PET/CT - Radiation Protection
Let us follow the course of F-18 from its production through application to degradation. F-18 is produced in cyclotrons. In Hungary four PET/CT devices operate, two out of which have their own cyclotrons; the isotopes to the other two devices are transported from these.
Let us examine a PET/CT centre with an own cyclotron. The cyclotron is equipped with strong shielding walls because of the very intense radiation generated during its operation. The environment of the cyclotron is constantly under radiation monitoring, the room is equipped with gamma and neutron detectors. All types of radioactive waste have a separate storeroom in the cyclotron room.
After the completion of the production of the F-18 isotope, it gets from the cyclotron to the synthesizer through a radiation shielded pipe system. Here semi-automatic devices perform the synthesis of the radiopharmaceutical so that there is as little contact with the staff as possible. When the production of the radiopharmaceutical (FDG) is complete, it gets to the dose calibrator through another shielded pipe. The dose calibrator is a device including a lead glass window, which automatically doses the activity appropriate for the examination. This is where the staff first gets in close proximity to FDG, since they transport it to the injection machine in a double-walled lead container. There is a shielded reception device designed specifically for this purpose and the injection is carried out by a machine, thus considerably decreasing the hand dose received by the staff. A separate container is reserved here for the remaining syringes, the lid of which opens in a rotating motion (vortex-free container), so that it does not stir up the air.
After injection, the patient is taken to a separate room where they have to wait one hour for the right activity distribution to form. During this time staff members have as little contact with the patient as possible. The patient receives all information before being injected. There are several separate rooms (all with appropriate radiation shielding walls) so that the patients do not receive further radiation from each other. No more than three patients are injected at the same time. Basically everything is signposted so that staff members have to give as few additional instructions as possible. The PET/CT device is separated from the staff with lead glass, this way they are protected during the examination while still seeing the patient perfectly. Although the PET/CT devices today are designed to prevent the patient from feeling claustrophobic, the direct visual contact is beneficial in case a problem occurs.
After the examination the patient has to wait a little while in another room. Basically it is advised that they should wait until they feel the urge to urinate. Since the kidneys excrete FDG, much activity accumulates in the bladder and by clearing it out into the toilet (active toilets must have separate drainage collectors) the patient may leave. The half life of FDG is 109.7 minutes, so no considerable activity can be expected in the street, seeing that the patient waited one hour for the activity distribution to form, the examination takes about half an hour and let us suppose that they spent another half an hour in the room created for this purpose. This way the patient can carry only half of the original activity and they also clear out part of it, so they do not pose a threat as far as radiation protection is concerned when they are out in the street. In spite of that, it is advised that the patient does not stay near children for a long time.
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