Air trapping on computed tomography: regional versus diffuse

Abstract

Chronic obstructive pulmonary disease (COPD) is diagnosed and classified by spirometry, assessing the presence of airflow obstruction and extent of forced expiratory flow deterioration. Spirometry, however, is unsuitable for characterising and quantifying the underlying pulmonary pathology of COPD, including alveolar destruction (emphysema) and airway remodelling (large-and small-airway disease). Accurate diagnosis of the pulmonary pathologies underlying COPD is seen as an important step towards better understanding its biology and "personalised" treatment. Computed tomography (CT) of the lungs provides an excellent opportunity to assess COPD in vivo and quantify its macroscopic pathology [1][2][3]. Emphysema and airway remodelling can be visually assessed and automatically quantified, and several studies have shown associations with morbidity and mortality [4][5][6][7]. In addition, CT can provide information on the spatial distribution of disease.In this issue of the European Respiratory Journal, KARIMI et al. [8] hypothesise that regional air trapping maybe a new imaging marker, in addition to emphysema, large-airway remodelling and diffuse air trapping.The small airways, commonly defined as those <2 mm in diameter, are regarded as the most important site of airflow obstruction in COPD [9]. Measuring small-airway disease on CT can be challenging as these cannot be visualised directly because they are too small and below the resolution of the CT scanner. Indirect measures are thus used to measure small-airway disease on CT. Air trapping on CT, sometimes also referred to as gas trapping, is often used to asses small-airway disease [10]. Pathophysiologically, it is defined as retention of air in the lung distal to an obstruction, but it is difficult to prove this by radiology-pathology correlation studies. To assess air trapping, both inspiratory and some form of expiratory scans are needed.The definition of regional air trapping on CT consists of three components. First, the trapped areas have a relatively high attenuation on expiratory CT scans. Fully inflated alveoli have an average attenuation of −850 Hounsfield units (HU) and with a good expiratory effort, the average attenuation increases by 150 HU. The second component is volume, as areas with air trapping do not lose much of their volume on expiration. The third component is morphology, as air trapping has a sharp delineation at the border of the secondary pulmonary lobule.The radiological diagnosis of regional air trapping is challenging for several reasons. First, proper expiration is needed for the air trapping to become visible. This needs compliance from patients or research participants, properly trained technicians, and/or spirometric gating. CT technicians tend to train patients to only a limited extent before the CT acquisition and they also sometimes start the scan too

Air trapping in small airway diseases: A review of imaging technique and findings with an overview of small airway diseases

“…Firstly, a full expiration phase CT is required, which can be challenging to obtain in itself. Patient compliance is required to achieve full expiration, which can be difficult for unwell patients and for people who are unable to follow commands 2 . Adequately trained CT radiographers are also essential to ensure that the timing of image acquisition is accurate.…”

“…Adequately trained CT radiographers are also essential to ensure that the timing of image acquisition is accurate. Other concurrent lung parenchymal diseases such as emphysema can also mask areas of air trapping on CT, posing a further diagnostic challenge for radiologists 2 …”

“…Another confounding factor is that air trapping can also be present in healthy subjects 2 . Varying studies have discovered air trapping present in up to 53% of healthy patients 5 and 64% of those with normal PFTs 5 .…”

“…With small airway diseases, this can result in geographic areas of differing (mosaic) attenuation. 1 The average attenuation of the lung parenchyma increases by 150 HU between inspiratory and expiratory phases, 2 with a change of less than 70 HU regarded as abnormal. 1 The affected areas of the lung also maintain most of their volume on expiration and are sharply delineated from the normal neighbouring lung parenchyma.…”

Pulmonary quantitative CT imaging in focal and diffuse disease: current research and clinical applications

“…lung volumes, notably because air trapping and emphysema may be admixed. 84 Volumetric non-rigid registration of inspiratory and expiratory scan overcomes such limitation because it allows biphase characterization of each voxel, and is particularly interesting for quantification of air trapping. This approach was named parametric response mapping (PRM) when used to define density clusters for topographic categorization of parenchyma into normal lung, air trapping and emphysema 85 ( Figure 3).…”