Usually, PET scans and fMRI scans are mutual exclusively preferred by brain researches, in that they either use one or the other technique, but not both, to assess brain function. I have always wondered if both techniques agree in "higher activity" area identification during task performance.
Dear Martin, thank you very much for indicating this insightful publication.
I had never thought of the intrinsic mechanisms for both techniques, which fundamentally differ on the biochemical level. PET, making use of precise metabolites, seems to have a broader range of applicability than fMRI, which solely depends on oxygen fluctuations within certain brain areas. It is interesting, though, that fMRI should assess fast metabolic adaptations, whereas PET with Fluordesoxyglucose assesses slow metabolic accommodation. As a quick guess, since both oxygen and glucose are used to obtain energy, where glucose is preferentially used for quick ATP yield, and oxygen normally comes later as the final electron acceptor in mitochondria, I would expect PET to deliver data on fast processes. On the other hand, it is true that both glucose and oxygen are delivered to cells through blood, and that brain cells have no glucose "stores" to look up to in case of need. Thus, availability always depends on blood flow, and as oxygen enters cells through (fast) diffusion, whereas glucose enters via (slow) protein transport, observations for fMRI and PET time scales possibly make more sense this way. Also, 18F-FDG isn't metabolized until it decays, which means until then neurons can't extract any energy from it, so the limiting step is indeed cell entry.
Anyway, thanks again for your suggestion, it was very helpful!
Brain perfusion can directly and qualitatively be measured with PET using oxygen 15. So yes, giving the right tracer, e.g. 15O, the same regions light up.
You create labeled water and quantify its amount in the brain. From there you can measure oxygen metabolic rates, flow, cerebral blood volume etc.
15O PET is actually used to validate fMRI measurements as it is a direct method.
Its short half lifetime of 2 minutes make it just more complicated to use compared to fMRI. You literally need to have people holding the door open for someone to run from the cyclotron bunker to the PET-scanner.
Nevertheless it is a very powerful tool.
FDG is of less commonly used in brain research as there is always a high glucose metabolism.
Bruno,
Some older studies comparing 15O water PET and fMRI mainly showed the same regions but with different centers of mass. This has been attributed to the difference in the methodologies. PET measures 150 Water as it is perfused into the areas of the brain that are active while fMRI measures the oxygen depleted hemoglobin in the blood after it has been extracted by the active cells. Depending on the area of activation, the directional flow of blood, and the spatial resolution of the imaging technique, the difference in the centers of mass has been roughly one centimeter.
More information can be found in this study:
http://www.ncbi.nlm.nih.gov/pubmed/10191171?dopt=Abstract&holding=npg
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