Positron Emission Tomography

PET can be used to measure brain metabolism, CBF, and the quantity of specific receptor types within the brain. The technique requires administration of a radionuclide that emits positrons during radioactive

Figure 8 Early acute brain hemorrhage. Oxyhemoglobin and deoxyhemoglobin are mixed in the acute stage of hemorrhage (<24 hr). The Tl-weighted image (left) shows only mild hypointensity and mass effect. The proton density image (center) shows hyperintensity in the area of hemorrhage. The T2-weighted image shows hyperintensity (straight arrow) as well as hypointensity (curved arrows), corresponding to oxyhemoglobin and deoxyhemoglobin, respectively. A hyperintense rim of edema is visible around the hematoma on proton density and T2-weighted imaging [adapted with permission from R. Bakshi and L. Ketonen, Brain MRI in clinical neurology. In Baker's Clinical Neurology (R. J. Joynt and R. C. Griggs, Eds.). Copyright Lippincott, Williams & Wilkins, 2001].

Figure 8 Early acute brain hemorrhage. Oxyhemoglobin and deoxyhemoglobin are mixed in the acute stage of hemorrhage (<24 hr). The Tl-weighted image (left) shows only mild hypointensity and mass effect. The proton density image (center) shows hyperintensity in the area of hemorrhage. The T2-weighted image shows hyperintensity (straight arrow) as well as hypointensity (curved arrows), corresponding to oxyhemoglobin and deoxyhemoglobin, respectively. A hyperintense rim of edema is visible around the hematoma on proton density and T2-weighted imaging [adapted with permission from R. Bakshi and L. Ketonen, Brain MRI in clinical neurology. In Baker's Clinical Neurology (R. J. Joynt and R. C. Griggs, Eds.). Copyright Lippincott, Williams & Wilkins, 2001].

decay. The scanner detects the gamma rays that are emitted when a positron with an electron and both are annihilated. This technique, annihilation coincidence detection, permits the location and quantity of radioactive decay to be mapped within the brain.

Using 15O tracer PET for imaging, quantitative determination of CBF, blood volume, the rate of oxygen metabolism, and the oxygen extraction fraction can be determined. Brain areas where there is inadequate CBF to satisfy metabolic demands ("misery perfusion''), areas where there is more than enough blood flow to meet metabolic requirements ("luxury perfusion''), and areas of frank ischemia leading to a reduction in brain metabolism can be mapped. Although these techniques revolutionized understanding of the pathophysiology of stroke, they are available only in specialized PET imaging centers that have a cyclotron available to generate 15O. Thus, clinical PET is seldom utilized in the management of acute stroke. [18F]fluorodeoxyglucose PET uses a much longer halflife radionuclide (18F vs 15O) to demonstrate a "patchy" pattern of metabolic compromise in patients with vascular dementia, corresponding to sites of repeated vascular injury.

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