Cystic Diseases of the Lung Idiopathic Pulmonary Fibrosis Idiopathic pulmonary fibrosis is a type of diffuse interstitial lung disease characterized by the presence of usual interstitial pneumonia in the histological examination

High-resolution computed tomography (HRCT) is the radiographic imaging technique that best detects the changes in lung structure that occur during interstitial diseases.1 It has been proved better than conventional chest x-rays and thick-section computed tomography in detecting and assessing the pattern and extension of the various interstitial lung diseases that present cysts.2 Correlation with clinical and functional conditions is better with HRCT than standard radiography.2,3 HRCT measurement of the extension of the assessed cystic disease is consistent with gas exchange alterations and severity of airway obstruction.4 However, HRCT gives a macroscopic view. The radiographic signs are nonspecific so conclusions on the etiology of the findings are normally based on associated and indirect signs. Although some diseases can have common findings, making differentiation difficult, others have specific characteristics that frequently indicate diagnosis. Although HRCT is central to the assessment of interstitial lung disease, clinical, functional, and analytical aspects must be taken into account in the final diagnosis.2 HRCT allows the study of the lung parenchyma with sufficient spatial resolution to assess the structures that make up the secondary lobules (less than 1 mm).5 A lung parenchyma scan with 1 mm or 1.5 mm thick sections2,5-7 is performed and reconstructed by using a high-spatial-frequency algorithm with acquisition data of 120kV and between 160 and 220 mA,5-8 sections being obtained during both inspiration and expiration.7,8 Lung cysts are defined as abnormal lung parenchymal spaces filled with air or liquid, with walls of varying thickness, and covered with epithelium.5 From the radiographic point of view, cysts can be defined as air-filled spaces of variable size with distinct borders and thin walls (less than 3 mm).6 The objective of this article was to review HRCT findings of a wide variety of diffuse cystic pulmonary diseases related to airway obstruction: honeycomb lung, histiocytosis X, lymphangioleiomyomatosis, tuberous sclerosis, and interstitial lymphoid pneumonia, as well as other diseases that imitate diffuse cystic processes such as emphysema and cystic bronchiectasis. This article does not cover infectious diseases or neoplastic processes that can produce lung cysts or localized congenital cystic diseases (cystic adenomatoid malformation). We do mention, however, pathological processes commonly encountered in daily practice that can imitate cystic diseases of the lung.