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CT Interpretation

Definitive distinctive changes may not occur until 6-8 hrs. 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

  • 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.

    Dense MCA artery sign

  • 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
  • Watershed infarcts between vascular territories often bilateral strokes between ACA and MCA territory and MCA and PCA
  • 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 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.
  • Left MCA Infarct

  • 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

NCCT 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 stoke 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 can capture and follow contrast as it enters the brain thus imaging the great 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. Multislice scanners allows different slices to be taken simultaneously. A time density curve for each pixel can be generated.

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 quiet 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 : MRI Basics

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