The following HU-ranges were sellekchem used to define differently aerated lung compartments: nonaerated (%Vnon,%Mnon), -100 to +100 HU; poorly aerated (%Vpoor,%Mpoor), -101 to -500 HU; normally aerated (%Vnorm,%Mnorm), -501 to -900 HU; hyperaerated (%Vhyper,%Mhyper), -901 to -1000 HU [2,3,18,23].In addition to the analysis of all CT slices covering the entire lung (whole-lung analysis), the calculation of volumes and masses characterizing the entire lung was performed by extrapolation from only 10 reference CT slices, as previously described [16-18]. Briefly, the most cranial and most caudal CT slices displaying lung tissue and eight equidistant CT slices between them were selected.

The extrapolation of quantitative CT data resulting from these 10 reference slices to the entire lung was performed as follows: mean values of each pair of consecutive slices were divided by nominal slice thickness and multiplied by the interval between the slice positions. All resulting products were summed up and a correction term was added [17,18]. All steps of the extrapolation procedure except for the identification of the most cranial and most caudal CT slices were performed automatically by dedicated software. Using the same approach, we also tested the extrapolation method for different slice thicknesses as well as for each possible number of reference CT slices between 15 and 5 slices.Examples for radiation doses were calculated using the CT-Expo software (Department of Diagnostic Radiology, Hannover Medical School, Hannover, Germany).StatisticsResults are reported as median and range (minimum and maximum values).

The agreement between extrapolation and whole-lung CT analysis was assessed according to Bland and Altman and is reported as bias and limits of agreement [24,25]. The GraphPad Prism 5 software was used for statistical analyses (GraphPad Software, La Jolla, CA, USA).ResultsThe median height (cranio-caudal distance) was 278 (222 to 320) mm for normal sheep lungs and 270 (218 to 308) mm for normal pig lungs. The median number of CT slices covering the entire lung of sheep was 24 (20 to 30) for 10 mm slice thickness and 55 (44 to 64) for 5 mm slices. The median number of CT slices covering the entire lung of pigs was 34 (25 to 41) for 7.5 mm and 54 (48 to 62) for 5 mm slice thickness.Values characterizing lung volumes and masses calculated from whole-lung analysis are given in Table Table1.

1. The classification as normal or opacified is supported by the respective amounts of nonaerated Drug_discovery lung in these groups. Animals with opacified lungs had relevant (up to 60%) amounts of nonaerated lung (Table (Table11).Table 1Lung volumes and masses quantified by whole-lung CT analysisBland-Altman plots illustrating the agreement of Vtotal and Mtotal obtained either by extrapolation from 10 CT slices or by whole-lung analysis are shown in Figure Figure11.