HRCT findings of IIPs may be broadly categorised into foci of increased opacity and foci of decreased opacity (table 1). The major imaging findings manifesting as increased opacity on HRCT imaging in patients with IIPs include ground glass opacity, consolidation, linear and reticular opacities, architectural distortion, airway thickening and, occasionally, centrilobular nodules. While each of these findings individually may be seen in the various IIPs, certain findings tend to predominate more than others in each disease, as will be discussed subsequently. The imaging findings in patients with IIP that manifest as decreased opacity on HRCT imaging include traction bronchiectasis, honeycomb lung and air trapping (also, because many of these patients are smokers, emphysema is a common finding).
Table 1HRCT findings commonly encountered in patients with idiopathic interstitial pneumonias (IIPs)*
The first step in the interpretation of HRCT imaging in patients with IIP is the assessment for the presence or absence of a pattern suggestive of UIP.1
If HRCT findings typical of UIP are found, one may confidently suggest the diagnosis of UIP and surgical lung biopsy may be avoided. In the absence of clear findings suggesting UIP, the HRCT scan should be examined for features that specifically indicate an alternative diagnosis, either another IIP or another interstitial lung process. The imaging differential diagnosis is then ordered accordingly, and the HRCT scan findings are correlated with the clinical history1
to suggest the most likely diagnosis. Occasionally a confident alternative diagnosis may be made in this situation (fig 3), but surgical lung biopsy is often required to firmly establish a diagnosis.
Figure 3Patient with suspected idiopathic interstitial pneumonia: alternative diagnosis established with high resolution (HR) CT imaging. Axial HRCT image through the pulmonary apices shows bilateral bizarre‐shaped cysts (arrows) and (more ...)