Mean frequency distribution of computed tomography numbers of scans performed at baseline and at wedge time in the perfused and nonperfused regions of the left lung. Comparison between baseline and wedge time frequency distribution of computed tomography numbers in the perfused region of the left lung (A) and in the nonperfused region of the left lung (B). Data refer to computed tomography scans acquired at end expiration, are clustered in ranges of 50 Hounsfield units (HU), and are expressed as means and SD. Two-way repeated-measure ANOVA on ranks was performed to compare baseline and wedge time values. Time × HU interaction: P = 0.018 and P < 0.001 for perfused and nonperfused lung regions, respectively. Tukey’s test was used for pairwise multiple comparisons. *, P < 0.05 versus baseline values.