ABSTRACT
Exposure to organic dusts can produce an immune-mediated inflammatory response in
sensitized individuals. The pulmonary disease caused by this response has been called
extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis. The clinical
phases associated with this process have been termed acute, subacute, and chronic.
There are corresponding imaging findings that are characteristic of each of these
phases, although there is some overlap between the phases. The acute phase is characterized
by confluent opacities that may mimic infection or edema. The subacute phase is characterized
by centrilobular nodules, areas of ground-glass attenuation, a mosaic perfusion pattern,
and air trapping on expiratory imaging. The chronic phase is characterized by subpleural
irregular linear opacities with associated architectural distortion. Honeycombing
may sometimes also be present. In the acute and subacute phases, the disease is predominantly
in the lower lungs, whereas in chronic EAA the findings are predominant in the mid
to upper lungs. Although the high-resolution computed tomography findings individually
are nonspecific, the combination of the findings coupled with the distribution of
the findings can often narrow the differential or allow a presumptive diagnosis of
EAA to be made.
KEYWORDS
Extrinsic allergic alveolitis - high-resolution CT - centrilobular nodules - mosaic
perfusion - air trapping
