Constrictive (Obliterative) Bronchiolitis
An example of the important use of combined inspiratory and expiratory HRCT scans to detect small airways disease is the detection of constrictive bronchiolitis.
Constrictive bronchiolitis involves the inflammation of bronchial tubes that leads to obstruction of the bronchiolar lumen. There are numerous causes of constrictive bronchiolitis, among which is chronic rejection in heart and lung transplantation.
Direct signs of constrictive bronchiolitis shown on HRCT include centrilobular branching structures and centrilobular nodules caused by peribronchiolar thickening and bronchiolectasis with inspissated secretions. Indirect findings include bronchiectasis and bronchiolectasis, mosaic pattern of lung attenuation, and air trapping (Fig 6).
In obliterative bronchiolitis, the mosaic pattern of lung attenuation is caused by hypoventilation of alveoli distal to bronchiolar obstruction, which leads to secondary vasoconstriction, which is seen on CT scans as areas of decreased attenuation. Uninvolved segments of lung show normal or increased perfusion with resulting normal to increased attenuation. The combination of low attenuation changes in involved regions with higher attenuation areas in uninvolved regions is referred to as a “mosaic pattern” of lung attenuation. buy birth control online
Air trapping is seen on CT scans as the failure of portions of lung to change volume or attenuation between inspiratory and expiratory images. Using paired CT scans in inspiration and expiration is useful for distinguishing small airways disease from primary vascular lung disease. In small airways disease, the lucent regions of lung seen at inspiration remain lucent at expiration due to air trapping, and show little increase in lung attenuation or decrease in volume. In contrast, the relatively opaque healthy lung will increase in attenuation and decrease in volume as expected. In primary vascular disease, because there is no air trapping or airways disease, the attenuation of both the hyperemic and oligemic lung at inspiration will increase in a similar fashion, and the volume of both will decrease at expiration.
Imaging-based methods that allow the detailed assessment of airway morphology and dynamic responses to pharmacologic interventions, characterization of patterns of parenchymal abnormalities, and quantitation of blood flow and ventilation parameters at the gas exchange interface are avidly being pursued. Lung diseases are often viewed as involving the whole lung equally, a situation reinforced by the results of PFTs, whereas regional lung disease is likely more common in most instances.
Figure 6. Inspiratory (top) and expiratory (bottom) HRCT scans in a patient with bronchiolitis obliterans. Note the mosaic attenuation pattern evident in the expiratory image (bottom), showing areas of relatively higher attenuation in regions of healthy lung, in contrast to areas of persistent low attenuation (arrows) representing regions of air trapping due to small airways disease. This patient had progressive dyspnea due to bronchiolitis obliterans associated with rheumatoid arthritis. (Courtesy of Wallace T. Miller, Jr, MD, University of Pennsylvania Medical Center.)