SPECT - Gamma Camera - Radiation Protection

In some respects SPECT devices and gamma cameras are more advantageous compared to PET, in other respects they are less favourable. The energy of the radiopharmaceuticals used in them is much lower than 511 keV, thus their penetration ability is lower than that of the annihilation photon generated after the decay of FDG. However, their half lives are longer than that of FDG, which is 109.7 minutes. The most frequently used SPECT isotope is Tc-99m, the gamma energy of which is 140 keV and its half life is 6 hours. It is produced in technetium generators containing Mo-99. Mo-99 decays to Tc-99 with a half life of 67 hours. Mo-99 can be produced in reactors by neutron activation. The generators can be kept on site (in the hospital), close to the place of examination. They are strongly shielded, so they can be kept behind a radiation hazard sign in themselves, without further radiation protection. The Tc-99 can be obtained from the generator by a process called milking: the generator can be washed by placing a vacuum vial on one side and a vial containing isotonic saline solution on its other side, thus the Tc-99 isotopes can be extracted from the generator.

Figure 1. Technetium generator

 
Because of the lower energies, in such examinations the staff cares directly for the patients even after injection, and patients can wait close to each other, they are not separated as those who wait for a PET scan. The generator and the injection room are as close to each other as possible, but due to their low energy, the isotopes are transferred from the generator to the injection room with relatively weak lead shielding and they are injected manually. The assistants control the gamma camera directly from beside the patient. In the case of SPECT, similarly to a PET scan, there is lead glass shielding between the patient and the specialist conducting the examination. There have been examples of a mother sitting next to her child during the child’s reflux examination throughout the entire duration of the SPECT study. This would be impossible in the case of a PET scan.

Moreover, those who visit the SPECT study and the gamma camera studies (e.g. medical students, medical physicists) can stay right beside the patient, while in case of a PET scan they must stay behind the lead glass with the entire group of assistants.