Positron Emission Tomography
Molecular imaging using positron emission tomography (PET) allows the non-invasive, accurate quantification of molecular amounts of matter in the brain of living human beings. This is achieved by radiolabeling a drug (“radiopharmaceutical”, “tracer”, “radiotracer”) that specifically binds to a target molecule of interest (for example receptor, transporter). Radioligands for nuclear medicine imaging are labeled with isotopes that decay by emitting positrons. In particular, atoms containing more protons than neutrons in their nucleus break down by positron emission (β+ decay). A positron is a positively charged electron (e+) that, after traveling a few millimeters in the tissue, decays into 2 photons (gamma quanta) with its antiparticle, the electron, in an annihilation reaction (so-called annihilation radiation).
Highly sensitive scanners have been developed to image the distribution and fate of such radioactively labelled molecules within living subjects. They are applied – usually by intravenous injection – to healthy subjects and patients, and their uptake, metabolism, catabolism and excretion can be observed and analyzed from outside the body with great precision.
Advantages / disadvantages compared to other methods
Nuclear medicine methods like PET have the advantage of considerably higher sensitivity compared to magnetic resonance (MR) tomographic imaging methods. They allow the quantification of extremely low amounts of substance in a concentration of 10-9 M to 10-12 M. However, compared to MR-based methods, they have a somewhat lower spatial and a considerably lower temporal resolution. PET/MR hybrid scanners combine the advantages of both methodologies and open new avenues for brain research.
Risks / Limitations
PET is associated with a radioactive exposure. This, however, is considered relatively low due to the short half-life of the applied isotopes (usually between 2 and 110 minutes). For example, the radiation exposure for a PET/MR examination with the D2 dopamine receptor ligand [18F] fallypride, which is being used at the CIMH, involves a maximum of 5 millisieverts (mSv) and thus corresponds to slightly more than twice the average natural radiation exposure of humans per year (which is 2.1 mSv according to the Federal Office for Radiation Protection, www.bfs.de). Depending on place of residence, food and living habits, the radiation exposure varies however, even in Germany, between 1 mSv and 10 mSv. In addition, for the quantification of a few radiotracers, which are used for research purposes, withdrawal of arterial blood is necessary.
Prof. Dr. Gerhard Gründer, Department of Molecular Neuroimaging
Zentralinstitut für Seelische Gesundheit (ZI) - https://www.zi-mannheim.de