CT Angiography and Perfusion

Learning objectives

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.

The Infarct core can be identified as that part of the brain which has already infarcted or is destined to infarct regardless of therapy. It has is defined as an area with prolonged MTT or Tmax, markedly decreased CBF and markedly reduced CBV. Note, that if one uses CBF alone to visually assess core size, it is easy to overestimate infarct core, as the penumbra often has reduced CBF also. So, even though some automated processes used CBF to define core, CBV is a safer parameter if 'eye-balling' the scan. The ischaemic penumbra, which in most cases surrounds the infarct core, also has prolonged MTT or Tmax but in contrast has only moderately reduced CBF and, importantly, near normal or even increased CBV (due to localised vasodilatation and autoregulation).

CT perfusion is less accurate if there are problems with cardiac output and arrhythmias or localised stenosis or poor placement of arterial and venous density regions of interest. This can lead to misleading flow maps and can affect MTT which can the overestimate ischaemia or global hypoperfusion and underestimated CBF. Seizures also can have ictal hyperperfusion, which may lead to an interpretation of hypoperfusion in the contralateral hemisphere mimicking infarct.

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.