Illustration of limits for segmentation of the sinonasal channel in transversal CT-slices. In a CT-slice at the level of 109 (A), the medial limitation for segmentation was a ‘hook’ protruding from the spiral lamella of the dorsal concha into the middle meatus (white arrow). The broken line shows the intrasinuidal limit for segmentation of the rostral sinonasal channel. In the CT-slice at the rostral level of 110 (B), the fusion of the spiral lamella of the ventral concha with the maxillary bone can be seen (black arrow). At the caudal level of 110/210 (C), the broken line displays the intrasinuidal limit for segmentation of the caudal sinonasal channel. Please note the differences of the ‘bulla’-like protrusion of the SCV (Bulla septi sinuum maxillarium) between left and right side. Numbered structures: (1) SMR, (2) SCV, (3) SF, (4) SMC.