The commonest technique for the assessment of air trapping at CT is based on post-expiratory thin-section CT obtained during suspended respiration following a forced exhalation. Each post-expiratory CT image is compared with the inspiratory CT that most closely duplicates its anatomical level to detect air trapping. Dynamic expiratory manoeuvres performed during helical CT acquisition have been described; these permit a small increase in the degree of expiration, which leads to a better detection of air trapping. This technique is recommended when patients have difficulty performing adequate suspended end-expiration. Ultra low dose MDCT with thin collimation over the lungs has become routine in many institutions to improve the conspicuity and the apparent extent of air trapping.
Multiplanar volume rendering slab associated with the technique of minimum intensity projection increases the contrast media between areas of normal lung attenuation and areas of lung hypoattenuation. This helps the depiction of mosaic perfusion pattern. Its applications on expiratory CT images can also facilitate assessment of the presence and extent of air trapping.
The extent of air trapping present on expiratory CT can be measured using a semiquantitative scoring system that estimates the percent of lung that appears abnormal. In the scoring system proposed by Stern et al., estimates of air trapping were made at each level and for each lung on a four-point scale: 0, no air trapping; 1, 1–25%; 2, 26–50%; 3, 51–75%; and 4, 76–100% of cross-sectional areas of lung affected. The air trapping score is the summation of these numbers for the different level studied. This scoring system allows good interobserver and intraobserver agreement. The extent of expiratory air trapping at CT has proved to be correlated with the degree of airflow obstruction at pulmonary function tests in patients with obliterative bronchiolitis.
Objective measurement of air trapping can be done using CT densitometry. In the density mask technique, all the pixels included in areas of air trapping are segmented by thresholding at –910 HU and are highlighted and automatically counted. Density changes between full inspiration and full expiration can be compared, and expiratory/inspiratory ratios can be calculated. The density mask has the advantage that it combines density measurement with the visual assessment of pathology. Using multidetector CT with thin collimation over the lungs performed at full expiration, an exhaustive assessment of the volume of air trapping may be provided as well as a 3D visualisation of distribution of air trapping.
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