Abstract

Clara Gaik Cheng Ooi
Hong Kong

Computed tomography (CT) has revolutionized imaging of the airways providing non-invasive anatomic evaluation of the large and medium sized bronchi, and an insight into the physiology of airways diseases particularly those involving the small airways. The recent advent of CT multidetector (MD) technology has further improved the resolution and speed of airway imaging.

CT techniques for airway imaging include inspiratory and expiratory high resolution CT (HRCT), low dose CT, volumetric CT and multidetector CT. In the majority of clinical settings, HRCT is the optimal technique to image most airway diseases including emphysema, which for all intents and purposes in this lecture, is included as part of the spectrum of airway disease. Small airways are arbitrarily defined as bronchi with diameter ?2mm. Normal or low dose volumetric technique should be evoked in circumstances when breath-holding may pose a problem in patients with shortness of breath. Retrospective reconstruction of volumetric data set into HRCT sections in MDCT scans should allow adequate evaluation of the airways in these patients. Expiratory HRCT scans allow air trapping to be detected particularly in the presence of mosaic attenuation (alternating areas of hypo and hyper attenuated lungs) and is an indirect sign of small airways disease. Expiratory scans should be obtained at end expiration to optimise results. However this may be difficult to achieve in some patients. Coaching the patient to expire optimally and to breath-hold in expiration prior to scanning will reduce repeat scans and ambiguous reports. Invagination or bowing in of the posterior membranous wall of the trachea is a fairly reliable sign of adequate expiration. For adequate evaluation of bronchial wall thickening, which is present in both small and large airways disease as a sign of airway inflammation, optimal window width and levels have been advocated: window level of between –250 and –700, and a width of not less than 1000HU.

Bronchiolitis can be classified according to histopathological appearances, which fall into these broad categories: cellular bronchitis (including follicular bronchiolitis and infectious bronchiolitis), respiratory bronchiolitis (respiratory bronchiolitis, Respiratory bronchiolitis-interstitial lung disease), constrictive bronchioltis and bronchiolitis obliterans with intraluminal polyps. HRCT classification can be based on either direct visualization of small airways in exudative forms of bronchiolitis (panbronchiolitis or infectious bronchiolitis) and indirect signs of constrictive bronchiolitis.

The aims of this lecture are therefore (1) To discuss the optimal CT technique for imaging large and small airways diseases. 2) Recognize imaging features of common diseases of the large airways including bronchiectasis and asthma, including pitfalls in diagnosis. 3) Recognize the HRCT patterns and differential diagnosis of small airway disease.