High resolution CT (HRCT) of the lung in adults. Defining the limits between normal and pathologic findings.

Under normal conditions, HRCT shows secondary lobule structures and allows for a precise evaluation, with inspiratory and expiratory scans, of central airways dimensions, lung area and parenchymal density. Frequently, asymptomatic smokers with normal lung function tests, present with mild abnormalities not visible at chest X-ray: bronchial thickening, bronchiolectasis, respiratory bronchiolitis and centrilobular or paraseptal emphysema. In the elderly, HRCT studies often show borderline findings such as age-related tracheo-bronchial calcifications and emphasise the progressive increase in the bronchoarterial ratio. Another frequent finding is expiratory air trapping, which can be associated with a variety of lung diseases, but can also represent a physiologic and temporary finding, during an episode of sub-clinical bronchospasm or related to local variation of bronchiolar dynamic compliance. The knowledge of the boundaries between normality and pathology is an essential prerequisite for the correct interpretation HRCT findings.