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Respiratory/Cardiovascular

Chest Anatomy

The thorax houses the lungs and airways, mediastinum with the heart and great vessels, pleural spaces, and the bony/muscular chest wall. Cross-sectional CT is the workhorse for parenchymal, mediastinal, and vascular assessment, while the chest radiograph remains the initial screen. A systematic compartmental approach (lungs, pleura, mediastinum, hila, chest wall, diaphragm) is essential for accurate interpretation.

Key Structures

  • Lungs: right lung has three lobes (upper, middle, lower) and the left two (upper with lingula, lower); 10 bronchopulmonary segments per side, each a functional unit with its own segmental bronchus and artery
  • Tracheobronchial tree: trachea bifurcates at the carina (~T4-5); right main bronchus is shorter, wider, more vertical (favoring aspiration); secondary and tertiary bronchi follow the segmental map
  • Fissures: major (oblique) fissures seen as sharp lines/avascular bands on CT; the right minor (horizontal) fissure separates upper from middle lobe
  • Mediastinum: divided into anterior (thymus, nodes), middle (heart, great vessels, trachea), and posterior (esophagus, descending aorta, sympathetic chain) compartments
  • Great vessels: ascending aorta, arch with brachiocephalic/left common carotid/left subclavian branches, descending aorta, main and lobar pulmonary arteries, SVC and azygos arch
  • Pleura and diaphragm: visceral and parietal layers with a thin normal fluid film; hemidiaphragms as domed muscular sheets with crura descending along the upper lumbar vertebrae
  • Chest wall: ribs, sternum (manubrium, body, xiphoid), costal cartilages, intercostal muscles and neurovascular bundle running in the subcostal groove

Imaging Planes & Modalities

  • Chest radiograph (PA and lateral): first-line for lungs, cardiac silhouette, pleura; the lateral localizes lesions and shows retrosternal/retrocardiac spaces
  • CT with lung and mediastinal windows: axial acquisition with multiplanar reformats; lung window (~ -600/1500 HU) for parenchyma, soft-tissue window (~40/400 HU) for mediastinum and nodes
  • CT pulmonary angiography: bolus-timed to the pulmonary arteries for embolism; contrast opacifies main to segmental/subsegmental vessels
  • HRCT: thin (~1 mm) sections, often prone and expiratory, for interstitial lung disease and air trapping
  • Cardiac-gated CT/MRI and echocardiography for cardiac chambers, valves, and pericardium; MRI adds tissue characterization
  • Coronal and sagittal reformats display fissures, diaphragm, apices, and craniocaudal extent better than axial alone

Landmarks & Normal Values

  • Carina at approximately T4-T5; tracheal diameter normally 15-25 mm (coronal) in adults
  • Aortic root/ascending aorta normally up to ~4 cm; descending aorta ~2.5-3 cm; main pulmonary artery up to ~2.9 cm (larger suggests pulmonary hypertension)
  • Main pulmonary artery to ascending aorta diameter ratio >1 suggests pulmonary hypertension
  • Azygoesophageal recess and aortopulmonary window are key mediastinal interfaces to scrutinize for nodes/masses
  • Right paratracheal stripe normally <4 mm; mediastinal nodes generally considered enlarged above ~1 cm short axis
  • Cardiothoracic ratio on PA radiograph normally <0.5

Anatomical Variants

  • Azygos lobe (aberrant azygos vein creating a fissure in the right upper lobe) present in ~1% of people
  • Accessory fissures (inferior accessory, superior accessory, left minor) that can mimic disease or wall off effusions
  • Aberrant right subclavian artery (arteria lusoria) arising as the last arch branch, coursing behind the esophagus
  • Right-sided aortic arch and other arch branching variants (bovine arch is the most common)
  • Tracheal bronchus ("pig bronchus") supplying the right upper lobe directly from the trachea
  • Persistent left SVC draining to the coronary sinus, often with a dilated coronary sinus

Pathology Correlation

  • Segmental anatomy predicts lobar/segmental collapse patterns and guides localization of pneumonia and tumors
  • Pulmonary embolism lodges at bifurcations; knowledge of arterial branching enables clot burden assessment and RV strain evaluation
  • Mediastinal compartment narrows the differential: anterior (thymoma, lymphoma, teratoma, thyroid), middle (adenopathy, foregut cysts), posterior (neurogenic tumors)
  • Lung cancer staging depends on nodal stations (IASLC map) and invasion of pleura, chest wall, mediastinum, and diaphragm
  • Pleural anatomy distinguishes free from loculated effusions and empyema (split pleura sign) from lung abscess
  • Diaphragmatic crura and hiatus relate to hiatal hernia and can mimic retrocrural adenopathy

Pitfalls & Mimics

  • Do not mistake normal thymus (in young patients) or a prominent thymic remnant for an anterior mediastinal mass
  • Motion and pulsation artifact near the heart/aorta can simulate dissection flaps or nodules; correlate with reformats
  • Mucus plugs and vessels-on-end can mimic pulmonary nodules on a single axial image; track on adjacent slices
  • The azygos arch and confluence can be misread as a paratracheal node or mass
  • Physiologic/dependent atelectasis mimics consolidation or ground glass; repeat prone or on inspiration
  • Companion shadows, rib fractures, and pleural fat can simulate pleural or parenchymal disease on radiographs

References

  • Webb, Higgins. Thoracic Imaging: Pulmonary and Cardiovascular Radiology
  • Muller, Silva. Imaging of the Chest
  • Fleischner Society Glossary of Terms for Thoracic Imaging
  • Netter. Atlas of Human Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.