Bronchiolitis is defined as inflammation of bronchioles that are less than 3 mm in diameter. This inflammation primarily involves the terminal bronchioles, the more distal respiratory bronchioles and, in some cases, the adjacent alveolar ducts and spaces.1
Bronchiolitis can be categorized as inflammatory or fibrotic. Inflammatory forms include cellular bronchiolitis, respiratory bronchiolitis, diffuse panbronchiolitis, and follicular bronchiolitis. Constrictive bronchiolitis and primary neuroendocrine cell hyperplasia are examples of fibrotic disorders.Bronchiolitis is a known complication of asthma and bronchitis and is now well documented in patients with emphysema.
In this article, we will describe the imaging techniques used to assess small-airway disease, with the main focus on the role of high-resolution CT (HRCT) in bronchiolitis. Our review will not cover bronchiolitis that is secondary to asthma, bronchitis, or emphysema. It also will not address proliferative bronchiolitis, or bronchiolitis obliterans with organizing pneumonia, because this entity produces a consolidative inflammatory response to many insults in the lungs and does not have a dominant imaging feature.1
In clinical practice, the most common imaging techniques that are used in the assessment of airway disease include projection imaging (classic chest radiography) and multichannel CT with HRCT images of the lung parenchyma. Projection imaging is less accurate than HRCT in making the diagnosis of emphysema, bronchiectasis, and small-airway disease. In patients who have small-airway disease that is secondary to asthma or bronchiolitis, findings on projection imaging are often normal.
Multichannel CT scanning should be performed at total lung capacity (TLC) and at functional residual capacity (FRC) in patients with suspected airway disease. Contiguous 5-mm images and 1.25-mm images should be reconstructed at 10-mm intervals from both the inspiratory and expirato-ry multichannel CT acquisitions. Most multichannel CT scanners use "beam modulation" to obtain high-quality images at the lowest effective dose for the patient.
Additional isotropic 0.625-mm-diameter cubic voxel 3-dimensional data sets can be reconstructed and viewed as 2-dimensional projection imaging or 3-dimensional volume-rendered sets of images viewed in any direction. The "post-processed" group of images is helpful in identifying subtle or confusing airway abnormalities and for surgical planning, if necessary.
The HRCT signs of bronchiolitis can be either direct or indirect. Direct signs include centrilobular nodules, bronchial wall thickening, and bronchiolectasis. Indirect signs include mosaic perfusion and hyperlucency on images obtained at TLC; mosaic or diffuse airtrapping on images obtained at FRC; vascular attenuation; and increased lung volumes.
Normal small bronchioles are usually not seen on HRCT; however, when inflammation surrounds them, they appear as nodules in the center of the secondary pulmonary lobule.2 Centrilobular nodules are the small (1 to 3 mm) nodules that develop in the central portion of the secondary pulmonary lobule; they are readily visible on HRCT scans in patients with some forms of bronchiolitis. Bronchiectasis refers to the lack of tapering and the dilatation of the bronchiole lumen in the small airways (diameter, less than 3 mm) in the periphery of the HRCT images.
Mosaic perfusion/hyperlucency refers to the variability in the attenuation of the lung secondary to airtrapping and/or decreases in perfusion. The term "mosaic" is used because the lung attenuation varies rapidly across a relatively lin- ear interface and may produce a polygonal or quiltlike appearance. Hyperlucency is the result of decreased attenuation or density in the areas of mosaic perfusion. Mosaic perfusion and hyperlucency apply to HRCT images that are obtained at TLC.
Mosaic or diffuse airtrapping refers to the appearance of the lungs on HRCT images that are obtained at FRC. The areas of decreased attenuation or density evident on the expiratory images correspond to obstructed small airways that produce hyperexpansion of the distal alveolar structures. Expiratory HRCT scans are considered an essential part of the workup in patients with bronchiolitis, because airtrapping may be evident only on these scans.3
In patients with constrictive bronchiolitis or bronchiolitis obliterans, expiratory CT images have been shown to provide the best correlation with indices of physiologic impairment.4 These areas of decreased density may be diffuse, patchy, or mosaic in patients with bronchiolitis, but the mosaic pattern of airtrapping is specific for bronchiolitis.5
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