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Chemistry LibreTexts

24.12: PET Scans

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    53991
  • Does your heart beat faster when you are scared? Do you have a tender feeling in your heart for that "special person"? Sorry to disappoint you, but that is actually simply a response generated by the brain. Scientists tell us that the seat of emotions is in the brain. Using PET scans and other techniques, they are looking at specific areas of the brain that process and store information dealing with strong emotions. They haven't localized the love site, but they're working on it.

    PET Scans

    One of the more interesting and useful approaches to radioisotope use in medicine is positron emission topography (PET), often referred to as a PET scan. This technique is especially useful in studying the processes in the brain. Many compounds do not enter the brain because of what is called the "blood-brain barrier", a filtering system to block material from being transported into brain tissue. This mechanism serves to protect the brain from a wide variety of harmful substances.

    In order to get a good picture of what is happening in the brain, radiolabels are attached to different compounds that will enter the brain. Since the brain uses about \(25\%\) of the glucose found in the body, this molecule is often labeled with a positron emitter such as \(\ce{F}\)-18 (half-life of 109.8 minutes) to study brain function in general. Other labels are attached to specific compounds that will localize in certain areas of the brain to look at specific structures.

    The PET scanner detects gamma emissions from the collision of a positron with an electron (see figure below). As the positron is released from the nucleus of the atom, it will collide with an electron. This meeting of matter (electron) with antimatter (positron) results in annihilation of both particles and the release of two gamma emissions that are \(180^\text{o}\) apart from one another. The apparatus detects these gamma rays and stores the data in a computer. From this information, a detailed picture of the brain can be developed.

    Figure 24.12.1: Scanner used to detect positron emissions.

    One useful application of PET scanning is in the diagnosis of Alzheimer's disease. This debilitating memory loss condition primarily occurs in elderly individuals. A protein known as beta-amyloid gradually forms deposits in the brain called plaque. Memory loss and impaired movement are the result of the plaque growth.

    The compound known as Pittsburgh compound B is often used to identify areas of plaque in the brain. The radiolabel is \(\ce{C}\)-11 (half-life of 20.38 minutes).

    Figure 24.12.2: Pittsburgh compound B used in diagnosis of Alzheimer's disease.

    The label attaches to plaque and can be observed using PET scans.

    Figure 24.12.3: Brain scans using Pittsburgh compound B to locate plaque.

    The computer translates the amount of isotope into a color scale, with red indicating a high level of radioactivity and yellow indicating somewhat less activity. We can see from the scans that the cognitively healthy individual shows the presence of very little plaque in the brain (see figure above). The individual with Alzheimer's demonstrates high concentrations of beta-amyloid in numerous areas of the brain.

    Other studies have been done looking at brain function in drug addicts. One of the theories about drug addiction involves the amount of dopamine action in the brain (a chemical that is a part of the system to transport nerve impulses). Studies of dopamine action have been helpful in understanding addictive processes.

    Figure 24.12.4: PET scan of dopamine binding in brains of normal and addicted individuals.

    The figure above shows the binding of chemicals that attach to dopamine receptors. The non-addicted individuals have large numbers of receptors for dopamine. The addicted persons show less binding to these receptors, indicating that fewer receptors are present. Since dopamine is somehow linked with the sense of pleasure, these data may help to bring a better understanding to the biochemical processes in drug addiction.

    Summary

    • PET scan techniques are results are described.

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.