Optimal imaging protocol for measuring dynamic expiratory collapse of the central airways

AIM: To compare measurements of expiratory collapse obtained using multidetector computed tomography (MDCT) of the central airways on routine axial and multiplanar reformatted (MPR) images. MATERIALS AND METHODS: Fifty volunteers with normal pulmonary function and no smoking history were imaged using a 64 MDCT system (40 mAs, 120 kVp, 0.625 mm collimation) with spirometric monitoring at end-inspiration and during forced expiration. Measurements of the trachea, right main (RMB) and left main bronchus (LMB) were obtained on axial and MPR images. Inspiratory and dynamic-expiratory cross-sectional area (CSA) measurements were used to calculate the mean percentage expiratory collapse (% Collapse). A paired t-test was used to assess within-subject differences and a Bland-Altman plot was used to assess agreement between the methods. RESULTS: Among 24 men and 26 women (mean age +/- standard deviation 50 +/- 15 years), CSA values were significantly greater on axial than MPR images (all p<0.001); however, the mean difference in % Collapse values for axial versus MPR were small: trachea approximate to 1% (55 +/- 19 versus 56 +/- 18, p = 0.338); LMB identical (60 +/- 20 versus 60 +/- 17 p = 0.856); and, RMB 4% (62 +/- 19 versus 66 +/- 19 p<0.001). On average, creation of MPR required 12 minutes of additional time per case (range = 10-15 min). CONCLUSION: Differences in mean % Collapse for axial versus MPR images were small and unlikely to influence clinical management. This finding suggests that MPR may not be indicated for routine assessment of central airway collapse.