11/13/24, 7\:32 PM Guide | CT head interpretation

CT head interpretation

Table of contents
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Introduction

The CT head scan is one of the most common imaging studies you can be faced with and the most frequently requested by
the emergency department. This article will cover some of the underlying principles of CT head studies and discuss a method
for their interpretation.

Underlying principles

Computed tomography (CT) scanning involves the use of X-rays to take cross-sectional images of the body. This is possible as
di
much, whilst other tissues will exert a more signi
beam is described in terms of an attenuation coe
voxel of tissue (voxel = volumetric pixel).
These values are frequently expressed as Houns
HU, whereas air under the same conditions has -1000 HU. Approximate values for various tissues are outlined in table 1 (these
are not set in stone – only rough estimates).
Table 1 ¹
Tissue Houns
Air -1000
Water 0
Cerebrospinal
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White matter +20 to +30
Grey matter +37 to +45
Coagulated blood +50 to +75
Bone +200 to +3000

Windowing

This gives rise to a dilemma. An article published in 2007 concluded that although a human observer could distinguish
between up to 900 shades of grey, most scan viewing platforms show images in 256 shades ²
. If we are trying to visualise a
range of units from -1000 to +3000 in terms of 256 shades of grey, for every incremental change in the greyscale there will be a
di
problem is negotiated with windowing.
Windowing (also known as grey-level mapping) is the process of changing the location and width of the available greyscale in
order to optimise discrimination between tissues. This is best explained visually.
Below we can see a greyscale (from white to black) being assigned to the whole range of HU (from air to cortical bone). We can
imagine that this may not provide su
A greyscale assigned the whole range of HU
Changing the width of the greyscale
Here we have changed the width (w value) of the greyscale – we are now visualising 200 HU in 256 shades. This gives us a
much better contrast between CSF, brain matter and blood. However, everything above blood will appear as white and
everything below CSF will appear as black.
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200HU in 256 shades
Changing the centre of the greyscale
Now we have changed the centre (c or l value) of the greyscale – we are getting the same contrast but at a di
Houns
An example of changing the centre of the greyscale
This business of windowing may seem unnecessary to discuss. However, almost everyone will
windowing on a scan at some point. Hopefully, some understanding of what this is actually doing will help you achieve the best
contrast in an image.
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Con

As with the interpretation of all studies, the
Check the following details\:
Patient name, hospital number and date of birth
Date and time the scan was acquired
Previous scans (if available) for comparison
The appearance of tissues on a CT scan is described in terms of ‘density’
. Darker structures are ‘hypodense or low density’
;
brighter structures are ‘hyperdense or high density’
.
Blood Can Be Very Bad is a mnemonic that can be used when faced with interpreting a CT head scan\:
Blood
Cisterns
Brain
Ventricles
Bone
Think of this approach as a framework for a quick review of a scan – it won’t turn you into an experienced radiologist! It’s
important to recognise that more subtle signs might still be overlooked. Furthermore, you should work through the entire
system even if you spot something obvious early on (e.g. if you see a large extradural haematoma, still check the cisterns, brain,
ventricles and bone for any other abnormalities).

Blood

Inspect for evidence of bleeding which may include\:
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).
Bear in mind that blood will have varying appearances depending on the age of the collection, with a more acute haematoma
appearing hyperdense compared to a chronic bleed. Some bleeds may also be very subtle and di
closely and this is one of the reasons why windowing is so important.

Extradural haematoma

An extradural haematoma is a collection of blood which forms between the dura mater and skull (they can also occur in the
spine although this is much rarer). 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. Extradural haematomas
need to be identi
brain tissue. As a result, intracranial pressure can rise rapidly and without prompt evacuation of the haematoma, brainstem
herniation can occur.
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Extradural haematoma [3]

Subdural haematoma

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.
Subdural haematoma [4]

Subarachnoid haemorrhage

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).
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Subarachnoid haemorrhage [5]

Intracerebral haemorrhage

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).
Intracerebral haemorrhage (intraventricular and intraparenchymal)

Cisterns

There are four key cisterns that which should be assessed for e
Ambient cistern\: surrounding the midbrain.
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Suprasellar cistern\: superior to the sella turcica.
Quadrigeminal cistern\: adjacent to the corpora quadrigemina.
Sylvian cistern\: across the insular surface and within the Sylvian
An example of some of the subarachnoid cisterns made more visible due to the presence of blood from subarachnoid haemorrhage [5]

Brain

Sulcal e

Sulcal e
associated with raised intracranial pressure.

Grey-white matter di

On a normal CT head scan, the grey and white matter should be clearly di
presence of oedema which may develop secondary to a hypoxic brain injury, infarction (e.g. ischaemic stroke), tumour or
cerebral abscess.
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Hypoxic brain injury [6]

Abnormal shifts of brain tissue

Look for abnormal shifts of brain tissue and/or herniation\:
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

Hypo/hyperdense foci

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.
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Pneumocephalus [7]
Hyperdense foci
Hyperdensity on a CT head may be due to the presence of blood, thrombus or calci
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.
Hyperdense right middle cerebral artery (MCA) [8]

Tumour

Radiological features
The radiological features of a tumour will vary depending on the histological diagnosis.
Any of the following may be noted in our around a tumour\:
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Surrounding haemorrhage\: may be hyperdense, isodense or hypodense depending on the maturity of the bleed.
Calci
Mass e
Oedema (hypodense)\: may be present in the brain tissue surrounding the tumour.
Contrast administration
Following intravenous administration of a contrast medium, lesions may show no change, or demonstrate some form of
contrast enhancement (e.g. homogenous enhancement, ring enhancement etc)\:
Homogenous enhancement occurs in a number of lesions including meningiomas and highly vascular tumours.
Ring-enhancement is typically associated with cerebral abscesses and some types of cerebral metastases (e.g. melanoma).
Cerebral metastases (before and after administration of contrast) [9]

Ventricles

Intraventricular haemorrhage and the choroid plexus

Intraventricular haemorrhage appears on a CT head as hyperdensity within the ventricular system.
However, not all hyperdensity in the ventricles represents acute bleeding\: the choroid plexus is frequently calci
appears bright on CT. Remember that blood is
high-density signal within the lateral walls of the ventricles it is likely to represent the choroid plexus.
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Intracerebral haemorrhage (intraventricular and intraparenchymal)

Hydrocephalus

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.
Hydrocephalus\: enlarged ventricles (ventriculomegaly) [11]

Ventricular e

Ventricular e
secondary to a mass or an intracranial haemorrhage. The shift in CSF that occurs in these cases follows the Monro-Kellie
doctrine.
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Ventricular e
Monro-Kellie doctrine
The cranium, enclosing the brain, forms a
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
Source\: geekymedics.com
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