In emphysema, the connective tissues supporting the shape and function of the lungs is progressively destroyed, resulting in expansion of the respiratory units. In panacinar emphysema, the entire respiratory acinus is expanded; this occurs more often in the lower lobes and anterior margins of the lungs. In centriacinar (or centrilobular) emphysema, only the respiratory bronchiole is expanded, with sparing of the distal alveoli; this occurs more commonly in the upper lobes. The most common cause of emphysema is smoking. Alpha 1-antitrypsin deficiency is a common inherited cause.

Centrilobular Emphysema
- Expansion of respiratory bronchiole, which worsens over time, with sparing of peripheral parts of the acinus
- Most common form of emphysema, usually associated with chronic airway obstruction
- Like respiratory bronchiolitis, strongly associated with cigarette smoking; may evolve from respiratory bronchiolitis
- There is no obvious or visible fibrosis, helping distinguish from interstitial pneumonias
- Diffusion capacity is decreased, but 30% of lung must be destroyed before pulmonary function tests deteriorate

Centrilobular Emphysema: Imaging Findings
- CXR clues: hyperinflation, flat hemidiaphragms, vascular disruption and increased branching angle
- Hard to detect until severe on CXR, preferential study is HRCT
- Occurs preferentially in the upper lobes, typical pattern is multiple smaller lucencies in center of secondary lobules
- Lucencies more pronounced centrally than peripherally in lung
- Large bullae (>1cm) are most common subpleurally
- Secondary lobule wall normally not seen, but may be seen in larger spaces secondary to relaxation atelectasis of adjacent lung
- Severity can be graded subjectively (<25% of lung affected is mild, 25-50% moderate, 50-75% marked, 75%+ severe)
- Objective grading is based on pixel density; greater than 2SD below average lung density (-750 to -850 HU), or around -900HU, is threshold for diagnosing emphysema
- Severe centrilobular emphysema and panacinar emphysema are not distinguishable
- DDx: Panlobular emphysema, Langerhans Cell Histiocytosis, late stage farmer's lung

Case #1. Severe bullous emphysema most prominent in right lung.

Case #2. Saber-sheath trachea (almost always associated with COPD) in setting of early emphysematous change.