Answer
With perfusion-weighted imaging (PWI), information about the perfusion status of the brain is available. The most commonly used technique is bolus-contrast tracking. The imaging is based on the monitoring of a nondiffusible contrast material (gadolinium) passing through brain tissue.
The signal intensity declines as contrast material passes through the infarcted area and returns to normal as it exits this area. A curve is derived from this tracing data (ie, signal washout curve), which represents and estimates the cerebral blood volume (CBV).
An arterial input function can be derived by measuring an artery in lower brain slices or by measuring gadolinium concentration that is proportional to the changes in T2 when gadolinium is used at low doses (< 3 mg/kg). Based on this arterial input function, quantitative maps of cerebral blood flow (CBF), CBV, mean transit time (MTT), time to peak (TTP), and various other hemodynamic parameters can be obtained. Considerable debate surrounds the choice of which PWI parameter should be used. Most centers in the United States use time domain parameters, such as MTT or TTP.
Arterial spin-labeled (ASL) PWI permits noninvasive quantification of CBF without the use of contrast agent. There are two distinct forms of ASL that are used clinically: 1) continuous ASL (CASL) and 2) pseudocontinuous ASL (PCASL). A 2015 guideline paper outlines the methodological details of these individual techniques. [1]
The use of DWI and PWI together has been shown to be superior to the use of conventional MRI in early phases and up to 48 hours after the onset of stroke. Using a combination of DWI and PWI is very important, because together they provide information about the location and extent of infarction within minutes of onset; when performed in series, they can provide information about the pattern of evolution of the ischemic lesion. This information may be of great importance in choosing the appropriate treatment modality and in predicting the outcome and prognosis. [2]
The lesion usually enlarges on serial DWI over a period of several days. It has been suggested that this enlargement can be halted if reperfusion (ie, resolution of original PWI lesion) occurs early enough. Lesions that are not large on initial PWI do not show this enlargement.
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Magnetic resonance imaging in acute stroke. Left: Diffusion-weighted MRI in acute ischemic stroke performed 35 minutes after symptom onset. Right: Apparent diffusion coefficient (ADC) map obtained from the same patient at the same time.
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Magnetic resonance imaging in acute stroke. Left: Perfusion-weighted MRI of a patient who presented 1 hour after onset of stroke symptoms. Right: Mean transfer time (MTT) map of the same patient.
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Magnetic resonance imaging in acute stroke. Diffusion-perfusion mismatch in acute ischemic stroke. The perfusion abnormality (right) is larger than the diffusion abnormality (left), indicating the ischemic penumbra, which is at risk of infarction.
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The diffusion-weighted MRI reveals a region of hypointensity in the distribution of the right middle cerebral artery. Flanking the anterior and posterior regions of this abnormality are regions of hyperintensities, which represent regions of new infarct. The contiguity of these regions suggests that they are extensions of the old infarct.
