الفهرس 
VASCULAR ANATOMY OF THE BRAINOnly 14 pages are availabe for public view
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Abstract
In ischemic stroke, a compromised blood supply leads to functional impairment, followed by structural disintegration of neurons in the absence of reperfusion. The initial phase of dysfunction is potentially reversible, prior to subsequent cell death. While some brain tissue may be irreversibly damaged, the other hypoperfused areas may be at risk but are potentially salvageable. Thrombolytic therapies aim to rescue this threatened brain tissue, termed ‘‘tissue at risk” or the ischemic penumbra.
Recent advances in both computed tomography (CT) and magnetic resonance (MR) technology and a new emphasis on thrombolytic therapy for acute stroke have
brought brain perfusion studies to the forefront of neuroimaging clinical practice and research.
Perfusion imaging approaches have focused on measurement of CBV, CBF, MTT, and TIP with bolus injections of intravascular contrast. The bolus of contrast is followed through brain parenchyma using rapid imaging techniques, allowing images to be acquired every 1 to 2 seconds. Curves reflecting the amount of contrast within a given pixel at any particular time point are then generated from the serial images acquired during the contrast bolus. The curves in this case actually are direct measurements of electron density in the case of CT and loss of signal in that case of MR.
The advent of multidetector-row CT (MDCT) has recently afforded a renewal of perfusion CT (PCT) techniques in the detection of stroke. PCT provides a comprehensive and accurate noninvasive survey to hemodynamic of the PCT relies on the extraction of functional rather than morphologic information from the CT data. Two PCT techniques are currently in practice: whole-brain PCT and dynamic PCT. Whole-brain PCT provides information related to cerebral blood volume (CBV), and thus about the infarct core, but not about the ischemic penumbra, which is the target of acute stroke treatment.
Dynamic PCT involves the dynamic acquisition of sequential CT sections in cine mode during the intravenous administration of iodinated contrast material. Dynamic PCT data analyzed according to the central volume principle has the major advantage of allowing the assessment of both cerebral blood flow (CBF) and CBV in a robust and reproducible quantitative way. Thus, the technique provides insight into cerebral vascular autoregulation and ischemic penumbra. However, this technique is currently limited by the evaluation of selected
portions of the brain, which are typically 40 mm thick.
Perfusion-weighted images are currently acquired using the dynamic-susceptibility contrast imaging technique. This is very similar to the computed tomography perfusion (CT perfusion) technique. A series of susceptibility-weighted images (known as T2*) are obtained every 1-2 sec during an injection of intravenous gadolinium contrast. Just as in CTP imaging, the central volume principle is used to calculate cerebral blood flow (CBF) and cerebral blood volume (CBV) on a voxel-wise basis.
CTP imaging techniques are relatively new compared with MR imaging–based methods. Because the general principles underlying the computation of perfusion parameters such as CBF, CBV, and MTT are the same for both MR imaging and CT, the overall clinical applicability of perfusion imaging by using either of these techniques is similar.
Differences in technique create several important distinctions. The most important advantage of CTP is the linear relationship between contrast concentration and attenuation in CT, which facilitates quantitative (versus relative) measurement of CBF and CBV. MR perfusion imaging (MRP) relies on the indirect T2* effect induced in the tissue by gadolinium; the T2* effect itself is not linearly related to the gadolinium concentration, making absolute measurement of CBF and CBV difficult.
One disadvantage of CTP is, until recently, the relatively limited coverage, whereas MRP is capable of covering the whole brain during a single bolus injection. But CT scanners are in developing, the newer 256-section scanners can provide whole-brain coverage, and in the next few years this coverage will be widely available. A second disadvantage of using CT rather than MR imaging for stroke assessment is the decreased sensitivity for detection of cerebral microbleeds compared with gradient-echo sequences. Microbleeds detected on T2*-weighted MR imaging, however, have been shown not to be a contraindication to thrombolysis.
Conclusion:
Structural, vascular, and physiological imaging of acute stroke increasingly informs both clinical trial design and individual patient management. It is likely that both CT and MRI-based techniques will be more widely applied in future, and the relative strengths and weaknesses of each imaging modality should be regarded as complementary rather than competing. Effectively, an ideal situation would be to have access to both imaging modalities to adjust to the various clinical situations and contraindications that present in the real world.