Synopsis
Use non-contrast head CT safely in acute neurological presentations, recognise major patterns and limitations, and continue urgent diagnostic pathways when a negative scan cannot exclude the suspected disease.
- Non-contrast CT is fast and excellent for acute haemorrhage, hydrocephalus, skull injury and substantial mass effect; it is less sensitive for early ischaemia and some posterior-fossa lesions.
- Review systematically: technical adequacy, extra-axial spaces, cisterns and sulci, ventricles, parenchymal symmetry and grey–white differentiation, posterior fossa, skull and extracranial tissues.
- Fresh blood is usually hyperattenuating, but anaemia, very small volume, lesion age and artefact can alter conspicuity; describe compartment and mass effect before inferring cause.
Key red flags
A fall in GCS, progressive focal deficit, new seizure or pupil asymmetry requires immediate reassessment and urgent imaging or repeat imaging despite an earlier reassuring scan.
Thunderclap headache with neck stiffness, vomiting, collapse or exertional onset remains concerning for subarachnoid haemorrhage even with a normal examination.
Anticoagulant exposure, bleeding tendency and recent trauma lower the threshold for urgent CT but do not identify the lesion or replace drug-specific haemostasis planning.
A normal early CT does not exclude acute ischaemia; for suspected SAH, a negative radiologist-reported CT within six hours may avoid routine LP, while a later negative scan can require LP at least 12 hours after onset.
Posterior-fossa and skull-base disease may be obscured by beam-hardening artefact; persistent brainstem or cerebellar signs need a more sensitive pathway.
Search sulci, basal cisterns, fissures, ventricles, brain parenchyma and extra-axial spaces. State side, compartment, volume impression, ventricular extension and associated oedema or shift.
Compare hemispheres for grey–white loss, focal hypoattenuation, sulcal effacement and a dense artery. Small or very early infarcts may remain invisible, especially in the posterior fossa.
Assess sulci, ventricles, basal cisterns, midline position and herniation corridors. Millimetres of shift alone do not determine clinical severity; lesion location and trajectory matter.
Define the disease, timing and image quality before calling CT reassuring. A negative result is meaningful only for the question and population in which its performance is known.
Investigation priorities
Identify acute blood, major mass effect, hydrocephalus, fracture and established infarction rapidly.
Management branches
An adult presents after sudden maximal-at-onset headache and initial non-contrast CT shows no blood.
- Confirm exact onset, neurological state, scan timing and technical adequacy, while maintaining urgent SAH assessment.
- If CT was obtained and radiologist-reported within six hours, use the NG228 pathway rather than adding routine LP automatically.