High-resolution CT of peripheral airways diseases

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ANATOMY AND PHYSIOLOGY

The complex function of the airways can be conceptually oversimplified by dividing the bronchial tree into three gross anatomically and functionally based zones: (1) conducting, (2) transitory, and (3) respiratory.10, 28 Originating at the trachea, the conducting zone continues for at least 16 generations, terminating at the terminal bronchioles. The function of the conducting zone as a transporter of oxygen is reflected in its structure; there are no alveoli to allow for gas exchange. Termed

HIGH-RESOLUTION CT SCAN MANIFESTATIONS OF SMALL AIRWAYS DISEASE

There are direct and indirect radiographic manifestations of peripheral airway diseases. Direct manifestations are uncommon because most airways of the tracheobronchial tree are radiologically invisible. Those diseased bronchioles that are visible with HRCT appear as dilated air-filled, branching, ring-like, or tubular structures in the lung periphery.23 Enhanced visualization of the bronchioles in this location is caused by wall thickening and dilatation. When the airway is obliterated by

ASTHMA

A reversible form of small airways obstruction, asthma affects both small and large airways. Asthma is very common, occurring in up to 5% of adults and 10% of children. The chest radiograph can be normal, but nonspecific findings include hyperinflation, hypoinflation with multiple focal areas of atelectasis distal to obstructed airways, pneumomediastinum, interstitial pulmonary emphysema, and complicating pneumonia. Obstruction of small bronchi and bronchioles by inflammatory exudate and

BRONCHIOLITIS OBLITERANS WITH INTRALUMINAL PROLIFERATION OF GRANULATION TISSUE POLYPS

Also termed proliferative bronchiolitis,5 this disorder is characterized by the presence of inflammatory or infectious exudative material within the lumen of the bronchioles and alveolar ducts. In most cases there is accompanying organizing pneumonia within the subtended distal components of the lobule (bronchiolitis obliterans with organizing pneumonia [BOOP]). Because the clinical,4 physiologic, and imaging13, 15, 16, 29 features of this conditions are inconsistent with other forms of small

CONSTRICTIVE BRONCHIOLITIS

Also known as obliterative bronchiolitis, constrictive bronchiolitis occurs when the bronchioles become concentrically obstructed because of peribronchiolar or submucosal fibrous proliferation, with little accompanying active inflammatory granulation tissue.5, 7, 15, 29 Unlike the restrictive features of BOOP, pulmonary function tests usually show airflow obstruction, or mixed restriction and obstruction. Usually irreversible, the disease severity depends on the extent of bronchiolar

CELLULAR BRONCHIOLITIS

Cellular bronchiolitis describes the process of active inflammation involving the bronchiolar wall or lumen. Acute infectious bronchiolitis is an example of cellular bronchiolitis, including viral, mycoplasma, tuberculosis, or Aspergillus infection.2, 15, 23 In infancy and childhood, respiratory syncytial virus is a common cause of acute infectious bronchiolitis. Although most episodes resolve without long-term sequelae, more severe cases of pulmonary parenchymal injury can lead to constrictive

PANBRONCHIOLITIS

Rare in North America and Europe, panbronchiolitis, also called diffuse panbronchiolitis, is an idiopathic inflammatory lung disease that is prevalent in the Asian population and most common in Japanese men. The disease is characterized by mononuclear cellular inflammation of the respiratory bronchioles. Patients present with nonspecific progressive dyspnea and productive cough. HRCT findings include branching linear and nodular opacities, bronchiolectasis, bronchiectasis, mosaic perfusion, and

RESPIRATORY BRONCHIOLITIS AND RESPIRATORY BRONCHIOLITIS-INTERSTITIAL LUNG DISEASE

Respiratory bronchiolitis, also known as smoker's bronchiolitis, is present histologically in most smokers.8 Characterized by mild chronic inflammation of the respiratory bronchioles and accumulation of pigmented macrophages within the respiratory bronchioles and adjacent alveoli, this disorder rarely is symptomatic and usually produces no chest radiographic abnormalities.8, 15, 29 Remy-Jardin et al19 correlated the HRCT findings in asymptomatic heavy smokers with pathologic specimens from

PITFALLS

There are pitfalls in the HRCT diagnosis of bronchiolar disease. The findings of thickened lobular septa and adjacent peribronchial nodules present in various granulomatous processes can mimic the TIB pattern of bronchiolar disease on HRCT.3 These granulomatous diseases include sarcoidosis, silicosis, and Langerhans' cell granulomatosis.3 Features that distinguish sarcoidosis from bronchiolitis include lymphadenopathy and perivenule nodularity.3

The lymphatic and hematogenous spread of pulmonary

SUMMARY

Bronchiolitis and bronchiolectasis are nonspecific inflammatory processes of the small airways that have a variable, but often characteristic, appearance on HRCT. Familiarity with the imaging features of these disorders is crucial in rendering an accurate radiographic diagnosis.

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