Computed Tomography of the Brain

Tuesday, September 25, 2018

CT Scan of the Brain

Indications



CT is the imaging modality most commonly used in triaging acute neurological disease. For non emergency indications CT is second best to MR, but is still widely used, often because it is more broadly available and simpler to interpret. The indications include the following:

Following major head injury (if the patient has lost consciousness, has impaired consciousness, or has a neurological deficit). The presence of a skull fracture also justifies the use of CT. NICE (National Institute for Health and Care Excellence) guidance has been issued on the use of imaging for the head injuries for adults and children, specifically CT, listing the criteria for assessment based on best relevant data and consensus recommendations.

In suspected intracranial infection ( the use of contrast enhancement is recommended).

  • For suspected intracranial haemorrhage and cases of ischaemic and haemorrhagic stroke.
  • In suspected raised intracranial pressure, and as a precaution before lumbar puncture once certain criteria are fulfilled. These would include reduced consciousness ( a Glasgow coma score of less than 15), definite papilloedema, focal neurological deficit, immune suppression and bleeding dyscrasia.
  • In other situations, such as epilepsy, migraine, suspected tumour, demyelination, dementia and psyschosis, CT is a poorer quality tool. If imaging can be justified, MRI is greatly preferable and is recommended by NICE in these situation except for the first episode of psychosis.

Brain CT scan Technique

  1. Most clinical indications are adequately covered by 3 mm sections parallel to the floor of the anterior cranial fossa, from the foramen magnum to the midbrain, with 7 mm sections to the vertex (or contigious 3 mm slices throughout). In all trauma cases, window width and level should be adjusted to examine bone and any haemorrhagic, space occupying lesions. Review of all trauma studies should be done on brain windows, bone and blood windows.
  2. In suspected infection, tumours, vascular malformations and subacute infactions, the sections should be repeated following intravenous (i.v) contrast enhancement, if MRI is not available. Standard precautions with regard to possible adverse reactions to contrast medium should be taken.
  3. Dynamic studies using iodinated contrast are increasingly being used as a routine in high velocity head trauma, the assessment of ruptured arteriovenous shunts and dural venous sinus thrombosis. CT angiography (CTA) on a typical 64 slice multidetector scanner is performed using 70-100 ml of contrast and 50 ml saline chaser, injected at 4 ml/s with a delay of 15s or triggered by bolus tracking with ROI in the aortic arch. Overlapping slices of 0.75 – 1.25 mm are reconstructed. CT venography (CVT) involves injecting 90-100 ml of contrast with a delay of 40s. Images are usually reviewed both as three dimensional rendered data and multiplanar reformats (MPRs).

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