ABSTRACT
While the chest radiograph (CXR) remains the initial imaging tool for the lungs,
high-resolution computed tomography (HRCT) is increasingly used to clarify
CXR findings, to detect lung disease in a symptomatic patient with a normal
CXR, and to monitor response to treatment and progression of many lung dis-
eases (1,2). Compared with CXR and conventional CT, the primary advantage of
HRCT is the high-spatial resolution enabling the detection of structures down to
0.2 to 0.3 mm (2,3). Regarded to be the most important subsegmental lung unit
smaller than a lobe or segment, the secondary pulmonary lobule is the smallest
anatomical unit regularly visualized at this resolution, and understanding of
lobular anatomy is essential to the interpretation of HRCT (3) (Fig. 1). At the
level of the secondary pulmonary lobule, the subtending lobular and intralobular
acinar bronchioles, and arterioles and interlobular septal veins and lymphatics
can all be readily seen in normal and, certainly, in abnormal situations (3). The
interlobular septa are typically picked up when abnormally thickened (3,4). The
superior ability to assess parenchymal details with HRCT provides a much
more accurate assessment of pattern and distribution of diffuse lung disease
compared with CXR (5). In one study, the accuracy of HRCT was reviewed in
129 patients with a variety of interstitial pneumonias; the positive predictive value
was 79% for HRCT diagnosis of cryptogenic organizing pneumonia (COP), 71%
for usual interstitial pneumonia (UIP), 65% for acute interstitial pneumonia (AIP),
63% for desquamative interstitial pneumonia (DIP), but only 9% for nonspecific
interstitial pneumonia (NSIP) (6). The relatively low diagnostic accuracy for NSIP
may be due to the lack of established CT features for NSIP at the time of the
study and a relatively higher number of atypical UIP patients in their studied
population (7). More recently, HRCT has been used to substantiate a diagnosis
of idiopathic pulmonary fibrosis (IPF) even in the absence of a surgical lung
biopsy; in a prospective study, the positive predictive value of a confident CT
diagnosis of UIP was 96% (8).
