What is the step‑by‑step approach to interpreting a head computed tomography (CT) scan in a patient presenting with headache, neurologic deficits, or possible trauma, infection, or cancer?

Systematic Approach to Head CT Interpretation

Interpret head CT scans using a structured, anatomic checklist method that systematically evaluates blood, brain, cisterns, ventricles, and bone—this approach reduces missed diagnoses and ensures comprehensive assessment regardless of clinical presentation. 1

Initial Assessment Framework

When approaching any head CT, establish the clinical context first:

• Trauma patients: Prioritize identification of hemorrhage, skull fractures, mass effect, and herniation that require immediate neurosurgical intervention 1
• Headache presentations: Focus on acute hemorrhage (subarachnoid, intraparenchymal), mass lesions, hydrocephalus, and signs of increased intracranial pressure 1
• Neurologic deficits: Search for acute ischemia, hemorrhage, mass effect, and structural abnormalities explaining focal findings 1

Structured Interpretation Checklist

Blood (Hemorrhage Assessment)

Systematically evaluate all potential hemorrhage locations:

• Epidural hematoma: Biconvex, hyperdense collection that does not cross suture lines but can cross midline 1
• Subdural hematoma: Crescentic collection that crosses suture lines but not midline; may be acute (hyperdense), subacute (isodense), or chronic (hypodense) 1
• Subarachnoid hemorrhage: Hyperdensity in sulci, cisterns, or fissures; requires immediate attention for possible aneurysm 1
• Intraparenchymal hemorrhage: Note location, size, surrounding edema, and mass effect 1
• Intraventricular hemorrhage: Blood in ventricular system; assess for hydrocephalus 1

Brain Parenchyma

Evaluate gray-white matter differentiation and parenchymal abnormalities:

• Loss of gray-white differentiation: Early sign of ischemia or diffuse axonal injury 1
• Hypodense lesions: Consider acute infarction, edema, or chronic injury 1
• Hyperdense lesions: Acute hemorrhage, calcification, or hypercellular tumors 1
• Mass effect: Midline shift >5mm, sulcal effacement, ventricular compression 1

Cisterns and CSF Spaces

Assess basal cisterns and extra-axial spaces:

• Cisternal effacement: Indicates increased intracranial pressure or mass effect; evaluate suprasellar, ambient, and quadrigeminal cisterns 1
• Sulcal effacement: Suggests edema, mass effect, or increased intracranial pressure 1
• Extra-axial fluid collections: Distinguish subdural from subarachnoid collections 1

Ventricles

Evaluate ventricular size, symmetry, and contents:

• Hydrocephalus: Enlarged ventricles with periventricular edema (obstructive) or without edema (communicating) 1
• Asymmetry: Suggests mass effect or trapped ventricle 1
• Intraventricular blood: Note location and amount; assess for obstructive hydrocephalus 1

Bone and Soft Tissues

Systematically review skull and scalp:

• Skull fractures: Linear, depressed, or basilar; note if crossing vascular grooves (epidural hematoma risk) 1
• Basilar skull fracture signs: Pneumocephalus, air-fluid levels in sinuses, soft tissue swelling 1
• Scalp hematoma: May indicate underlying fracture or injury site 1
• Paranasal sinuses: Fluid levels, opacification, or fractures 1

Critical Findings Requiring Immediate Action

Identify and communicate these findings immediately to the clinical team:

• Acute hemorrhage with mass effect or midline shift >5mm 1
• Herniation (uncal, subfalcine, tonsillar) 1
• Depressed skull fracture with underlying brain injury 1
• Acute hydrocephalus 1
• Large acute infarction with mass effect 1

Common Pitfalls to Avoid

Avoid these frequent interpretation errors:

• Missing subtle subarachnoid hemorrhage: Always examine the sylvian fissures, interpeduncular cistern, and sulci at narrow window settings 1
• Overlooking small epidural hematomas: Carefully review the convexities and temporal regions where fractures cross vascular grooves 1
• Failing to assess for herniation: Always evaluate the position of the uncus, cingulate gyrus, and cerebellar tonsils 1
• Missing basilar skull fractures: Review bone windows systematically; look for indirect signs like pneumocephalus or sinus opacification 1
• Inadequate assessment of posterior fossa: CT has limited sensitivity for posterior fossa pathology; maintain high suspicion and consider MRI if clinical concern persists despite negative CT 1

Window Settings for Optimal Interpretation

Use appropriate window settings for different pathologies:

• Brain windows (width 80, level 40): Standard for parenchymal assessment 1
• Bone windows (width 2000-4000, level 300-700): Essential for fracture detection 1
• Subdural windows (width 200-300, level 70-100): Improves detection of isodense subdural hematomas 1

When CT is Insufficient

Recognize CT limitations and when MRI is necessary:

• Persistent unexplained neurologic deficits despite normal CT require MRI for detection of diffuse axonal injury, small contusions, or posterior fossa lesions 1
• CT detects only 10% of diffuse axonal injury because >80% of lesions lack macroscopic hemorrhage 1
• MRI is superior for detecting subtle findings adjacent to calvarium or skull base, small white matter lesions, and nonhemorrhagic contusions 1, 2
• Suspected vascular injury (dissection, pseudoaneurysm) requires CTA or MRA 1

Documentation and Communication

Structure your report to include:

• Technique: Contrast administration, slice thickness, reformations 3, 4
• Comparison: Prior studies if available 3, 4
• Findings: Systematic description using the checklist approach above 3, 4
• Impression: Clear statement of key findings, differential diagnosis, and management recommendations 3, 4
• Critical results: Direct communication for urgent findings requiring immediate intervention 3, 4