الفهرس 
Vascular anatomy of the brainOnly 14 pages are availabe for public view
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Abstract
Perfusion imaging allows the blood flow to the tissue to be imaged. It is currently widely applied to the management of acute ischaemic stroke. Using either Computerised Tomography or Magnetic Resonance Imaging techniques, perfusion maps can be created in a short enough time to allow their routine use in clinical practice. Perfusion Imaging enables the physician to directly estimate the tissue at risk which can be salvaged with reperfusion, enabling appropriate patient selection.
CT perfusion imaging is one of the techniques widely applied for assessing perfusion abnormalities of patients with acute stroke. It is useful for prediction of the initial severity of clinical symptoms, the extent of final infarction, and clinical outcomes. .
CT perfusion imaging is carried out after the injection of iodinated contrast medium and monitoring the first pass of the contrast agent bolus through the cerebral vasculature. Contrast agent time-concentration curves are generated. CBF, CBV and MTT are calculated by a complex mathematic process gives the color coded maps.
There are different protocols for CTP. The variations among protocols aim to image the largest volume of brain tissue, at the same time, keep the radiation exposure within reasonable limits.
Perfusion MRI as well as perfusion CT is based on the estimation of the contrast medium bolus passage through microcirculation of a vessel’s net. Also, perfusion MRI estimates parameters of the regional blood flow (rCBV, rCBF, MTT); however, unlike CT methods, perfusion MRI deals with relative blood flow indicators in comparison with the unaffected side.
In order to provide improved ability to measure changes in the concentration of gadolinium as it passes through the cerebral vasculature, most PWI techniques in current clinical use rely on dynamic susceptibility contrast (DSC) imaging, in which image contrast is based on gadolinium’s magnetic susceptibility effect, rather than its T1 relaxivity effect.
Perfusion MRI allows evaluation of the hemodynamic status of acutely ischemic tissue and has greatly improved evaluation of acute stroke. The CBV abnormality correlates highly with the DWI abnormality,which is thought to represent the ischemic core. With proximal emboli, CBF and tissue transit time maps demonstrate the operational ischemic penumbra, additional tissue with altered perfusion that is at risk of progressing to infarction. Diffusion and perfusion are useful in predicting tissue viability, and are also useful in predicting HT and clinical outcome.Most importantly, it has now been shown how DWI–PWI mismatch criteria may be used to successfully guide thrombolytic therapy.
The sensitivity of CT perfusion is very high, even when the examination is conducted within the first minutes (and hours). In these moments, it is possible to detect a significant decrease of the blood flow in the affected brain area .At the same time, it is possible to calculate the quantitative indicators of brain perfusion. This may play an important role in treatment selection and patient management. Numerous researchers demonstrated that this method is more sensitive in the first hours after stroke onset than even MRI perfusion.
The choice therefore is based on local availability and logistics. Due to the wide availability of CT and its high sensitivity to the presence of haemorrhage, most patients being considered for thrombolysis will have a CT first. If an angiogram and a perfusion image can be obtained in an additional 10 minutes without moving the patient it would be the ideal scenario.