Answer
The three-dimensional (3D) time-of-flight (TOF) technique is based on flow-related enhancement; it is the preferred MRA technique. However, it has some limitations, especially flow signal dropout secondary to turbulent flow in the tortuous and stenotic vascular segments, which makes interpretation of stenosis in these areas difficult. These are common predilection sites for atherosclerosis. In addition, in slow-flow regions, the spin saturation of the scan causes overestimation of stenosis. In contrast-enhanced studies, it provides more information than standard angiography, especially in detecting critical stenosis of extracranial vessels, but it is less reliable in intracranial critical stenosis. Always keep in mind that MRA is a flow-dependent technology; absence of flow signal does not mean literally a complete occlusion but rather that flow is below a critical value.
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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.
