CT Anatomy

CT Anatomy - Scan Essentials & Brainy Views

• Hounsfield Units (HU): Quantitative scale for radiodensity.
  • Air: -1000 HU (black)
  • Fat: -100 to -50 HU (dark grey)
  • Water: 0 HU
  • CSF: +15 HU (darker than brain parenchyma)
  • Acute Blood: +50 to +75 HU (bright/hyperdense)
  • Bone: +700 (cancellous) to +3000 HU (cortical) (white/hyperdense)
  • Contrast Media: Significantly ↑ HU values.
• Windowing (Window Width/Window Level): Optimizes image contrast for specific tissues.
  • Brain Window (e.g., W:80 L:40): Differentiates grey/white matter, detects infarcts, hemorrhage.
  • Bone Window (e.g., W:2500 L:500): Visualizes fractures, bony lesions, cortical detail.
  • Lung Window (e.g., W:1500 L:-600): Assesses lung parenchyma, nodules, emphysema.
  • Soft Tissue/Abdomen Window (e.g., W:400 L:50): Views solid organs, fluid collections.
• Key Axial CT Brain Structures:
  • Ventricles: Lateral, third, fourth (CSF-filled, appear dark).
  • Basal Ganglia: Caudate nucleus, putamen, globus pallidus.
  • Thalamus.
  • Falx cerebri & Tentorium cerebelli (dural folds).
  • Major Lobes: Frontal, parietal, temporal, occipital (general identification).

⭐ Acute hemorrhage (e.g., intracranial bleed) typically appears hyperdense (bright) on non-contrast CT scans, with HU values around +50 to +75 HU due to clotted blood components.

CT Anatomy - Chest Checkpoints

• Lungs & Pleura:
  • Right Lung: 3 lobes (Superior, Middle, Inferior); 2 fissures (Oblique, Horizontal).
  • Left Lung: 2 lobes (Superior, Inferior); 1 fissure (Oblique). Lingula (LUL) analogous to RML.
  • Bronchopulmonary Segments: Functional units; typically 10 right, 8-10 left.
• Key Mediastinal Structures (Cranial to Caudal Scan):
  • Vessels: SVC, IVC, Aorta (ascending, arch with branches, descending), Pulmonary Artery (main, R/L branches).
  • Airways: Trachea (C-shaped rings) bifurcates at Carina (T4/T5 level) into Main Bronchi. 📌 R bronchus: Shorter, Wider, More Vertical ("Right, Robust, Reaches").
  • Digestive: Esophagus (posterior to trachea, anterior to vertebrae).
• Lymph Node Stations (IASLC):
  • Brief ID: Hilar, Paratracheal, Subcarinal, Aortopulmonary window.

⭐ The azygos vein arches over the right main bronchus to drain into the SVC, a key landmark visible on CT imaging near the carina an important checkpoint to identify right sided structures correctly and avoid misinterpretation with pathology like lymphadenopathy or mass.

CT Anatomy - Gut Guide & Organ Odyssey

• Solid Abdominal Organs:
  • Liver: RUQ; segments by portal/hepatic veins. Key for resection.
  • Gallbladder: Inferior to liver.
  • Spleen: LUQ, posterior to stomach.
  • Pancreas: Retroperitoneal; head in duodenal C-loop, body, tail to spleen. Uncinate process posterior to SMV.
  • Kidneys: Retroperitoneal, bean-shaped. Cortex, medulla, pelvis visible.
  • Adrenal Glands: Superior to kidneys, Y/V-shaped. Often subtle.
• Major Vessels:
  • Aorta: Anterior to spine; branches: Celiac, SMA, IMA, Renals.
  • IVC: Right of aorta.
  • Portal Vein: Posterior to pancreatic neck; SMV + Splenic Vein.
• GI Tract Basics:
  • Stomach: LUQ, J-shaped.
  • Small Bowel: Central; jejunum: feathery valvulae.
  • Colon: Peripheral, haustra.
• Pelvic Organs:
  • Bladder: Anterior.
  • Female: Uterus (pear), Ovaries (almond, lateral).
  • Male: Prostate (below bladder), Seminal Vesicles (posterior).

⭐ The left renal vein passes anterior to the aorta and posterior to the SMA (Nutcracker phenomenon risk).

CT Anatomy - Vascular Views & Trauma Traces

• CT Contrast Phases & Timing:
  • Non-Contrast (NCCT): Baseline; shows acute blood (hyperdense), calcification.
  • Arterial Phase (~20-30s post-IV contrast): Peak arterial opacification (e.g., CTA for aneurysms, dissection); hypervascular tumors.
  • Portovenous Phase (~60-70s): Optimal liver parenchyma & portal vein enhancement; best for hypovascular liver lesions.
  • Delayed Phase (~3-15 min): Evaluates contrast washout (e.g., RCC), fibrosis, collecting system/urothelial lesions.
• Key Vascular CTA Targets:
  • Circle of Willis: Aneurysms, stenosis, thrombosis.
  • Pulmonary Arteries: Pulmonary embolism (filling defects).
  • Aorta & Major Branches: Dissection, aneurysm, stenosis.
• Trauma CT (correlates with eFAST findings):
  • Detects free fluid (hemoperitoneum, hemothorax) & free air (pneumoperitoneum, pneumothorax).
• MSK CT Basics:
  • Defines cortical bone, medullary cavity; details complex fractures, joint integrity.

⭐ On non-contrast CT, acute hemorrhage typically appears hyperdense, measuring 30-60 HU (Hounsfield Units).

High‑Yield Points - ⚡ Biggest Takeaways

• Hounsfield Units (HU): Air (-1000), Water (0), Bone (+1000). Fat is negative, soft tissue positive.
• Windowing (Width/Level) is crucial for tissue differentiation (e.g., Lung, Bone, Soft tissue windows).
• IV contrast enhances vessels & pathology; note enhancement patterns.
• Orientation via axial, sagittal, coronal planes is fundamental.
• Recognize common artifacts: beam hardening, motion.
• Acute hemorrhage is hyperdense; infarcts/chronic bleeds are hypodense.
• Lung window for parenchyma; mediastinal window for soft tissues/vessels.