Influence of certain physical and physiological factors on the in vivo measurement of lung density

The effects of multiple photon scattering and the assumption of infinitesimal beam geometry on the results obtained with a clinical lung densitometer were investigated. In phantom materials, it was possible to account almost completely for the multiple scatter contribution by measurement of transmission for the 170Tm source. A theoretical analysis based on a simplified model showed that the influence of finite geometry was small. Measurements in six normal volunteers showed that lung density is a function of the fractional air content of the lung. To relate density to fractional water content of the lung, measurements must be made at a fixed lung volume. The height in the lung at which measurements are made appears to be unimportant. The results of these experiments enable predictions to be made concerning the clinical usefulness of lung density measurements