CT lung density is proportional to the air-to-tissue ratio. Therefore, the established HU threshold can be used to assess local hyperinflation, emphysema, or air entrapment. The key to using lung density to infer the presence and progression of disease is to standardize the volume in which the lung is imaged. Emphysematous lung stimulates total lung capacity (TLC) and instructs the patient to count the number of voxels below a threshold ranging from -950 HU to -900 HU, corresponding to low attenuation lung areas. It is evaluated by to assess regional air entrapment, patients hold their breath at either functional residual capacity (FRC) or residual capacity (RV), count voxels below approximately −850 HU, and expiratory lung volume. Imaging is performed at air volume. Demonstrating the difference in air entrapment area between a normal subject (A) and a patient with severe asthma (B), 3 of quantitative multi-detector array computed tomography (MDCT) airway and parenchymal measurements in the same severe patient. This latter technique may allow visualization of local associations between markers of airway remodeling (e.g., lumen area and wall thickness) and distal air entrapment. In addition, functional small airways were assessed using image matching techniques called parametric response mapping (PRM) and disease probability mapping (DPM). Merge inspiratory and expiratory CT scans to detect attenuation changes in equivalent spatial domains. Matched voxels can be assigned to air-trapped, normal, and hyperinflated lung categories. This technique has the potential to classify the radiological component of asthma and chronic obstructive pulmonary disease (COPD) into emphysema-like and air-entrapped phenotypes and monitor disease progression and response to therapy. Small airway instability leading to premature airway closure was recently used by the Inner City Asthma Consortium, along with air entrapment spirometry, as a risk indicator for subsequent exacerbation and need for further asthma treatment. However, the relationship between asthma severity and degree of air entrapment from CT is complex. Air entrapment has been found to be correlated with asthma severity and significantly higher in uncontrolled asthma during exacerbations. He found no significant association between air entrapment and severe, intractable asthma, according to American Thoracic Society criteria (patients with symptomatic asthma requiring high-dose inhaled or oral corticosteroids). I found however, they reported that subjects with air trapping were significantly more likely to have asthma-related hospitalizations, intensive care unit visits, and ventilator history compared with subjects without air trapping. Air entrapment assessed by CT in patients with asthma was also associated with airway hyperreactivity, suggesting that small airways are primarily responsible for this feature of asthma. Air entrapment typically occurs in patients with obstructive pulmonary disease, including bronchiolitis obliterans (i.e., bronchiolitis obliterans), asthma, and COPD, and in some interstitial lung diseases, particularly sarcoidosis. Partial bronchial obstruction by a tumor or foreign body may be the cause, in which case it is usually confined to the lungs, lobes, or parts of the lungs. Congenital causes of air entrapment include bronchial atresia, congenital emphysema, and bronchomalacia. Some degree of air entrapment is common in normal individuals, usually affecting a small portion of the lung (less than 25% of the cross-sectional area of ​​the lung in the plane of the scan), most commonly in the upper segment of the lung. You can see it affects the lower lobes of the lungs, the anterior middle lobes or the tongue, or the dependent regions of individual lung lobes, especially the lower lobes.