Obliquely oriented superior accessory fissure of the lower lobe of the lung: CT evaluation of the normal appearance and effect on the distribution of parenchymal and pleural opacities

PURPOSE: To review computed tomographic (CT) and radiographic features of an oblique superior accessory fissure in the lower lobe of the lung. MATERIALS AND METHODS: CT scans in 34 patients with a prospectively identified accessory fissure of the lower robe were reviewed and correlated with chest radiographs (31 patients). The fissure and its relationship to segmental bronchovascular structures were evaluated on transverse scans. Three-dimensional (3D) shaded surface display (SSD) reconstructions were obtained from spiral volume data (six patients). RESULTS: Thirty-four patients had 36 accessory fissures (26 right, 10 left). Four of the 36 accessory fissures were manifested by a normal fissure line; two, by slight thickening or minimal linear atelectasis; 16, by thicker linear or subsegmental atelectasis; two, by contiguous tumor infiltration; one, by adjacent consolidation; and 11, by intrafissural extension of pleural fluid. Analysis of bronchovascular structures revealed that this fissure was the superior accessory fissure. The 3D SSD reconstructions demonstrated an oblique orientation. On frontal radiographs, presence of this fissure correlated with a curvilinear band of atelectasis coursing inferomedially and obliquely from its intersection with the lateral aspect of the major fissure toward the infrahilar region on the right and the heart border on the left. Chest radiographs also showed intrafissural extension of pleural fluid. CONCLUSION: A superior accessory fissure of the lower lobe, more common on the right than on the left, can be identified at transverse CT. Contrary to previous descriptions, it may be oblique, as findings on transverse images suggest and 3D SSD reconstructions confirm. Its presence correlates with characteristic patterns of atelectasis and pleural fluid collection on conventional radiographs.