CT lung densitometry in assessing intralobular air content. An experimental and clinical study

To introduce a parameter called "intralobular air content percentage" to replace the CT number of the lung and to establish a proper protocol for its assessment.

We calibrated the HU (Hounsfield unit) scale for low densities with foam and evaluated the influence of certain acquisition and reconstruction parameters on the accuracy of CT densitometry of the lungs. The reproducibility of the results obtained in human experiments and the intralobular air content percentage of normal and diseased lung tissue were assessed.

Air content could be reliably derived from the calibrated CT number of an area within a secondary lobulus. The mean intralobular air content of normal lungs varied from 77.8% to 88.0% in full inspiration. Helical or axial recording with a 10-mm slice thickness, a standard or soft algorithm and high tube currents and voltage settings, was suitable for the measurements.

Before absolute lung density measurements (as a HU number or an air content percentage), the CT equipment has to be calibrated for low densities. The intralobular air content percentages of cooperative patients were reliably reproducible.