Objective: The purpose of this study was to evaluate the effect of radiation dose reduction on the quantification of air trapping on expiratory CT. Materials and methods: This study was conducted as a retrospective evaluation of inspiratory and expiratory CT studies performed in routine clinical practice before and after alteration of the scanning protocol for expiratory CT at our institute. Eighty-six patients who had a clinical diagnosis of chronic obstructive pulmonary disease (COPD) and underwent inspiratory and expiratory CT and pulmonary function testing (PFT) were included. For the quantitative analysis, CT scans were obtained at six evenly spaced levels from the lung apices to the bases. The area of segmented lung without emphysema between －500 to －950 HU was obtained from the summation of six slices. The relative area between －900 and －950 HU for the area of the segmented lung (RA900-950) was calculated on both the inspiratory and expiratory scans. Comparisons of the RA-change between the standard-dose group (200 mA) and the low-dose group (80 mA) were performed by Mann-Whitney U test. Results: There was no significant difference between the standard-dose group and the low-dose group in the mean RA-change, and RA-change in both the standard-dose and low-dose groups correlated significantly with the results of PFT. In addition, there were no prominent differences in the correlation coefficients between the two groups. Conclusions: Low-dose CT could evaluate air trapping objectively and was not inferior to standard-dose CT for this purpose.
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