bronchiolitis obliterans organizing pneumonia (BOOP) is a
disease characterized histologically by the presence of granulation
tissue plugs within respiratory bronchioles and alveolar ducts, and
organizing pneumonia extending into the surrounding alveoli.26 CT
scans show patchy GGO (in 8 to 75% of patients), nodules, or areas
of consolidation with a predominantly peripheral (50% of patients),
bilateral, and nonsegmental distribution (figure 10).27-29
Collagen vascular diseases are multisystem disorders
characterized by vascular changes, fibrosis, and inflammation of
connective tissue. Specific diseases include progressive systemic
sclerosis (scleroderma), systemic lupus erythematosus,
polymyositis/dermatomyositis, rheumatoid arthritis, and Sjogren's
syndrome. GGO is seen on CT scanning in 63 to 100% of these
patients,30 and is a sign of active inflammation in the absence of
significant honeycombing, bronchiectasis, or other signs of lung
Pulmonary contusion results from trauma to the chest wall and
lung, with bleeding into the air spaces and lung interstitium.
Generally, the cause is a compression injury with significant
kinetic energy absorption adjacent to the site of chest wall
injury. The CT scan appearance of lung contusion is that of
ill-defined areas of GGO, consolidation, or both, usually with a
peripheral, non-anatomic distribution (figure 11).32,33
Desquamative interstitial pneumonitis is characterized by
alveolar filling with macrophages. The HRCT scan findings consist
of GGO with a lower lung zone (73%) and a peripheral (59%)
predominant distribution (figure 12). Usual interstitial
pneumonitis, or idiopathic pulmonary fibrosis, results in a similar
distribution of GGO on CT scanning but typically with more areas of
honeycombing and traction bronchiectasis (figure 13).
Pulmonary toxicity has been associated with numerous drugs and a
variety of radiographic and CT patterns. CT scanning shows nodular
areas of GGO and consolidation, often with a peripheral
Pulmonary eosinophilia occurs with a variety of conditions or
diseases, or can be idiopathic. Chronic idiopathic eosinophilic
pneumonia is characterized by multiple dense areas of opacity on
chest radiographs and CT scans. In one study of patients with
chronic eosinophilic pneumonia, the most common HRCT finding was
GGO, usually adjacent to areas of consolidation, with a peripheral
distribution.37 Acute idiopathic eosinophilic pneumonia is
characterized by diffuse GGO and micronodules on chest radiographs
and CT scans, often in a bronchovascular distribution.38
Sarcoidosis is a multisystemic disorder of unknown cause
characterized by the presence of noncaseating granulomatous
inflammation affecting various sites of the body, with a propensity
to involve the respiratory tract. The most common HRCT scan
findings of pulmonary sarcoidosis are irregularly thickened
bronchovascular bundles (88%) and small nodules along vessels
(50%).39 GGO is present in 75% of patients with sarcoidosis, which
corresponds histologically with many granulomatous lesions, with or
without perigranulomatous fibrosis, in the interstitium and
alveolar septa around small vessels. Sarcoidosis can result in a
predominantly peripheral distribution of GGO and/or consolidation,
or a diffuse or patchy pattern of only GGO (figures 14 and 15).
Bronchovascular and centrilobular patterns of GGO
-Processes that can result in GGO in a predominantly
bronchovascular distribution include eosinophilic pneumonia and
sarcoidosis. A predominantly centrilobular distribution of GGO has
been described with both extrinsic allergic alveolitis and
respiratory bronchiolitis. All reported cases of respiratory
bronchiolitis have occurred in cigarette smokers.40-42 In the
majority of these patients, HRCT shows GGO, which is often
extensive, as the predominant finding.43 Pigmented macrophages
within respiratory bronchioles and adjacent alveolar ducts and
alveoli lead to the bronchovascular distribution of GGO.
GGO from infiltrative lung disease is a nonspecific finding on
HRCT scans of the chest, correlating histologically with partial
filling of air spaces, inflammatory or fibrotic interstitial
thickening, or increased capillary
blood volume. GGO also can represent mosaic perfusion secondary
to chronic vascular disease, or air trapping from small airways
disease. It is important to correlate the HRCT pattern of GGO with
clinical history and associated HRCT scan findings in developing a
differential diagnosis. AR
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Dr. Collins is in the Department of Radiology at the University
of Wisconsin Hospital and Clinics, in Madison, WI. Dr. Stern is in
the Department of Radiology at Harborview Medical Center,
University of Washington in Seattle, WA.