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.

Chest Anatomy

🧩 Key Structures

🎞️ Imaging Planes & Modalities

📏 Landmarks & Normal Values

🔀 Anatomical Variants

🧠 Pathology Correlation

💡 Pitfalls & Mimics