CT Head
Computed tomography (CT) of the head uses an X-ray tube and detector array rotating around the patient to produce axial images of the brain, ventricles, skull, scalp and facial structures in a few seconds.
Multidetector scanners acquire a volume of isotropic data in a single rotation (spiral scanning), with multiplanar (coronal and sagittal) reconstruction, bone and soft tissue reconstruction algorithms, and optional three dimensional rendering.
Non-contrast CT head is the first line imaging test in emergency neurology: it takes 5 to 10 minutes for the whole visit (the scan itself is under a minute), and shows acute haemorrhage (blood is hyperdense), mass effect, hydrocephalus, fractures, and early signs of infarction.
Intravenous iodinated contrast is added for suspected tumour, abscess, infection, vascular malformation and venous thrombosis, and may be combined with CT angiography and CT venography.
Typical effective dose is 1.5 to 2.5 mSv for a routine adult head scan (higher for contrast enhanced and perfusion studies). Doses are adjusted for children (age based settings, with an effective dose 1 to 3 times that of adults depending on settings, and tube current modulation).
CT head is among the most frequently performed imaging examinations. Traumatic brain injury affects about 50 to 60 million people each year worldwide, and stroke affects about 12 million people.
CT detects acute intracranial haemorrhage with a sensitivity of over 95% in the first 6 hours (hyperacute and acute blood is bright), identifies the 80 to 85% of strokes caused by infarction by excluding haemorrhage, and shows skull fractures.
Sensitivity for acute ischaemic stroke is low in the first hours (about 40 to 60% for early signs in the first 3 hours. 70 to 80% by 24 hours) and for small posterior fossa infarcts, so MR is more sensitive.
For subarachnoid haemorrhage CT sensitivity is about 95 to 98% within 6 hours of onset (when interpreted by a neuroradiologist on a modern scanner), falling to 85% at 24 hours and 50% at 5 days.
Indications. Head injury with reduced consciousness, post-traumatic amnesia, vomiting, focal deficit, seizure, skull fracture signs, or high risk features (age over 65, anticoagulant use, dangerous mechanism). Suspected stroke, transient ischaemic attack (to exclude haemorrhage before thrombolysis or thrombectomy, together with CT angiography and perfusion). Sudden severe headache (thunderclap, suspected subarachnoid haemorrhage).
Acute confusion, coma of unknown cause, and new focal neurological deficit. Suspected raised intracranial pressure, hydrocephalus (and shunt malfunction), mass lesion, brain abscess, cerebral venous thrombosis, and ventriculitis (with contrast). Seizure (first seizure, new focal onset), brain tumour staging, and follow up of known lesions.
Suspected congenital anomaly, craniosynostosis (three dimensional CT), and skull base disease. Post-operative neurosurgical assessment. Evaluation of brain death (with clinical tests).
Pre-operative planning and image directed surgery. Not for routine investigation of uncomplicated tension headache or migraine without red flags, or mild head injury that meets low risk criteria.
Contraindications and precautions. No absolute contraindications. Pregnancy: avoid unless essential; if needed, CT head can be done with abdominal shielding unnecessary (the dose to the foetus is negligible with a head scan), as the benefit of diagnosis outweighs risk.
Children: scan only if clinically justified, with dose optimised settings, and with preference for MR or observation in minor head injury that meets low risk criteria.
For contrast:
- previous severe reaction to iodinated contrast (premedication or alternative)
- renal impairment (estimated glomerular filtration rate under 30 mL each minute each 1.73 square metres: weigh risk and benefit and hydrate; contrast is not withheld in an emergency)
- metformin (follow local advice depending on renal function)
- thyroid disease (usually no action; hyperthyroidism with caution)
- myasthenia gravis
- extravasation risk
Non-cooperative or agitated patients may need sedation, with airway monitoring. Immobilisation and cervical spine protection are maintained in trauma. Metal in the head (aneurysm clips, shunts, dental implants) causes artefacts but does not preclude CT.
Preparation and technique. No fasting is needed for non-contrast CT (4 to 6 hours of fasting for contrast, depending on local practice). The patient lies supine, with the head positioned in a head holder, the gantry angled or the head tilted parallel to the orbitomeatal line to avoid scanning through the lens.
A scout image sets the field of view.
Scanning is from the skull base to the vertex, in axial slices of 2.5 to 5 mm for review (thin 0.5 to 1.25 mm for reconstructions).
A soft tissue algorithm is used for brain (window width about 80 HU, level about 40 HU), and a bone algorithm (width 2000 to 4000 HU, level 500 to 700 HU).
Subdural windows (width 150 to 250 HU) improve detection of small extra-axial collections, and stroke windows (narrow, width 30 to 40 HU) enhance subtle early infarcts.
Contrast: 50 to 100 mL of non-ionic iodinated contrast intravenously, with scanning after a delay of 60 to 90 seconds (angiography: bolus tracking in the arterial phase; venography: 40 to 60 seconds delay). Dose is minimised by the lowest tube current and voltage that give adequate images, with iterative reconstruction.
