Validation of lung densitometry threshold at CT for the distinction between senile lung and emphysema in elderly subjects

Background and Aims. An ageing lung is characterised by distal airspace enlargement without alveolar wall destruction: therefore the anatomical distinction between senile lung and emphysema is clear-cut. In clinical settings the definition of precise boundaries between normalcy and pathology is more difficult with the risk of overdiagnosis. CT is an important diagnostic advancement in the field of COPD. Most methods for the evaluation of emphysema are based on the detection and measurement of areas characterised by a density level below a threshold assumed to characterize parenchymal destruction. Methods. Our retrospective study included 47 healthy subjects (65-91 years), 36 never smokers and 11 former smokers. As a reference sample we recruited 9 patients with emphysema (69-81 years). Thoracic scan was performed by single slice spiral CT and acquired without contrast enhancement. For each scan and on both lungs we sampled eighteen regions of interest in the upper, middle and lower field. Mean lung density (MLD) and lower limit of normal (LLN) of density distribution were calculated. Results. MLD for the whole study sample was -846±41 HU. -901 HU was the LLN of density distribution in the study sample. No significant correlation was noted between age and MLD. In the emphysematous sample the average lung density was -946±18 HU. The mean coefficient of variation was 3% in the healthy sample and 2% in the emphysematous one. The difference between groups was significant (p<0.0001). In one healthy subject only we measured a value slightly below the threshold reported in literature for conventional CT; no emphysematous value fell above the LLN. Conclusions. This study highlights the fact that in the elderly the threshold level of lung density commonly adopted in diagnostic algorithms of emphysema is fully applicable. When applying this method to older subjects the risk of misinterpreting areas of physiologic non-destructive reduction of density as emphysema is low.