Effect of lung orientation on measurement and resolvability of airways using computed tomography

Volumetric data sets in the lung are formed by stacking sequential computed tomography (CT) slices. The resulting volumetric image file, however, is nonuniformly resolved; the in-plane resolution is greater than that in the z-axis (slice thickness) dimension. The purpose of this study was to determine the effect of branch orientation within the image volume on the measurement and resolvability of airway branches. An isolated canine lobe was sequentially scanned at three orientations with a Siemens Somotom Plus S CT Scanner using a 124.4 mm field of view, 137 kVp, 220 mAs, a 2 mm slice thickness, and a 1 mm table feed. A grid size of 2562, resulted in an in- plane pixel dimension of 0.49 mm. The lobe was inflated to an airway pressure of 20 cm H2O with the main bronchus of the lobe aligned approximately perpendicular to the scan plane. The entirety of the lobe was scanned at this orientation. The inflated lobe was then reoriented 90 degree(s) clockwise (as if the lobe was sitting on a clock face) and again the entirety of the lobe was scanned. The lobe was rotated a third time 90 degree(s) in plane from the first lobe alignment and again rescanned. Airway trees were segmented for each orientation and there were significant differences in the number of resolved branches among the three segmented trees. Measurement of airways over four millimeters in diameter was not affected by orientation. Airways smaller than two millimeters in diameter showed surprising similarity in measured diameter, but all airways were not resolved at all orientations. The angle of orientation of the individual airways with respect to the scan axis was therefore calculated to determine the angle at which a branch of given diameter was no longer resolved. There was surprising similarity of measured diameter in these smaller airways with orientation, if the branch was resolved. The orientation at which branches were resolved was quite variable, suggesting a high degree of randomness in the segmented branches at this level of branch resolution.