How would you structure a CT head interpretation?
- Confirm details
- Blood
- Cisterns
- Brain
- Ventricles
- Bone
How does a CT work?
Demonstrates the extent to which a material can be penetrated by an X-ray beam
Distilled water at standard temperature and pressure has 0 Hounsfield units HU, whereas air under the same conditions has -1000 HU.
What would you check about patient details?
Patient name, hospital number and date of birth
Date and time the scan was acquired
Previous scans (if available) for comparison
What would you look for when looking for blood?
- Extradural haematoma (extra-axial)
- Subdural haematoma (extra-axial)
- Subarachnoid haemorrhage (SAH): may be very subtle. Remember a SAH can extend into the ventricular system so always look at the posterior horns as blood may collect in the dependant portion.
- Intracerebral haemorrhage (intra-axial): this may be intraventricular (within the ventricles) and/or intraparenchymal (within the brain tissue).
How would you spot an extradural haematoma on a CT?
An extradural haematoma is a collection of blood which forms between the dura mater and skull
Extradural haemorrhage is often preceded by a clear history of trauma, therefore you should look carefully for evidence of an associated fracture.
The majority of cases of extradural haematoma result from trauma to the middle meningeal artery
Intracranial pressure can rise rapidly and without prompt evacuation of the haematoma, brainstem herniation can occur.
How would you spot a subdural haematoma on a CT?
A subdural haematoma forms between the dura and the arachnoid mater and typically develops secondary to trauma (as a result of tearing of bridging veins).
In elderly patients who have experienced a fall, the inciting traumatic event may be less obvious.
How would you spot a subarachnoid haematoma on a CT?
A subarachnoid haemorrhage involves bleeding into the subarachnoid space (between the arachnoid and pia mater).
This space normally contains CSF and the vasculature of the brain.
The most common cause of subarachnoid haemorrhage is trauma, however, they can also develop spontaneously (e.g. aneurysmal rupture).
How would you spot an intrracerebral haematoma on a CT?
Intracerebral haemorrhage involves bleeding within the brain secondary to a ruptured blood vessel. Intracerebral haemorrhages can be intraparenchymal (within the brain tissue) and/or intraventricular (within the ventricles).
How would you assess the cisterns?
There are four key cisterns that which should be assessed for effacement, the presence of blood and asymmetry:
- Ambient cistern: surrounding the midbrain.
- Suprasellar cistern: superior to the sella turcica.
- Quadrigeminal cistern: adjacent to the corpora quadrigemina.
- Sylvian cistern: across the insular surface and within the Sylvian fissure.
What would you say about the brain?
- Sulcal effacement
- Grey-white matter differentiation
- Abnormal shifts of brain tissue
- Hypo/hyper-dense foci
- Tumours
What is sulcal effacement?
Sulcal effacement is the term used to describe the loss of the normal gyral-sulcal pattern of the brain, which is typically associated with raised intracranial pressure.
What causes poor differentiation between grey and white matter?
Suggests the presence of oedema which may develop secondary to a hypoxic brain injury, infarction (e.g. ischaemic stroke), tumour or cerebral abscess.
What are common abnormal shifts in brain tissue?
- Subfalcine: beneath the falx cerebri
- Uncal: inferomedial displacement of the uncus
- Transcalvarial: brain shift through the calvarium
- Transtentorial: may be superior or inferior
- Tonsillar: downward displacement of the cerebellar tonsils into the foramen magnum
What is a hypodense foci?
Hypodensity on a CT head may be due to the presence of air, oedema or fat:
- Oedema is often seen surrounding intracerebral bleeds, tumours and abscesses.
- Pneumocephalus (air within the cranial vault) may be noted after neurosurgery or adjacent to the inner table in cases of calvarial fractures.
What is a hyperdense foci?
Hyperdensity on a CT head may be due to the presence of blood, thrombus or calcification:
A hyperdense middle cerebral artery (MCA) is sometimes noted in total anterior circulation strokes (TACS) and indicates the presence of a large thrombus within the vessel.
What are the radiological features of a tumour?
- Surrounding haemorrhage: may be hyperdense, isodense or hypodense depending on the maturity of the bleed.
- Calcification: hyperdense on CT and typically associated with meningiomas.
- Mass effect: displacement of tissue due to the tumour or associated bleeding/oedema.
- Oedema (hypodense): may be present in the brain tissue surrounding the tumour.
What would you say about the ventricles?
- Intraventricular haemorrhage and the choroid plexus
- Hydrocephalus
- Ventricular effacement
- Other intraventricular pathology
How does intraventricular haemorrhage present?
Hyperdensity within the ventricular system.
However, not all hyperdensity in the ventricles represents acute bleeding: the choroid plexus is frequently calcified and often appears bright on CT
How would you spot hyddrocephalus on a CT?
Hydrocephalus is a term that describes the abnormal accumulation of CSF in the ventricles of the brain. It can be broadly divided into communicating (i.e. non-obstructive) and non-communicating (i.e. obstructive). An early sign of hydrocephalus on a CT head is dilation of the temporal horns.
What does ventricular effacement describe?
Ventricular effacement describes a thinning in the appearance of the ventricles. This may result from cerebral oedema secondary to a mass or an intracranial haemorrhage. The shift in CSF that occurs in these cases follows the Monro-Kellie doctrine.
What is the Monro-Kellie doctrine?
The cranium, enclosing the brain, forms a fixed space comprising three components: blood, cerebrospinal fluid, and brain tissue.
These components remain in a state of dynamic equilibrium, therefore any increase in any one of them results in a compensatory decrease of the other two.
Once the other compartments have reached their point of maximum compensation, any further increase in the size of one results in increased intracranial pressure.
What are other intraventricular pathologies?
Cysts
Tumours
Infective lesions
What would you say about the bone?
Assess the bones of the skull using the appropriate windowing.
Look for fractures of the calvarium and skull base. Subtle areas of low density within the inner table of the skull may represent small locules of air in the soft tissue windows. Careful evaluation to look for subtle fractures here is essential.
Superficial soft tissue injury may be associated with underlying fractures.
