Dark bronchus sign | Radiology Signs

Answer:

The dark bronchus sign is the appearance on expiratory CT of a bronchus that remains conspicuously black against lung parenchyma which has failed to increase in attenuation as it should. It is a marker of air trapping from small airways disease: normal lung becomes denser and reduces in volume on expiration, so the contrast between the air-filled bronchial lumen and the surrounding lung falls, whereas lung distal to obstructed bronchioles cannot empty, stays lucent and preserves the contrast. The sign is best assessed on paired inspiratory and expiratory acquisitions, and its regional counterpart is mosaic attenuation, in which lobular areas of trapped, lucent lung alternate with normally dense lung and the vessels within the lucent areas are smaller because of reflex hypoxic vasoconstriction. The differential of mosaic attenuation must be worked through systematically, since the same pattern can arise from small airways disease, from infiltrative lung disease producing ground-glass in the denser areas, or from chronic thromboembolic pulmonary hypertension producing perfusion mosaicism; the expiratory series is what separates them, because only air trapping accentuates on expiration. Common causes are constrictive bronchiolitis after transplantation or infection, hypersensitivity pneumonitis, asthma, cystic fibrosis, bronchiectasis of any cause and diffuse idiopathic pulmonary neuroendocrine cell hyperplasia.

Why is it called so?

The bronchus stands out as a dark hole because the lung around it has failed to become denser on expiration.

Pathophysiology

During normal expiration alveolar gas leaves the lung, the parenchyma decreases in volume and its attenuation increases by roughly 80 to 300 HU, so bronchial walls and lumina become less conspicuous. Inflammation and fibrosis of the membranous and respiratory bronchioles narrow their lumina and cause dynamic collapse on expiration, so the gas distal to them cannot escape. That lung retains its inspiratory volume and attenuation, and the bronchus within it remains sharply outlined. Reduced ventilation in the affected lobules triggers hypoxic pulmonary vasoconstriction, which diverts blood to normally ventilated lung and produces the small-calibre vessels characteristic of true air trapping, a feature that helps distinguish it from ground-glass opacity in the mosaic pattern.

Alternative names: Dark bronchus appearance; expiratory air trapping sign

Other associated named signs: Mosaic attenuation, the head cheese sign of mixed infiltrative and obstructive disease, and tree-in-bud of cellular bronchiolitis