The term small airways is generally used to refer to airways 3 mm or less in diameter, the vast majority of which represent bronchioles.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

Cliccare qui per andare alla sezione Riferimenti</sup> Bronchioles are divided into membranous bronchioles that function purely as an air-conducting system and respiratory bronchioles that contain alveoli within their walls and are thus involved in respiration. Bronchioles measure approximately 0.5 to 1 mm in diameter. The small airways contribute very little to resistance because, being so numerous, their overall cross-sectional area is large. Consequently, there may be considerable destruction of the small airways before the patient becomes symptomatic and there is any detectable abnormality of pulmonary function.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

Cliccare qui per andare alla sezione Riferimenti</sup> Abnormalities of the small airways may originate within these airways themselves or result from extension of diseases involving either the larger bronchi or the lung parenchyma.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

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In order to appreciate the high-resolution CT (HRCT) scan changes found in small airway diseases it is necessary to have an understanding of the anatomy of the normal secondary pulmonary lobule. The secondary pulmonary lobule is defined as the smallest portion of lung surrounded by connective tissue septa.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

Cliccare qui per andare alla sezione Riferimenti</sup> It measures 10 to 25 mm in diameter. The bronchiole and branch of the pulmonary artery supplying the secondary pulmonary lobule are located in its center, whereas branches of the pulmonary veins are located peripherally within the interlobular septa. The lobular bronchiole divides into three or more terminal membranous bronchioles that each divide into respiratory bronchioles that then branch into alveolar ducts. Thus, the bronchioles are centrilobular structures being clustered near the center of a secondary pulmonary lobule. This accounts for the characteristic centrilobular distribution of bronchiolar abnormalities on HRCT scan. The bronchioles within a secondary pulmonary lobule measure less than 1 mm in diameter<sup>11 Kuhn C. Normal anatomy and histology Pathology of the Lung New York: Thieme (1995).  1-36

Cliccare qui per andare alla sezione Riferimenti</sup> and have a wall thickness of less than 0.1 mm. This is below the limit of visibility on HRCT scan and thus normal centrilobular bronchioles are too small to be visualized.<sup>20 Murata K., Itoh H., Todo G. , e al. Centrilobular lesions of the lung: Demonstration by high-resolution CT and pathologic correlation Radiology 1986 ;  161 : 641-645

Cliccare qui per andare alla sezione Riferimenti</sup> Normally, the only centrilobular structure to be identified on HRCT scan is the pulmonary artery branch. Any additional centrilobular opacities are abnormal.

HRCT scan is currently the best imaging technique for assessment of diseases of the bronchioles.<sup>9 Hartman T.E., Primack S.L., Lee K.S. , e al. CT of bronchial and bronchiolar diseases Radiographics 1994 ;  14 : 991-1003

Cliccare qui per andare alla sezione Riferimenti</sup> Window settings influence the apparent size of structures, particularly small ones, such as bronchioles.<sup>15 McGuinness G., Naidich D., Leitman B.S. , e al. Bronchiectasis: CT evaluation AJR Am J Roentgenol 1993 ;  160 : 253-259

Cliccare qui per andare alla sezione Riferimenti</sup> Measurement of airway diameter is most accurate at a window level of −450 H<sup>28 Webb W.R., Gamsu G., Wall S.D. , e al. CT of a bronchial phantom: Factors affecting appearance and size measurements Invest Radiol 1984 ;  19 : 394-398  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup>; however, this is not generally used in clinical practice because it gives suboptimal demonstration of the lung parenchyma. A window level of −700 H and a window width of 1000 to 1500 H is recommended for optimal depiction of the airways and lung parenchyma. It should be appreciated, however, that this results in an underestimation of true bronchial diameter.<sup>16 McNamara A.E., Müller N.L., Okazawa M. , e al. Airway narrowing in excised canine lungs measured by high-resolution computed tomography J Appl Physiol 1992 ;  73 : 307-316

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There is no universally accepted single protocol for assessing small airways disease.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

Cliccare qui per andare alla sezione Riferimenti</sup> We recommend and use a combination of 1-mm collimation scans at 10-mm intervals through the lungs at end-inspiration and end-expiration scans at five equally spaced levels from the top of the aortic arch to just above the right hemidiaphragm.<sup>19 Müller N.L., Miller R.R. Diseases of the bronchioles: CT and histopathologic findings. State of the art review Radiology 1995 ;  196 : 3-12

Cliccare qui per andare alla sezione Riferimenti</sup> The end-expiratory scans are essential for the detection of air-trapping.