Chest X-Ray: Lung Anatomy

• Understanding normal anatomy is key before being able to understand what is abnormal

• Anatomy review
  • Visceral pleura hugs the lungs (forms fissures), parietal pleura lines chest wall
  • Lobes of the lung: right lung has upper, middle, and lower lobes, left lung has upper and lower lobes
  • Fissures: right lung has horizontal/minor fissure and oblique major fissure, left lung has one huge oblique major fissure 
    •  Invisible or fine white lines, if thickened may represent excess fluid from a process such as congestive heart failure
  • Trachea -> carina -> main bronchus -> lobar (secondary) bronchi -> segmental (tertiary) bronchi -> subsegmental bronchi -> bronchioles -> secondary lobules (centrilobular artery and bronchus) -> respiratory bronchioles -> alveoli 
    • Cannot usually see bronchi on chest x-ray
    • Conduction zone = trachea, bronchi, bronchioles, terminal bronchioles
    • Respiratory zone = respiratory bronchioles, alveolar ducts, alveoli 
    • Lung parenchyma = alveoli, ducts, and respiratory bronchioles
    • Type 1 pneumocytes do gas exchange, type 2 pneumocytes produce surfactant, alveolar macrophages ingest and process debris
  • Hemidiaphragms: left is obscured by heart, on lateral radiograph can follow the right hemi all the way across
  • Left main pulmonary artery arches over the left main bronchus 
    • The right pulmonary artery will be anterior and inferior to the bronchus/left pulmonary artery on lateral radiograph
    • Each pulmonary artery may appear as slightly opaque compared to the lumen of the bronchus 
    • Pulmonary vessel markings taper peripherally and will show up as white lines thicker at the base of the lung 
  • Lungs have supply from pulmonary arteries and bronchial arteries (from aorta)
  • Pulmonary veins drain into left atrium
  • Innervation: parasympathetic from vagus, sympathetic from thoracic ganglia 
  • Lymph drainage goes to hilum 
  • Physiology: parasympathetic causes vasoconstriction, bronchoconstriction, gland secretion, sympathetic does opposite and opens things up 

• Lateral radiograph is important to view areas of the lung that may not be obviously abnormal on frontal view 
  • Normal features of lateral radiograph:
    • Space behind the sternum (lack of could point to mediastinal masses)
    • Absence of a major shadow from the hila (presence could point to sarcoidosis)
    • Consistent height of vertebrae
    • Sharp posterior (requires less fluid to visualize) costophrenic angles (blunted/opacity filled could point to pleural effusion)
    • Continuous right hemidiaphragm (and slightly higher)

• As with all studies/indications, when looking at an image, do not forget to look at all of the structures in field of view; even if you are reading a chest x-ray to rule out pneumonia/pneumothorax, remember to view all of the other structures, such as the thoracic vertebrae!

References: Herring's Learning Radiology, Radiopaedia, Mandell's CORE Radiology