CT Head Scanning Indications

Computed axial tomography was first used at the Atkinson Morley Hospital in London in 1972. CT scans rapidly became the mainstay of the diagnosis of structural brain disease until the advent of magnetic resonance imaging during the late 1980s. However, CT remains an extremely valuable method of investigation, particularly in the acute situation and because of the very high expense of MRI systems. CT scans have a relatively high radiation dose so should be avoided in pregnancy. Claustrophobia is less of a problem compared to MRI.

• CT remains the investigation for the diagnosis and management of many central nervous system diseases.
• MRI is superior in the posterior fossa and parasellar region and for the assessment in multiple sclerosis, epilepsy and tumours.
• CT is superior to MRI in the assessment of head injury.
• Indications for CT imaging, CT angiography, and CT venography include:

Cranial

• Acute stroke:
  • CT scan as soon as possible and within 48 hours to rule out haemorrhage.
  • CT scanning is reliable in excluding primary intracerebral haemorrhage as a cause of acute stroke, provided it is performed within about a week of onset.
  • A CT scan is particularly important in those patients who are being evaluated for thrombolysis or carotid endarterectomy.
  • The CT signs of ischaemia are more subtle and detection of acute infarction is variable depending on the timing of the examination.
  • The area of infarction matures over the first week or more with the development of progressively better defined low attenuation and loss of volume in the damaged area.
• Transient ischaemic attack: can help to distinguish infarction from haemorrhage and also differentiate from other causes such as extracerebral haemorrhage or glioma.
• Acute subarachnoid haemorrhage: CT will provide evidence of subarachnoid haemorrhage in 98% of cases if performed within 48 hours. CT is indicated in acute headache with focal neurological signs, nausea, vomiting or Glasgow Coma Scale below 14. MRI is better for inflammatory causes of acute headache.
• Acute head injury: CT Head in every severe head injury (Glasgow Coma Scale less than or equal to 8) and every moderate head injury (Glasgow Coma Scale 9-12). The role of CT scanning in mild head injury is controversial.² NICE now recommends:³

  CT Scan in Head Injuries3
  Selection of adults for CT Scan	Selection of children (under 16 years) for CT Scan
  Urgent Scan if any of the following (results within 1 hour) GCS <13 when first assessed or GCS<15 two hours after injury Suspected open or depressed skull fracture Signs of base of skull fracture** Post-traumatic seizure Focal neurological deficit >1 episode of vomiting Coagulopathy + any amnesia or LOC since injury A CT scan is also recommended (within 8 hours of injury) if there is either: More than 30 minutes of amnesia of events before impact Or any amnesia or LOC since injury if Aged ≥65 years Coagulopathy or on warfarin Dangerous mechanism of injury RTA as pedestrian RTA - ejected from car Fall > 1m or >5 stairs	Urgent scan if any of the following: Witnessed loss of consciousness >5 minutes Amnesia (antegrate or retrograde) >5 minutes Abnormal drowsiness ≥3 Discrete episodes of vomiting Clinical suspicion of NAI Post-traumatic seizure (no PMH epilepsy) GCS <14 in emergency room (Paediatric GCS<15 if aged <1) Suspected open or depressed skull fracture or tense fontanelle Signs of base of skull fracture** Focal neurological deficit Aged <1 - bruise,swelling or laceration on head >5cm Dangerous mechanism of injury (high speed RTA, fall from >3m, high speed projectile).
  **Signs of basal skull fracture: haemotympanum, 'panda' eyes (bruising around the eyes), CSF leakage (ears or nose) or Battle's sign (bruising which sometimes occurs behind the ear in cases of basal skull fracture).

• Space occupying lesions: suspected tumour or mass, e.g. cerebral abscess. MRI is more sensitive for early tumours and posterior fossa lesions but CT is usually adequate for supratentorial lesions. MRI may miss calcification.
• Suspected hydrocephalus or shunt revision. MRI may be more appropriate for children. Ultrasound is first choice for infants.
• Chronic headache. CT or MRI are not usually useful if there are no focal neurological signs but are more likely to detect an abnormality if there is:
  • Recent onset and a progressive worsening of symptoms and frequency or a change in their pattern
  • Association with the onset of epilepsy (especially focal epilepsy)
  • Change in personality
  • Associated dizziness, lack of coordination, tingling or numbness
  • History of recent head injury, or falls (to exclude subdural haemorrhage)
• Intracranial infection: To exclude raised intracranial pressure prior to lumbar puncture (but only if there is considered to be a high risk of coning). In cases of childhood bacterial meningitis, CT is accurate in the diagnosis of intracranial complications of bacterial meningitis and is indicated mainly in children with persistent neurological dysfunction such as complex seizure disorder, and is of little value in children with prolonged fever alone.⁴
• Detection or evaluation of calcification: For example, the radiological hallmark of an oligodendroglioma is calcification, which is best detected on CT scanning. Calcification may be invisible on MRI.
• Other: mental status change, increased intracranial pressure, headache, acute neurological deficits, congenital lesions (e.g. craniosynostosis, macrocephaly, and microcephaly), evaluation of patients with psychiatric disorders and brain herniation. In the assessment of psychosis, CT scan should be reserved for those with recent onset, rapid unexplained deterioration, focal neurological signs, recent head injury before onset or if there has been urinary incontinence or gait disturbance early in the illness.
• Secondary indications (e.g. when access to MRI is not available): diplopia, cranial nerve dysfunction, seizures, apnoea, syncope, ataxia, suspicion of neurodegenerative disease, developmental delay, neuroendocrine dysfunction, encephalitis, vascular occlusive disease or vasculitis (including use of CT angiography and/or venography), aneurysm, cortical dysplasia, and migration anomalies.

Extracranial

• Middle or inner ear symptoms, including vertigo. If felt necessary following specialist assessment. MRI is much better, especially for acoustic neuromas.
• Sinus disease if there has been failure of maximal medical treatment, complications, e.g. orbital cellulitis or suspicion of malignancy.
• Congenital anomalies, benign and malignant neoplasms, trauma, vascular malformations, evaluation of palpable masses, planning and follow-up of radiotherapy.⁵
• Orbital lesions, including eye trauma in which there may be an associated facial fracture. Ultrasound may be appropriate for intraocular lesions. CT scan may also be indicated for strong suspicion of an intraocular foreign body that has not been shown on x-ray.
• Fractures of the temporal bone, skull, and face.
• Evaluation of the skull base including primary and secondary bone lesions.
• Cranio-maxillo-facial surgery: The CT scan delineates lesions in the oral and maxillofacial complex to aid in planning surgical treatment. CT-based 3D models allow precise preoperative diagnosis and operation planning.⁶
• Secondary indications (e.g. when access to MRI is not available): Evaluation of lesions involving the orbit, larynx, pharynx, oral cavity and soft tissue spaces of the face.