Thoracic Imaging 2003 - Society of Thoracic Radiology

and often with lobular sparing. GGO predominates in the mid
and lower lungs, but may occur in all lung zones. It is thought
to represent the interstitial inflammatory infiltrate seen histologically.
Ill-defined centrilobular nodules, less than 5-mm in
diameter, are found in approximately 55% of cases. These are
usually ground glass in attenuation, occur in all lung zones, and
often spare the extreme apices. GGO and centrilobular nodules
correlate with disease activity, and may resolve after cessation
of antigen exposure. Mosaic attenuation is seen on inspiratory
HRCT images representing normal lung parenchyma interspersed
with areas of GGO and areas of air-trapping. The latter
will become more evident and exaggerated on expiratory HRCT
images, and represents a manifestation of bronchiolitis. A combination
of ground glass opacity, low attenuation areas and normal
lung (the "head-cheese" sign ala Rick Webb!) on inspiratory
sections is common in HP. Less commonly, consolidation
or linear interstitial opacities due to fibrosis are seen.
Differential diagnosis includes atypical / viral pneumonias,
desquamative interstitial pneumonia (DIP), non-specific interstitial
pneumonitis (NSIP), and less commonly sarcoidosis.
Centrilobular nodules are also found in respiratory bronchiolitis
but tend to be more patchy, sparse and upper lung predominant.
The nodules in sarcoidosis and the pneumoconioses are usually
more solid and less ground glass like than in HP.
Chronic: In this stage the findings are a combination of
those found in the subacute phase with superimposed fibrosis.
Therefore, patchy GGO and centrilobular nodules are common,
seen in 66% of cases. Mosaic attenuation and air trapping on
expiration are also seen. In addition, there may be thickened
interlobular septa, irregular interfaces and traction bronchiectasis,
found in decreasing order of frequency. Honeycombing is
less common, occurring in less than 25% of cases. The distribution
of the fibrosis is patchy and most often shows a mid- or
lower lung predominance. Relative sparing of the costophrenic
recesses is seen in a majority of cases. Emphysema is reported
in up to 50% of cases on HRCT.
Differential diagnosis of these HRCT features includes an
overlap with usual interstitial pneumonitis (UIP/IPF), NSIP and
DIP. Clinical history, serology, the mosaic patchy distribution,
air trapping and sparing of the costophrenic recesses help distinguish
HP from these entities.
Summary: HP is an immune-mediated interstitial lung disease
caused by recurring or heavy exposure to a variety of antigens.
It has non-specific clinical features and physical findings.
A detailed occupational and recreational history is essential for
diagnosis. The constellation of HRCT findings may be the first
to suggest the diagnosis during the patient work-up, and distinguish
this disease from interstitial pneumonias like DIP, NSIP
and UIP.
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