A C wall thickness, may be missed. MPR was relatively less in- formative than SSD in detecting airway lesions. As Kauczor et al. emphasized, SSD images are less reliable in detecting peripheral (segmental) airway stenosis than those obtained by MPR (4). In a study by LoCicero et al., the longitudinal extent of abnormalities was best demonstrated by MPR and SSD images, whereas the transverse extent of disease and relationships to adjacent structures were most clearly revealed by axial CT (3). In this study, SSD proved to be the best technique for lo- calizing tracheobronchial lesions. In this respect, RCT was less informative than TCT, MPR and SSD images, especial- ly in the localization of a lesion close to the bilateral sec- ond carina. The partial volume effect of RCT may account for this difference. It has been widely accepted that MPR and SSD images provide more accurate assessment of the extent of tracheo- 140 Chen et al. B Fig. 8. An outpouching lesion at the anastomotic site after sleeve left upper lobectomy in a 67-year-old man. SSD (A), routine enhanced CT (B), and coronal MPR image (C) show an outpouching lesion at the anastomotic site (arrow). For these three tech- niques, the respective score for relationship was 0, 4, and 3, respectively. bronchial narrowing than axial images (6, 9), particularly in the case of a lesion located in the left main bronchus, and in our study, SSD was the most informative modality for evaluating the extent of an airway lesion. MPR has been shown to be more precise than axial imaging in demonstrating the length of a stricture, especially when the lesion gradually narrows or, is located obliquely (1, 3, 9). Because the structure of the tracheobronchial tree is geo- metrically complex, SSD imaging is particularly useful in this respect. Accurate assessment on the basis of single plane images such as those obtained by TCT or MPR is dif- ficult; curved reformation, though, is able to render com- plex 3-D structures in a single flat plane and may be of val- ue in identifying the course of an extremely tortuous air- way (10). Such reformation to some extent distorts the true anatomical relationship, however, and may even gen- erate an artificial lesion. SSD provides the most reliable in- formation on the extension of airway lesions, and using a state-of-the-art CT scanner, takes only slightly longer to process than MPR, especially in cases involving irregular or asymmetric airway narrowing. When a single or double threshold is used, only a few voxels are visible, and for this reason, a limitation of SSD imaging is that the technique fails to provide information with which to evaluate the relationship between an abnor- mality and adjacent structures. Our experience has shown, however, that when the volume-rendering technique is used, 3-D imaging can display airway narrowing and adja- Korean J Radiol 1(3), September 2000