•          Standard emphysema metrics, including median lung CT number, are points on the cumulative frequency distribution of lung CT number histogram.

•          Mean lung CT number and mean lung CT number are highly correlated. (different but highly correlated)

•          Mean lung CT number correlated with bulk lung density

•          Median-mean lung CT number does not depend on:

–        Reconstruction kernel

–        Slice thickness

–        Image noise (radiation dose or mAs)

•          Precision of median-mean lung CT number or bulk lung density is preferred instrumental precision claim because is simple and can be supported with phantom measurements.