Diagnostic Aerosols: Current Status and Future Prospects

Abstract

The inhalation of diagnostic aerosols directly into the respiratory tract (RT) can mimic the entry of air, inhaled allergens, and toxins. Such inhalation has also the advantage for diagnosis of diseases of the RT in that the active ingredient has, at least initially, a high ratio of local/systemic concentration. Nonradioactive aerosols have been used in the bronchial reactivity testing and as contrast media in bronchography. More recent developments include the assessment of airway dimensions and presence of airway diseases by means of the aerosol bolus technique. Radiolabeled aerosols have been used for anatomical (static) imaging of RT and in the kinetic mode to follow the clearance processes. Gamma scintigraphy has been used extensively in the differential diagnosis of pulmonary embolism and ventilation defects. Detection and localization of airway obstruction are also possible using radioaerosols. Transport processes in the RT can be followed by delivering radiolabeled soluble probes by inhalation to study pulmonary and mucosal permeability, or by employing insoluble carriers to investigate mucociliary clearance. The three-dimensional functional heterogeneity of RT makes the use of three-dimensional imaging desirable in order to improve the anatomical resolution and the signal/noise ratio. Regional, rather than whole lung, measurements are preferable for similar reasons. It is envisaged that a wider availability of multiple-detector single photon emission computerized tomography and positron emission tomography cameras will enhance the spatial and temporal resolution of these diagnostic modalities. The specificity of the tests used in anatomical (static) imaging may be improved using immunoscintigraphic methods. The differential diagnosis based on measurement of permeability may be advanced in the future by selection of more appropriate carriers of the radiolabels. It should be remembered that these advanced methods will not yield reproducible results without paying attention to the fundamental properties of aerosols, in particular, their instability to evaporation/condensation and deposition before entry to the patient's RT. Design of appropriate aerosol technology for use in diagnostic procedures poses an interesting challenge to aerosol scientists.