Three-section expiratory CT: insufficient for trapped air assessment in patients with cystic fibrosis?

To estimate the effect of the number of computed tomography (CT) sections on trapped air (TA) assessment in patients with cystic fibrosis (CF) by using an established scoring system and a new quantitative scoring system and to compare CT and pulmonary function test (PFT) estimates of TA in a cross-sectional and longitudinal study.

In this institutional review board-approved pilot study, 20 subjects aged 6-20 years (12 female and eight male; median age, 12.6 years) contributed two expiratory CT studies (three-section baseline CT, volumetric follow-up CT) and two PFT studies over 2 years after parental informed consent was obtained. From follow-up CT studies, seven sets were composed: Set 1 was volumetric. Sets 2, 3, 4, and 5, had spacing of 2.4, 4.8, 9.6, and 20.4 mm, respectively, between sections. Sets 6 and 7 contained five and three sections, respectively. Longitudinal follow-up was performed with three sections. All images were deidentified and randomized, and TA was scored with the Brody II system and a new quantitative system. Statistical analysis included the Wilcoxon signed rank test, calculation of Spearman and intraclass correlation coefficients, and use of three-section and linear mixed models.

For the Brody II system, the intraclass correlation coefficient for set 1 versus those for sets 2 through 7 was 0.75 versus 0.87; however, mean scores from sets 6 and 7 were significantly lower than the mean score from set 1 (P = .01 and P < .001, respectively). For the quantitative system, the number of sections did not affect TA assessment (intraclass correlation coefficient range, 0.82-0.88; P > .13 for all). CT and PFT estimates were not correlated (r(s) = 20.19 to 0.09, P = .43-.93). No change in TA over time was found for CT or PFT (P > .16 for all).

The number of sections affected Brody II estimates, suggesting that three-section protocols lead to underestimation of TA assessment in patients with CF when using the Brody II system; CT and PFT estimates of TA showed no correlation and no significant change over time.