The CT head scan is one of the most common imaging studies that you can be faced with and the most frequently requested by A&E. This article will cover some of the underlying principles of CT head studies, and discuss a method for their interpretation.
Underlying principles and terminology
Computed tomography (CT) scanning involves the use of x-rays to take cross-sectional images of the body. This is possible as different tissues interact with X-rays in different ways. Some tissues will allow the passage of these X-rays without influencing it much, whilst other tissues will exert a more significant effect. The extent to which a material can be penetrated by an x-ray beam is described in terms of an attenuation coefficient which assesses how much a beam is weakened by passing through a voxel of tissue (voxel = volumetric pixel).
These values are frequently expressed as Hounsfield Units (HU). Distilled water at standard temperature and pressure has 0 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).
|White matter||+20 to +30|
|Grey matter||+37 to +45|
|Coagulated blood||+50 to +75|
|Bone||+200 to +3000|
Table 1 ¹
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 viewing 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 difference of approximately 15 HU. In short, there will not be enough contrast to reliably discern between structures. This 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 sufficient contrast to differentiate between grey and white matter, and coagulated blood.
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.
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 different range of Hounsfield units. This process of changing the centre and width of the greyscale is windowing.
This business of windowing may seem unnecessary to discuss. However, almost everyone will find themselves fiddling with the 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. Windowing can differentiate blood from calcification which can be very important in the management of such patients.
As with the interpretation of all studies, the first step is to confirm you have the correct patient and scan.
Check the following:
- Patient name, hospital number and date of birth
- Date and time the scan was acquired
- Always look at previous scans so that you have something to compare to
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 a CT head scan. 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).
Check for evidence of:
- Extradural haematoma (Extra-axial)
- Subdural haematoma (Extra-axial)
- Subarachnoid haemorrhage – 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.
- Intraventricular haemorrhage
- Intraparenchymal haemorrhage (Intra-axial)
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 difficult to spot unless you look closely and this is one of the reasons why windowing is so important.
An extradural haematoma is a collection of blood which forms between the dura mater and skull (they can occur in the spine although this is much rarer). Extradural haemorrhage is often preceded by a clear history of trauma and looking for a fracture is important as they can be extremely subtle.
The majority of cases result from trauma to the middle meningeal artery. These types of bleed are extremely important as they cannot cross skull sutures and hence they can compress the brain rapidly and without prompt evacuation of the haematoma, these patients can herniate their brainstems.
A subdural haematoma forms between the dura and the arachnoid mater. These also tend to be associated with a history of trauma. However, in elderly patients who have experienced a fall, the inciting traumatic event may be less obvious. These types of bleeds are normally due to damage to the bridging veins.
Subarachnoid haemorrhage is a bleed 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 may also be spontaneous (typically aneurysms).
Intracerebral haemorrhage describes a bleed within the parenchyma of the cerebrum. Intraparenchymal haemorrhage can be used to describe a bleed within the tissue of other areas of the brain (e.g. pons, cerebellum etc.).
There are four key cisterns that may be checked for effacement, blood and asymmetry:
- Ambient – surrounding the midbrain
- Suprasellar – superior to the sella turcica
- Quadrigeminal – adjacent to the corpora quadrigemina
- Sylvian – across the insular surface and within the Sylvian fissure
Check for sulcal effacement – this is when the normal gyral-sulcal pattern is no longer clearly visible, and is a sign of raised intracranial pressure
Assess the grey-white matter differentiation – loss of the insular ribbon sign
- The insular ribbon is the pattern of normal grey-white matter differentiation seen at the insular cortex. Although this sign is documented in literature, it isn’t always seen in practice and shouldn’t be relied upon heavily.
- Loss of this normal grey-white differentiation may be an early sign of MCA infarction
Abnormal shifts of brain tissue
Look for abnormal shifts of brain tissue and 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
Hypodensity may be due to air, oedema or fat:
- Oedema is often seen surrounding intracerebral bleeds and tumours
- Pneumocephalus (air within the cranial vault) may be seen after neurosurgery or adjacent to the inner table in cases of calvarial fractures.
Hyperdensity may be due to acute blood, thrombus or calcification:
- A hyperdense middle cerebral artery (MCA) is sometimes seen in total anterior circulation strokes (TACS) and indicates the presence of a large thrombus within the vessel
Features will vary depending on the histological diagnosis.
- Surrounding haemorrhage (may be hyperdense, isodense or hypodense depending on maturity of the bleed)
- Calcification (hyperdense) – often seen in meningiomas and along the falx
- Mass effect (shift of tissue) – due to tumour or surrounding bleeding/oedema
- Oedema (hypodense) – often surrounding bleeding and tumours
- Following IV 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 is demonstrated by a number of lesions including meningiomas and highly vascular tumours.
- Ring-enhancement is seen in certain conditions including Metastasis, Abscess, Glioblastoma, Infection, Contusion, Demyelination and Radiation necrosis. MAGIC DR (mnemonic)
- Please note: radiological appearances of lesions can vary dramatically
Intraventricular haemorrhage and the choroid plexus
- Intraventricular haemorrhage appears as hyperdensity within the ventricular system
- Not all hyperdensity in the ventricles represents acute blood – the choroid plexus is frequently calcified and often appears bright on CT. Remember blood is fluid and hence will be dependent within the ventricles. If you see high density within the lateral walls of the ventricles this is likely to represent the choroid plexus.
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. no obstruction) and non-communicating (i.e. an obstruction is present). The earliest sign of hydrocephalus is to look at the temporal horns.
Ventricular effacement describes the 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.
- Intracranial volume is occupied by brain, CSF and blood
- These contents are incompressible and the volume is fixed
- An increase in the volume of one compartment (e.g. brain – cerebral oedema) must be accommodated for by a decrease in the other compartments (e.g. CSF – ventricular effacement)
- Past this point of compensation a further increase in volume of one compartment will result in an increased intracranial pressure (ICP)
Other intraventricular pathology
- Infective lesions
Assess the bone using the appropriate windows.
Look for fractures of the calvarium and skull base. Subtle areas of low density within the inner table of the skull may represent small locals 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.
CONTENT REVIEWED BY
Dr Kunal Patel
1. Hounsfield Scale. Available from: https://en.wikipedia.org/wiki/Hounsfield_scale
2. Kimpe T, Tuytschaever T. Increasing the Number of Gray Shades in Medical Display Systems—How Much is Enough?. Journal of Digital Imaging 2007;20(4):422-432.
3. Hyperdense MCA. By James Heilman, MD (Own work) [CC BY-SA 4.0], via Wikimedia Commons
4. Subdural haemorrhage. By James Heilman, MD (Own work) [CC BY-SA 3.0], via Wikimedia Commons
5. Extradural haemorrhage. By Hellerhoff (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
6. Subarachnoid haemorrhage. By James Heilman, MD (Own work) [CC BY-SA 4.0], via Wikimedia Commons
7. Meningioma. By Hellerhoff (Own work) [CC BY 3.0], via Wikimedia Commons
8. Cerebral metastases. By The original uploader was Marvin 101 at German Wikipedia (Original text: benutzer:marvin_101) (Own work) [CC BY-SA 2.0 de], via Wikimedia Commons
9. Choroid plexus. Aaron G. Filler, MD, PhD, FRCS.
10. Hydrocephalus. By Lucien Monfils (Own work) [CC BY-SA 3.0], via Wikimedia Commons.
11. Pneumocephalus. By James Heilman, MD (Own work) [CC BY-SA 4.0], via Wikimedia Commons
12. Hypoxic brain injury. By James Heilman, MD (Own work) [CC BY-SA 4.0], via Wikimedia Commons
13. Skull fracture. By James Heilman, MD (Own work) [CC BY-SA 4.0], via Wikimedia Commons