Ground glass opacity on CT scanning of the chest: What does it mean?

Causes of a diffuse pattern of
GGO on CT scanning
• Acute rejection of lung
transplantation
• Adult respiratory distress syndrome
• Edema
• Extrinsic allergic alveolitis
• Hemorrhage
• Infectious pneumonia
Table 1 FIGURE 2. Acute rejection of lung transplantation.
A 38-year-old man presents with increasing
shortness of breath 3 weeks after
process. We have categorized the etiologies
of GGO according to the most commonly
seen patterns of distribution:
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 radiographic
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 gravitydependent
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 find-
bilateral lung transplantation. HRCT (1.0 mm
collimation) shows diffuse bilateral GGO, correlating
with a pathologic diagnosis of severe
acute rejection.
FiGURE 3. Early adult respiratory distress
syndrome (ARDS). A 12-year-old boy with
acute shortness of breath and hypoxemia,
requiring intubation and mechanical ventilation
after receiving chemotherapy for lymphoma.
HRCT (1.0 mm collimation) shows
diffuse bilateral GGO. Pulmonary artery
wedge pressure was normal, and transbronchial
lung biopsy and bronchoalveolar
lavage showed no evidence of infection.
The patient went on to develop severe
ARDS with complications of barotrauma.
ings 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
FIGURE 4. Extrinsic allergic alveolitis.
Imaging of a 50-year-old farmer with an
acute onset of increasing shortness of
breath. HRCT (1.0 mm collimation) shows
diffuse bilateral GGO, correlating with the
clinical diagnosis of acute farmer’s lung.
FIGURE 5. Diffuse alveolar hemorrhage. A
45-year-old man presents with increasing
shortness of breath 2 months after bone marrow
transplantation. Helical CT scan (10 mm
collimation) shows diffuse bilateral 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
septal thickening. 13
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 mor-
18 APPLIED RADIOLOGY, December 1998