The most common reasons for performing CT head is after head injury or a suspected stroke. There are NICE guidelines on the indications and urgency for imaging in these cases.
Trauma is one of the most common presentations in A&E.
The main worry with head injuries is a bleed in one of several areas:
Fractures, particularly involving the skull base, are also a concern. CT head (without contrast) is the preferred modality. Acute bleeds should be discussed with neurosurgery.
There may also be an associated neck injury, in particular a cervical spine fracture, which may be imaged by either plain films or CT.
The purpose of urgent CT imaging in suspected stroke is mainly to check whether there is an intracranial bleed or an alternative cause for the symptoms such as a tumour.
- If there is a bleed, the patient can be discussed with neurosurgery for possible urgent operative management.
- If there is no bleed, the patient can be thrombolysed (if indicated).
Plain CT heads may look normal very early in an ischaemic stroke, as areas of ischaemia do not appear hypodense area until hours later. MRI brain, particularly the diffusion-weighted (DWI) sequence, can detect an ischaemic stroke earlier and shows it more clearly than CT.
The other major reason to perform neuroimaging is to look for a mass lesion, particularly tumours. Brain tumours are exceedingly rare, and when they occur, are much more likely to be secondary metastases than a primary lesion.
The total volume of brain matter + CSF + intracranial blood must be a constant. If any one component increases, the others must decrease.
i.e. If there is a large haematoma or brain tumour within the skull, the CSF must be pushed out to compensate, causing effacement (compression) of ventricles. However, sometimes the pressure on the brainstem may cause blockage to CSF flow and a hydrocephalus.
Suspected spinal cord compression is investigated with MRI spine.
Cauda equina syndrome is particularly concerning, as there is a risk of permanent disability if not treated urgently.
For paraspinal pathology, MRI delineates soft tissues well, which is particularly helpful in looking at collections and abscesses.
CT of the spine is used mainly to look at the bones – for example, vertebral collapse, crush fractures, or lytic lesions. MRI can also detect bony injuries well, but CT is the preferred modality in this setting.