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What is Positron Emission Tomography? 

        PET was developed in the  mid- 1970s, and it was the first scanning method to give functional information about the brain.  PET measures the emission of positrons from the brain after a small amount of radioactive isotopes, or tracers,  have been injected into the blood stream.  A common example is a glucose-relative with embedded fluor-18.   With this molecule, the activity of different regions of the brain can be measured.  The result is a three-dimentional map with the brain activity represented by colors. 

        This technique has proven extremely useful in finding out just how we think, and it is used in research on brain functioning.  However, the half-lives of the isotopes used are very short, meaning that on-site production is required.  This can be done by the help of physicists and a cyclotron, but few institutions can afford these facilities.  There are therefore not more than 100 or so PET laboratories around the world. 

        The major advantage of PET is that it can show changes that occur fairly quickly, on the order of seconds.  The isotopes used can also be embedded in certain substances in order to measure the uptake of that part in different regions of the brain.  This way, specific binding of neurotransmitters can be mapped out. 

        Other situations which a PET scan can help diagnose are: 
            -Cardiovascular diseases 
            -Early brain changes/diseases 
            -Tumors or anatomical abnormalities 
            -The cause of mental disorders 
            -Areas affected by a clot or stroke 

        To prepare for a PET scan, a patient should eat a light meal the evening before.  No food or drink should be ingested on the morning of the scan.   Depending on the medical reason for the  scan, other preparations may also be needed.  During the procedure, a small amount of radioactive material will be injected into one of the patient's veins.  The patient will then be asked to lie down on a table that slides into the PET machine so that pictures of the brain can be taken.  There is no real risk involved with the radioactivity of the isotope.  However, there is a possiblity an individual will show allergic reactions to the substances injected.  A patient with known allergies should discuss this with a doctor before undergoing the PET scan. 



  

What is Single Photon-Emission Tomography? 

        SPECT is closely related to PET, but uses isotopes with longer half-lives that can be stored on site.  It was developed 10-20 years earlyer than PET, but its resolution is much lower, yielding less detailed images.  (A typical SPECT will have a resolution of 9x9x9 mm.)  The areas of usage are similar to those of PET, except for functional research where rapid changes are to be observed.  SPECT has in particular been used to study the neurotransmitter dopamine.