Characterizing Lung Aerosol Penetration

Radioactive aerosol work has identified factors which crucially affect patterns of inhaled particle deposition within the lung. For many sizes and types of particle, gamma camera images have been analysed in terms of quantitative distribution indices - often a 'penetration index' (PI) or a 'central to peripheral' deposition ratio (C/P). With insoluble particles, retention 24 hours post inhalation - often described as 'alveolar deposition' (AD) - offers a different method of measuring depth of lung particle penetration. Studies of insoluble particle clearance have indicated the extent to which the imaging indices (PI or C/P) or retention values (AD) can predict the ensuing rates of particle clearance. A standardised approach to lung zone definition for PI or C/P measurement is important to facilitate data intercomparisons between different centres. An important step towards such standardisation can be gained from expressing aerosol lung penetration relative to that of a reference radioactive gas itself inhaled under standardised conditions. Information gained from the imaging indices is not identical to that from AD measurement. Sometimes differences between the two approaches may serve to illuminate mechanisms at work. In other instances it may be the combination of imaging and retention data that best characterises lung aerosol penetration.