Analysis of the nonaerated lung volume in combinations of single computed tomography slices – is extrapolation to the entire lung feasible?

Introduction

The nonaerated lung volume (V_non) can be quantified from computed tomography (CT) images. Analysis of the CT slices covering the entire lung is time-consuming and thus limits potential clinical and experimental applications. This could be improved by analyzing only a few representative CT slices. The number and anatomical location of CT images required for analyses that are representative for the entire lung, however, is discussed controversially.

Methods

The percentage of V_non (%V_non) relative to the total lung volume was quantified in CT-image series (n = 21) of sheep with gross anesthesia-induced atelectasis. This was performed for different combinations of number and anatomical location of CT slices and the results were compared with the %V_non of the entire lung (lung). The combinations were: one juxtadiaphragmatic slice (juxta), three apical, hiliar and juxtadiaphragmatic slices (3 old), and three consecutive juxtadiaphragmatic slices (3 new). The correlation between %V_non and the arterial oxygen partial pressure (PaO₂) was examined for all combinations. The PaO₂ was measured at the time of the CT and transformed logarithmically (lnPaO₂) to linearize the relation between PaO₂ and %V_non. Linear regression and Bland–Altman plots were used for statistical analysis.

Results

The R-squared (R²) values for the correlation between lnPaO₂ and %V_non of lung and the slice combinations juxta, 3 new and 3 old were 0.61, 0.60, 0.57 and 0.55, respectively. The %V_non of lung correlated best with the %V_non of slice combinations juxta and 3 new (R² = 0.96 and 0.95, respectively). Comparison of these slice combinations with lung also resulted in the least bias in the Bland–Altman analyses (6.3 and 5.9%, respectively). R² for the correlation between lung and 3 old was 0.93, and the bias for lung vs 3 old in the Bland–Altman analysis was 6.8%.

Conclusion

Depending on the precision required, the use of single juxtadiaphragmatic CT slices can help to speed up the analysis process and thereby propel the clinical implementation of CT-derived information. Our data suggest that juxtadiaphragmatic slices may be better suited than the 'traditional' combination of apical, hiliar and juxtadiaphragmatic slices.