Welcome to Stroke Education.CO.UK

Learning objectives
  • Role of CTA Imaging in stroke
  • Limitations
  • Basic Interpretation

Introduction

  • CT Angiography can be performed by giving a 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.

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.

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