Positron Emission Tomography (PET)

Positron Emission Tomography (PET), is a diagnostic test used by physicians to create an image of the brain, which in contrast to MRI, emphasizes function rather than structure. PET scans require intravenous injection of a unstable substance which emits positrons, most often fluorodeoxyglucose (FDG). FDG is very similar in structure to glucose (a common sugar), the fuel used by most every tissue in the body. FDG is rapidly absorbed by parts of the body that use a lot of energy, especially the brain. A short time after the injection of FDG, the patient is placed in the PET scanner. When FDG undergoes radioactive decay, it releases a particle known as a positron. When this positron collides with a nearby electron in a cell, a special type of light known as a photon is released. For each collision, there are two photons released in opposite directions. A PET scanner is essentially a set of photon cameras which detect the emitted photons. Using computer-aided techniques, the origin of the photons (and thus positrons and FDG) can be determined. This allows physicians to identify areas of the body which exhibit high sugar uptake. PET scans are especially useful in the assessment of epilepsy. During a seizure (ictal), brain regions that produce seizures exhibit high metabolic activity and use lots of sugar, which usually appears bright on PET scans. Conversely, in between seizures (interictal), those same regions of brain appear very inactive (dark). The image below illustrates a horizontal cross-section of brain, with the front of the brain at the top of the image, and the back of the brain at the bottom. In this PET scan, high metabolic activity is indicated by the bright red/yellow areas, and low metabolic activity is indicated by the dark areas. In particular, the dark spot indicated by the arrow and circle is a region of low metabolic activity (low glucose usage) and represents an area of brain that was responsible for this patient's infantile spasms (seizures). Removal of that brain area has since resulted in seizure-freedom for this patient.






















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