Positron Emission Tomography (PET)
The PET technique can be used to investigate transport, diffusion and accumulation processes, especially in multi-phase mixtures. Therefore at least one of the substances of interest must be radioactively labeled with a positron emitting radionuclid. This is done be insertion of β+ active atoms into molecules of a non-radioactive precursor by means of a chemical synthesis. As PET nuclids (tracers) mainly C-11 is used for labeling of organic substances and F-18 for labeling of salts. Further, Cu-64 can be used. Due to the comparatively short half-life period of PET tracers the radioactive substances must be produced close to the experimental site. At the FZD this is possible at a cyclotron PET nuclid production facility. Maximum permitted working activites in our measurement laboratory are 1.6 MBq for C-11 and F-18, and 8 MBq for Cu-64. This enables the study of the dynamics of substance transport, diffusion, migration and accumulation process with a time resolution down to 1 s. Increasing the activity of up to 500 MBq is in principle possible, however, it requires improved radiation protection measures.
At the annihilation of positrons and electrons a characteristic 511 keV gamma radiation is produced, whereby each annihilation event generates two gamma quanta that are emitted diametrically, i.e. they escape from the loaction of the event at an angle of 180°. By means of a PET scanner device such annihilation events can be measured. Therefore, a multitude of gamma scintillation detectors is arranged about the volume of interest. A gamma quantum impinging on a scintillation detector generates an electrical signal after photoelectric conversion by a scintillator-photomultipier combination. By analysing the pulse hight of this signal scattered quanta that lost energy by the Compton effect can be excluded from the measurement. Further, a special coincidence electronics tests, if there are two simultaneous detector responses. Each coincidence is considered as an annihilation event and then the geometry of the active volume of the two registering detectors defines the spatial channel (line of response) where the annihilation event took place. By means of post-measurement image or volume reconstruction algorithms on a computer we can reconstruct the two- or three-dimensional distribution of the positron emitter from the set of LORs that has been recorded.
We have developed a special PET scanner for fluid-dynamic experiments. The device surrounds a cylindric measurement volume of 100 mm diameter and 1 m height. This can be, for instance, a vertical bubble column, a column reactor or another vessel filled, for instance, with liquid or particles. The device enables recording of annihilation radiation in 6 measurement planes with 16 detectors in each plane. A tomographic reconstruction is possible either for the complete volume or just within one or more planes. The measurement planes spacing can be 200 mm at minimum and a complete scan of the whole volume is possible, even with a small plane spacing, by moving the object with a motor driven vertical axis up and down as required. It is further easily possible to change the measurement geometry and adapt it to a particular measurement problem. The spatial resolution of the PET scanner is determined by the size of the BGO scintillation crystals (30 mm x 40 mm active area) and is between 10 mm and 15 mm in-plane. The axial resolution is essentially determined by the plane separation.
- Zippe, C.; Hoppe, D.; Fietz, J.; Hampel, U.; Hensel, F.; Mäding, P.; Prasser, H.-M.; Zippe, W.
Messung von Phasen- und Konzentrationsverteilungen in Blasensäulen mit Positronen-Emissions-Tomographie
Chemie Ingenieur Technik 77(2005) 10, 1581-1587
- Zippe, C.; Hoppe, D.; Fietz, J.; Hampel, U.; Hensel, F.; Mäding, P.; Prasser, H.-M.; Zippe, W.; Berührungslose Messung von Phasen- und Konzentrationsverteilungen in Blasensäulen mit positronenemittierenden Radionukliden, Wissenschaftlich-Technische Berichte, FZR-389, Forschungszentrum Rossendorf 2003