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Terry's questions are very pertinent and succinct. Perhaps I can add something extra to the replies which other people have already contributed. Be prepared to question my remarks, however, as I am not a PET-practicioner. One of the main obstacles to understanding PD is the inability to examine brain tissues in live people, be they patients or not. Consequently, alternative methods are sought. X-ray and magnetic resonance imaging primarily show bone and tissue structures, while PET imaging shows chemical quantity and activity. (There are variations and exceptions to this statement, but it is by and large true. Another technique, ultrasound, has very recently been reported here on PIENET to have value for PD as well.) For PD, PET imaging shows the amount of dopamine in relevant portions of the brain. Dopamine is produced in the substantia nigra, and is conveyed through nerve cell 'projections' to the putamen. As the producing cells die/change, the quantity of dopamine in the putamen decreases, and this can quite clearly seen in PET images and can be measured quantitatively. Since it is widely asserted that about 80% of the dopamine-producing cells have already died/ceased functioning by the time the physical symptoms of PD appear, PET imaging can reveal the dopamine depletion in early stages of the outward manifestations of PD. Hence, the combination of X-ray/CAT scan, MRI scan, and PET scan can help a physician to diagnose a patient's physical symptoms as being due to PD and to distinguish them from other causes. As the diagnosis of PD is often difficult and fraught with anxiety for the patient, a PET scan can be a useful diagnostic tool to alleviate the uncertainty surrounding diagnosis. The extension of this idea into a universal screening test for PD (prior to the onset of physical symptoms) is, at present, not practical as no-one can yet define who is 'at risk'. It is appropriate, however, to consider this approach for research studies on selected groups of people into, for example, genetic or environmental associations as causes of PD. PET imaging can also be used in research into the efficacies of experimental therapies and drugs as a way to observe the changes in dopamine quantity, or to observe the progression of PD (see, for example, the segment about Dr. Fahn in the posting by George Pospisil on October 5 about NIH funding of Udall Centers of Excellence). However, for the 'regular' PD patient, PET imaging is not a routine event as on-going treatment strategies do not rely on its information. As you all know, your medication regime or decision to undergo a surgical procedure is based on your physical symptoms (and other health issues) and the PET data does not materially add to that picture. I might add that the process of acquiring a PET image can be quite taxing to the patient, particularly those with moderate to advanced stages of PD, as it requires the patient to be begin the procedure in a pure 'off' state (by withholding l-dopa medication for many hours prior to starting) and then requiring him/her to be motionless for 45-90 minutes while the body is crying out with discomfort from stiff muscles. There is also the matter of a low dose of ionizing radiation, and the invasiveness of arterial injections. Altogether, not an easy procedure, so hats off to all those who have done, and will do, it for the sake of their own betterment, and that of others through research. In summary, then, PET imaging is a wonderful and different diagnostic, monitoring, and research tool for PD, but as of yet it does not enter into the treatment equation. Roger Buxton