Next: >> Stroke Imaging with MRI


CT imaging forms the basis of stroke care as it can rapidly differentiate between Ischaemic and Haemorrhagic stroke and some stroke mimics. All stroke physicians need to develop competencies in reading CT scans and identifying what is normal and what is not. The best way to learn is to look at all the scans which you request, to attend a weekly neuroradiology meeting and to spend time learning the cross-sectional anatomy and vascular supply. Looking at scans is part of good stroke practice and the correlation and sometimes lack of correlation between clinical findings and imaging can be vitally important. Looking at imaging isn't just for medical staff but something that should be of interest to all involved in stroke care. In some centres specialist nurses and therapists attend the neuroradiology meetings and this helps to educate all.

It wasn't always this way. I inherited a copy of Brain's clinical neurology from the 1970s and there isn't a single cross sectional image in the whole book as it didn't exist. Prior to CT there as no reliable way to diagnosing haemorrhage versus ischaemia other than at post mortem. Various clinical scoring systems where developed but were unreliable. A calcified pineal good give some idea of where the midline was and tell if it had shifted but it is hard to think that was very useful and I suspect craniectomies were done for suspected haemorrhage which never were. It was really a forlorn attempt at gathering information. No wonder the feeling of diagnostic futility led to a very negative view of stroke. How times change. It's still a surprise that even with the amazing advances in imaging that we still have to deal with diagnostic uncertainties and the answers are not always clear.

History of CT Imaging

The first CT scanner was developed in 1972 by Sir Godfrey Hounsfield who was a researcher at EMI. It's a fascinating fact that it was the wealth derived by record sales from the Beatles who were signed to EMI that funded early CT research. Hounsfield went on to win a Nobel prize. It was Hounsfield who came up with the idea that one could determine what was inside a box by taking X-ray readings at all angles around the object. He then built a computer that could take input from X-rays at various angles to create an image of the object in "slices". Applied medically this became computed tomography. Hounsfield built a prototype head scanner and tested it first on a preserved human brain, then on a fresh cow brain from a butcher shop, and later on himself. On 1st October 1971, CT scanning was introduced into medical practice with a successful scan on a cerebral cyst patient at Atkinson Morley Hospital in Wimbledon, London, United Kingdom.

It wasn't until the mid to late 80's that CT was available initially in teaching hospitals for assessing stroke patients. Prior to this there was no way to discern ischaemic and haemorrhagic strokes ante-mortem. The CT scanner consists of an X-ray tube and on the opposite side of the cylinder where the patient lies is a set of x-ray detectors. Through a process of data acquisition the X-ray tube and detectors rotates 360 degrees around the patient in the x and y plane and acquires a vast amount of imaging information which then undergoes image reconstruction. The tube also moves in the z axis so building up a 3 d image. Spiral CT was developed in the early 1990s and improves scan speed and flexibility. The x-ray tube and detectors rotate continuously about the patient while the scan table advances the patient continuously through the gantry. Between the x-ray tube and the detectors there is a loss of attenuation as it travels through structures and this is known as the attenuation coefficient which reflects the degree to which the x-ray intensity is reduced by the material. These values are scaled to give values known as Hounsfield units seen in the table below. The overall appearance can be altered by varying Window level and window width. Different windows can be used to look at different structures as shown below. Helical scans mean that there is continuous movement of the patient through the gantry whilst imaging. Multislice scanners mean that anything from 16-64 slices may be acquired at one time. For CT head slice thickness, is typically 5 mm for a standard head CT and between 0.625-1.25 mm for CTA. Although there is a wide number of Hounsfield numbers the human eye can only differentiate less than 100 different levels of grey.

Different Hounsfield units

MediumHounsfield UnitsAppearance
Fat-80 to -100Black
CSF +5Black
White matter +30Dark Grey
Grey matter +40Light Grey
Acute haemorrhage +70White
Bone+400 to +3000Bright White

Radiation Exposure

CT involves exposure to ionising radiation and so care must always be taken in its use. In the UK access to scanning is guided by IRMER guidelines and only trained professionals can request scans which includes appropriately trained doctors, specialist nurses and radiographers and radiologists. In many trusts have protocols (See indications for urgent CT scan)and agreements that allow specialist stroke nurses to arrange urgent Head CT scans without medical input. This is a vital time-saving step in those who need acute therapies and experience tends to show that this works well. CT scans should only be done when there is a clear clinical need and the result will help to direct therapies or provide useful information that can alter management. A typical head CT involves exposure to 2.1 milli-Sieverts (mSv) but can often be more. For comparison 10 millisieverts (mSv) of radiation is the rough equivalent of 200 chest X-rays. An increased risk of cancer has been identified among long-term survivors of the Hiroshima and Nagasaki atomic bombs who received exposures of 10–100 milliSieverts so care must be taken. Obviously denser structures need greater doses of radiation. Risk can be measured here.

Natural Background3.1 mSv/year
Domestic pilots2.2 mSv/year
Average US Exposure6.2 mSv/year
CXR (AL and lateral)0.10 mSv/year
Chest CT7.0 mSv/year
Chest Abdomen Pelvis CT21.0 mSv/year
Brain CT (standard)2.0 mSv/year
CTA/CTP Head16.4 mSv/year


Clinically there are no real contraindications for CT if clinically indicated. There are no concerns about pacemakers or defibrillators or metal clips or recent surgery. It is probably unwise if the patient is moribund and too unwell for any intervention and better served by palliation but that's a specialist call. Scans are incredibly quick nowadays and patients can be closely monitored. The only real difficulty is with agitated or confused patients in whom scan quality will be degraded with movement artefact. A doctor can be at the side of the patient during a CT with appropriate lead apron or behind a screen for protection if needed. CT does not prevent monitoring equipment of infusions or drips. It may be occasionally necessary to intubated and ventilate a patient prior to scanning.

Non contrast CT scan in Ischaemic stroke

Non contrast CT scan is the standard imaging modality for hyperacute stroke care. It is fast, cheap, accessible, and very sensitive for haemorrhage and there is no problem with pacemakers or monitoring equipment. Claustrophobic or monitored patients can be scanned relatively easy. Done early many scans will be normal despite significant clinical findings. Stroke is primarily a clinical diagnosis and not a radiological diagnosis.

Indications for urgent CT scan

Clinical Indications for urgent CT i.e. within 1 hour of arrival at hospital
  • Acute stroke suspected eligible for acute therapies needs done immediately
  • Acute neurology on Anticoagulant treatment or known bleeding tendency needs done immediately
  • Depressed level of consciousness
  • Unexplained progressive or fluctuating Symptoms
  • Papilloedema
  • Neck stiffness or fever
  • Severe headache at onset

CT and Windowing

This involves mechanism by which one can select for different Hounsfield numbers. Used well it can help increase the contrast between various structures. For example a window of 2000 will show most CT numbers. A wide window (W) 400-2000 HU is useful for showing low and higher density tissues. A narrow window 50-350 HU is better at showing soft tissue. Window Level (L) is that midpoint of the CT range. Brain is W80:L40, Stroke is W8:L32 or W40:L40.

Brain window to best show white/grey matter differentiationW=80 L=40
Bone windows for bone pathologyW=3500 L=700
Subdural windows for small or isodense subdurals W=250 L=70

What may be missed on a CT Scan

CT is incredibly useful but not infallible and its limitations should be respected. A normal CT scan is entirely compatible with a large ischaemic stroke when within 6 hours of onset. There may be some subtle early changes. CT is also very poor at picking up changes in tissue density in bony spaces such as temporal bones and posterior fossa due to what is called beam hardening as the x-ray beam is distorted by the dense bone traversed. Metallic objects such as aneurysm clips or piercings that weren't removed can cause a radiating streak pattern. Partial volume artefacts can cause streaks or bands of alternating dark and light stripes and like beam hardening also affect the posterior fossa. CT scanning cannot differentiate between hypodensity due to infarction or haemorrhage after 1-2 weeks and MRI with GRE or T2* must be used. Subacute ischaemic stroke after several days can show haemorrhage and irregular patterns of density and haemorrhagic changes which can resembled tumour. Follow up interval scanning will be needed.

Caution : What may be missed on a CT scan
  • Isodense Subdural haematoma
  • Brainstem or Posterior fossa pathology though more recent scanners are much improved
  • Low attenuation lesions near skull missed 'beam hardening'
  • Suspected Haemorrhage over 2 weeks ago (get GRE MRI)
  • Multiple sclerosis plaques

Look at as much brain imaging as possible, the ones that you request and more. The key is to see a wide variety of normality and to build up some pattern matching skills and experience in identifying important structures and lesions. You will have an advantage over the radiologist who has only the clinical details on the card whereas you have the patient.

Imaging Anatomical Landmarks

It is important to develop your ability to study imaging and correlate it with brain structure and function. There is a standard anatomical vocabulary with terms such as axial, sagittal and coronal which are commonly used in both CT and MR to describe different cross sectional images. Modern day imaging actually allows the brain to be imaged and reconstructed in any plane but these are the standard and familiar views.


Make sure you understand the terms axial, sagittal and coronal. Axial is simple horizontal slices from above down. Sagittal is in the same plane as cutting the brain in the midline or parallel to this (parasagittal). The falx cerebri that separates the brain hemispheres lies in the sagittal plane. Coronal is the plane of my daughter's hair band. A bit like a tiara or 'corona'. It is vital to have a good inner representation of the cerebral vessels as the enter the skull and join the circle of Willis and the circle of Willis as well and the position of the vessels. A few simple points will help greatly. The circle of Willis (COW) is at the level of the midbrain. Find the midbrain and then look for the vessels. This is a good place to start looking for the hyperdense MCA as the MCA leaves the COW laterally within the sylvian fissure.

Normal Cross sectional CT anatomy

CT Axial From Top (Vertex) to Bottom (Base of Skull) Comments
A place to look for evidence of Subdural collections of blood which acutely are dense and bright white and become darker with time. Appreciate the sulcal patterns here high up near the primary motor cortex.
Look for subdurals at this level. Look for any density that could be blood in the sulci which could be seen with a convexity type subarachnoid. This is also the level where parafalcine positioned anterior cerebral artery infarction may be seen.
As above
As above
This is a cross section through outer grey matter and sulci and gyri of the cortex and the dark inner matter which is the the centrum semiovale and is a mass of white matter, superior to the lateral ventricles/corpus callosum, present in each of the cerebral hemispheres under cerebral cortex. It has a semi-oval shape and contains projection, commissural, and association fibers. Inferiorly these fibres are continuous with the corona radiata.
This is the corona radiata which is a white matter sheet that continues ventrally as the internal capsule and dorsally as the centrum semiovale. This sheet of both ascending and descending axons carries most of the neural traffic from and to the cerebral cortex.
Can see midline falx separating hemispheres. The lateral ventricles are visible with some asymmetrical calcified choroid plexus. Surrounded by the fibres of the corona radiata
As above. The beginning of the cerebellum is seen in the posterior fossa.
Level of the midbrain. This is V shaped. Sometimes the Circle of Willis (COW) and MCAs is best seen at this level depending on the slice taken. If not seen check the slice above or below. The cerebellar hemispheres are seen.
Level of the upper pons and temporal lobes can be seen. The COW is at this level. The slightly dense basilar artery can be seen lying in front of the pons in the midline. This patient had no symptoms and this is a normal scan.The cerebellar hemispheres are seen.
Can see temporal lobes as well as frontal lobe. Basilar artery and pons. Cerebellum posteriorly.
Can see temporal lobes as well as frontal lobe. Basilar artery and lower pons. Cerebellum posteriorly.
Basilar/Vertebral artery. Medulla and Cerebellum posteriorly. Frontal and temporal lobes.
Medulla and lower cerebellum posteriorly. Frontal and temporal lobes.
Medulla seen as it becomes spinal cord at foramen magnum

CT Interpretation

Definitive distinctive changes may not occur until 6-8 hours. In the meantime more subtle signs are seen. At about 6 hours and sometimes earlier there may be loss of grey-white matter differentiation - seen at the cortical surface due to localised changes such as cytotoxic oedema within the grey matter which has a higher metabolic requirement and so becomes oedematous quicker. These signs are subtle and can be missed by even the most experienced

    Early signs (< 6 hrs) of Acute Ischaemic Stroke
    • May be entirely normal initially
    • Cortical Sulcal effacement - suggests some increased oedema
    • Loss of Grey/White differentiation in the basal ganglia
    • Loss of insular ribbon sign is similar to loss of grey white differentiation with localised cytotoxic oedema. Vascular supply here is more vulnerable due to poor collateralisation and so this may show first.
    • Obscuration of the Sylvian fissure: Similar to insular ribbon sign
    • Hypoattenuation seen on CT is highly specific for irreversible ischaemic brain damage and infarction if it is detected within first 6 hours.
    • Hyperdense MCA sign or more distal MCA "dot sign" it may be normal is a sign of clot (thrombotic or embolic) (not a contraindication to lysis) but shows extent of possible infarct which depends also on collateral flow.
    • Obscuration of the lentiform nucleus (loss of the normal attenuation difference of the globus pallidus and/or putamen with respect to contiguous white matter structures

    Later signs (6-24 hrs) of Acute Ischaemic Stroke
    • Watershed infarcts between vascular territories often bilateral strokes between ACA and MCA territory and MCA and PCA may suggest carotid disease
    • Clearly delineated wedge shaped hypodense region involving cortex and adjacent white matter related to the occluded artery anatomy and collaterals at 12 hours.
    • May be some haemorrhagic transformation. Estimated incidence of haemorrhagic transformation is up to 40% in the subacute period even when not thrombolysed.
    • Lacunar infarcts may be seen deep within white matter and within the basal ganglia.
    • Occasionally due to collateralisation or perhaps reperfusion of the MCA the cortex remains unaffected but subcortical areas infarct and become hypodense and this is seen with a subcortical striato-capsular type of stroke.
    • Late changes over days and weeks is most marked Hypodensity due to cytotoxic oedema initially and Vasogenic oedema secondarily and best seen days 3-10.

    Late signs ( > 24 hrs) of Acute Ischaemic Stroke
    • Fogging - density of ischaemic tissue reaches same intensity as normal brain tissue and so evidence of infarction not seen
    • Late changes over weeks and months shows continue as the infarcted zone has density of CSF and there is loss of volume.
    • A hypodense caudate suggests MCA occlusion proximally taking out lenticulostriate arteries. Depends on leptomeningeal anastomoses of ACA and PCA
    • Haemorrhagic transformation may occur

Non contrast CT false negatives (there is a stroke) usually in infarcts when done early or in those who present 7-10 days after stroke and there is a visible hypodensity but no blood and so aetiology of perhaps a small bleed may be missed. In these cases a gradient echo will show haemosiderin deposition around the margins suggesting haemorrhage as he cause.

NCCT false positives are seen particular in older hypertensive patients where Lacunar infarcts are common and most often asymptomatic but appear on scans done for a myriad of reasons so unless there is corresponding new neurology do not diagnose acute stroke but do treat for "stroke disease".

CT of Left Posterior cerebral artery infarct

ASPECTS scoring

The Alberta Stroke program Early CT score (ASPECT) scoring system is used to assess MCA infarction at two different axial slices corresponding with two different levels and one subtracts 1 from total of 10 for each area affected. A score of 0 suggests extensive MCA infarction and correlates inversely with NIHSS. A score of 10 is normal. An ASPECTS score less than or equal to 7 predicts a worse functional outcome at 3 months as well as symptomatic haemorrhage.

  • Lentiform nucleus level
    • M1, M2, M3
    • Lentiform
    • Caudate nucleus
    • Internal capsule
    • Insular cortex
  • Centrum semiovale level
    • M4, M5, M6

The ASPECTS score has some clinical correlation. A normal brain has a score of 10 and as more areas are affected the score falls. A sharp increase in dependence and death occurs with an ASPECTS of 7 or less. A common misunderstanding of ASPECTS scoring is to assess only two standardised cuts, i.e. one ganglionic cut and one supraganglionic cut. Be sure to include the assessment of all axial cuts of the brain NCCT scan. The ASPECT score is not needed prior to thrombolysis but it does give an element of quantitative rigor to assessing the CT analysis and may be mentioned in any discussion with a stroke physician so useful to have heard of it.

Aspects Cross section

CT Angiography (CTA)

Can be performed by giving single IV bolus of contrast through good IV access. Helical CT scan with multislice abilities and short acquisition times can capture and follow contrast as it enters the arterial and then venous phase through the brain thus imaging the arterial and venous vessels. Scan acquisition is done such that vessels are imaged at the point of peak opacification. Can give good imaging of circle of Willis and branches as well as extra cranial vessels. Three dimensional imaging can be reconstructed. Can be useful in determining diagnoses e.g. conforming a basilar artery stroke or in planning further intravascular procedures depending on whether clot is seen occluding major vessels. Post-acquisition software analysis can reconstruct very useful 3D images of the vascular structure without other soft tissues known as a Maximum intensity projection. In terms of ability to detect aneurysms it is 94-98% sensitive the only difficulty being in aneurysm less than 3 mm in diameter where the pick up rate is about 70%. CTA may be undertaken in acute stroke to identify the ongoing presence of thrombus when there is consideration for either intra-arterial thrombolysis or mechanical management of the thrombus. CTA is also useful when looking for arterial evidence of arterial dissection or pseudoaneurysm formation. CTA can also identify the presence of vasospasm. However in almost all cases it is second best to CT angiography and there has to be a clinical assessment of risks and benefits. In many cases CTA is sufficient.

CT Perfusion

The brain volume can be mapped during perfusion in a CT slice following an injection of IV contrast. The first pass is measured as the contrast perfuses the brain and can be done along with CTA. Modern scanners can take 10 and more images per second. Modern multislice scanners with fast acquisition times of up to 16 to 64 slices at one time allows different slices to be taken simultaneously during distinct phases of arterial and then venous passage of contrast. A time density curve for each pixel can be generated. The software can calculate relative cerebral blood volume CBV (CBV) and the mean transit time (MTT) which can be displayed in a colour map. Cerebral blood flow can be calculated from CBF=CBV/MTT.

The volume of blood per unit of brain 4-5 ml/100 g, Flow to grey matter is 50-60 ml/100 g/min. Transit time is from arterial inflow to venous outflow can be measured as can Time to peak enhancement - beginning of contrast injection to the maximum contrast in the area under study. CT Perfusion shows the volume of viable brain at risk due to reduced flow. This can help to demonstrate the penumbra.

CT perfusion has been explored as useful tool in acute large vessel occlusive stroke disease and it may be used alongside MRI DWI to assess extent of stroke and possibly to direct therapies. It is still very much a research tool and not commonly used outside the teaching hospital. Its place in the hyperacute stroke protocol remains unclear.

CT in Haemorrhagic Stroke

Blood stands out well on a NCCT as 'hyperdense' as it absorbs and attenuates x-rays well compared with water and CSF and normal brain tissue. Within 24 hours of a haemorrhagic stroke the sensitivity is about 99% which then falls off as blood is reabsorbed and changes it characteristics. After 1-2 weeks it is quite possible that there is no sign of blood but only an area of apparent reduced attenuation. At this stage the only way to have any evidence of haemorrhage is to use MRI Gradient Echo which will be discussed later.

Haemorrhage is almost always unilateral and asymmetrical. Several 'bright' structures may be seen on a CT scan including basal ganglia calcification and choroid plexus calcification. Bright lesions may be seen to be the tip of bony skull prominences by ascending and descending skull slices. Intraparenchymal blood should be easy to see and describe. Once spotted then the next things to look for are signs of midline shift. Changes in the midline and obvious bulging of pressure into ventricles can be significant. If there is bleeding into ventricles then hydrocephalus can happen so simple signs such as enlargement of the IIIrd ventricle all become important. Always make sure that the circumference of the brain is looked at - there can easily be a small subdural present and if chronic it becomes hypodense and the blood loses its brightness and it gradually has the consistency of brain and then CSF. Look for asymmetry and pressure effects.

Subarachnoid haemorrhage is quite simple to spot - there is hazy blood through the folds on the surface of the brain and this also extends along the natural folds including the sylvian fissure and the interhemispheric fissure and in the prepontine spaces in front of the brainstem where free subarachnoid blood can gravitate as the patient is lying supine in the scanner. If the cause is a ruptured berry aneurysm then the site of most blood collection may give clue to the artery involved. Again look for developing hydrocephalus and if there is a haematoma then for pressure effects.

Cerebral Digital Subtraction angiography (DSA)

This is the gold standard test for studying cerebral vasculature. It is used mainly in tertiary centres to get the best possible image of cerebral vasculature. A catheter is inserted at the femoral artery and threaded up iliac and descending aorta to the aortic arch. From here it may be threaded up from the vertebral artery to the circle of Willis and either subclavian or carotids systems cannulated depending in the vasculature to be examined. Radio-opaque iodinated contrast is injected and X-rays are taken to show the passage of the contrast. It is useful post haemorrhage in diagnosing small aneurysms, arteriovenous malformations and vasculitis where it may show occlusion or narrowing or beading. There is a small approximately 1% risk of stroke. Other than that there is a small risk of vascular injury at insertion site, haemorrhage and infection. Care must be taken with contrast in those with renal impairment.

Next: >> Stroke Imaging with MRI

Last updated 03/03/2018

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