Thoracic Imaging and Cross-sectional Anatomy

Thoracic Imaging Modalities - Thorax Tech Tour

CXR Anatomy & Views - Shadows & Silhouettes

• Standard Views: PA (preferred for minimal magnification), AP, Lateral, Decubitus.
• Systematic Review (ABCDE) 📌:
  • Airway: Trachea (midline), carina, main bronchi.
  • Bones: Ribs (fractures, lesions), clavicles, scapulae, vertebrae.
  • Cardiac/Mediastinum: Cardiothoracic Ratio (CTR) < 0.5 (PA view). Assess size, contours (aortic knob, main pulmonary artery).
  • Diaphragm: Right hemidiaphragm higher than left. Sharp costophrenic angles.
  • Everything Else/Fields: Lung zones, vascularity, pleura (effusions, pneumothorax), soft tissues.
• Silhouette Sign: Loss of border between structures of same radiographic density (e.g., fluid, soft tissue); indicates lesion is anatomically contiguous with that structure.

⭐ Silhouette Sign: Right middle lobe (RML) pneumonia typically obscures the right heart border on a PA CXR because the RML is anterior and adjacent to the right heart border.

CT Thorax: Mediastinum - Mediastinal Maze Map

Mediastinal compartments on CT:

• Anterior: Sternum to anterior pericardium. Contents: Thymus, lymph nodes, fat.
• Middle: Contains pericardium, heart, great vessel origins, trachea, main bronchi.
• Posterior: Posterior to pericardium, anterior to vertebrae. Contents: Esophagus, descending aorta, azygos/hemiazygos veins.

Key Axial CT Levels & Structures:

• Thoracic Inlet/Supra-aortic:
  • Brachiocephalic (BC) veins & arteries, L. Common Carotid (LCC), L. Subclavian (LSA) arteries, trachea, esophagus.
• Aortic Arch Level:
  • Aortic arch, SVC, trachea, esophagus. Aortopulmonary (AP) window (nodes, ligamentum arteriosum).
• Main Pulmonary Artery (MPA)/Carinal Level:
  • MPA, Ascending & Descending Aorta, SVC, carina, R/L main bronchi.
• Cardiac Level:
  • RA, RV, LA, LV, proximal coronary arteries, pericardium.

Mediastinal lymph nodes: Short axis > 10mm is suspicious.

📌 Mnemonic for anterior mediastinal masses: "5 T's" - Thymoma, Teratoma/germ cell tumors, Thyroid (ectopic/goiter), Terrible Lymphoma, Thoracic Aorta (aneurysm; though often extends).

⭐ The aortopulmonary (AP) window, located between the aortic arch and pulmonary artery, is a crucial site for detecting metastatic lymph nodes, especially from left lung cancers.

CT Thorax: Lungs & Pleura - Lung Lobes Logic

• Lobar Anatomy:
  • Right: 3 lobes (RUL, RML, RLL).
  • Left: 2 lobes (LUL, LLL); Lingula (LUL) like RML.
• Fissures:
  • Major (Oblique): Bilateral; separate lower from upper/middle lobes.
  • Minor (Horizontal): Right only; separates RUL from RML. Often incomplete.
• Segmental Anatomy (Simplified) 📌
  • RUL: Apical, Posterior, Anterior (APA)
  • RML: Lateral, Medial (LM)
  • RLL: Superior, Anterior basal, Medial basal, Lateral basal, Posterior basal (SAML P)
  • LUL: Apico-posterior, Anterior; Lingula: Superior, Inferior (APASI)
  • LLL: Superior, Anteromedial basal, Lateral basal, Posterior basal (SALP)
• Bronchovascular Bundles:
  • Artery & bronchus central per segment (bundle).
  • Veins intersegmental, peripheral.
• Pleura:
  • Visceral (lung) & Parietal (chest). Thickness < 1-2mm.
  • Azygoesophageal recess: Posterior mediastinum, right of esophagus.

⭐ Right minor fissure often incomplete/absent (~20-50%); complicates RML pathology diagnosis (e.g., atelectasis).

High‑Yield Points - ⚡ Biggest Takeaways

• PA view is standard for CXR; AP view magnifies the heart.
• CT lung windows assess parenchyma; mediastinal windows show vessels and soft tissues.
• The silhouette sign localizes opacities by obscuring adjacent structures.
• Air bronchograms within an opacity suggest an intrapulmonary lesion.
• Costophrenic angle blunting on CXR indicates pleural effusion; lateral view is more sensitive.
• CT Angiography (CTA) is gold standard for pulmonary embolism.
• The left hilum is typically higher than the right hilum.