Can you teach me how to read a non‑contrast head CT?

How to Read a Non-Contrast Head CT (NCCT)

Use a systematic, anatomy-based search pattern that covers all visible structures—not just the brain parenchyma—to minimize diagnostic errors and improve detection of subtle but critical pathologies 1.

Core Interpretation Framework

The most effective approach involves a disciplined, compartmentalized review that extends beyond obvious abnormalities. Here's the algorithmic structure:

1. Start with Technical Assessment

• Window settings matter: Review images in both brain windows (to assess parenchyma) and bone windows (to detect fractures)
• Ensure adequate image quality and coverage from skull base to vertex
• Note the timing of the scan relative to symptom onset, as this affects what you can detect

2. Systematic Anatomical Search Pattern

Work through these compartments sequentially 1, 2:

A. Intra-axial (Brain Parenchyma)

• Cerebral hemispheres: Look for areas of abnormal density
  • Hyperdense = acute blood, calcification, or hypercellular lesions
  • Hypodense = edema, infarction, or chronic injury
  • Loss of gray-white differentiation suggests early ischemia
• Deep gray nuclei (basal ganglia, thalamus): Particularly important for prognosis—infarcts here predict worse outcomes 3
• Brainstem and cerebellum: Critical areas where subtle findings can be life-threatening
• Periventricular regions: Check for hydrocephalus or periventricular white matter changes

B. Extra-axial Spaces

• Subdural space: Look for crescent-shaped collections along the convexity
• Subarachnoid space: Blood appears hyperdense in sulci and cisterns
  • Intraventricular hemorrhage is a strong predictor of poor outcome 3
• Epidural space: Biconvex collections that don't cross suture lines

C. Vascular Structures 4

Examine vessels even without contrast:

• Hyperdense vessel sign: Suggests acute thrombus (e.g., hyperdense MCA = acute stroke)
• Vessel size asymmetry: May indicate stenosis or occlusion
• Filling defects: Can be visible in major sinuses suggesting thrombosis
• Check for vascular calcifications that might indicate atherosclerosis

D. Ventricular System

• Size and symmetry of lateral, third, and fourth ventricles
• Midline shift >5mm suggests mass effect requiring urgent intervention
• Effacement of sulci and cisterns indicates increased intracranial pressure

E. Skull and Extracranial Structures

• Bone windows are mandatory: Fractures are easily missed on brain windows alone 5
• Paranasal sinuses: Opacification may indicate trauma or infection
• Mastoid air cells: Fluid suggests skull base fracture
• Soft tissues: Scalp hematomas, foreign bodies

3. Context-Specific Interpretation

For Trauma 5:

• NCCT is the gold standard initial study for acute head trauma (GCS 9-15 when imaging indicated)
• Look specifically for:
  • Hemorrhage (epidural, subdural, subarachnoid, intraparenchymal)
  • Skull fractures (require bone windows)
  • Pneumocephalus
  • Midline shift
  • Cisternal effacement
• Critical pitfall: Small subdural hematomas can be isodense to brain in subacute phase (7-21 days)—look for sulcal effacement and mass effect

For Stroke 4:

• Early signs within 3-6 hours:
  • Loss of gray-white differentiation (insular ribbon sign)
  • Hyperdense vessel sign (acute thrombus)
  • Sulcal effacement
  • Subtle hypoattenuation in vascular territory
• Critical pitfall: Normal NCCT does NOT exclude acute ischemic stroke in the hyperacute phase

For Hemorrhage Expansion Risk 6:

• Irregular hematoma shape
• Heterogeneous density (swirl sign, black hole sign)
• Fluid levels within hematoma
• These predict expansion and guide management decisions

4. Common Diagnostic Pitfalls to Avoid

• Don't stop at the obvious finding: A large subdural may distract from a small epidural or skull fracture
• Always use bone windows: Up to 50% of fractures are missed on brain windows alone
• Check all visible structures: Incidental findings in sinuses, orbits, or soft tissues may be clinically significant
• Beware of beam-hardening artifacts: Posterior fossa is particularly prone to artifacts that can obscure pathology
• Consider timing: Subacute blood (7-21 days) can be isodense to brain and easily missed

5. When NCCT is Insufficient

Know when to escalate 5:

• Order CTA if you suspect vascular injury (penetrating trauma, skull base fracture, unexplained neurologic deficit)
• Order MRI for subacute/chronic trauma with unexplained deficits, or when NCCT is negative but clinical suspicion remains high
• Repeat NCCT for new neurologic deficits or clinical deterioration

Technical Optimization

Adjust your viewing approach 1:

• Scroll through thin slices, not just thick reconstructions
• Use multiple window settings (brain, subdural, bone)
• Magnify areas of concern
• Compare side-to-side for asymmetry
• Correlate with prior imaging when available

The key to mastery is developing a consistent search pattern that you execute every time, regardless of the clinical indication—this prevents satisfaction of search errors where you stop looking after finding one abnormality.