Mean frequency distribution of CT numbers of scans performed on sheep with experimental acute respiratory distress syndrome expressed as percentages of tissue mass and grouped into intervals of 50 HU. Data are presented as mean ± SE. (A) Comparison between 60 and 140 tube current-exposure time products (mAs). (B) Comparison between 60 and 15 mAs. (C) Comparison between 60 and 7.5 mAs. *P < 0.05 vs. 60 mAs by paired t-test or rank-sum test as appropriate. Vertical dashed lines delimit lung compartments as defined in Materials and methods. CT = computed tomography; HU = Hounsfield units.