Development of PET

Positron emission tomography is an imaging technique in which the patient is given an isotope that undergoes positron decay, and after a reasonable time (~1 hour) the isotope accumulates in the regions of the body we wish to examine. 511-keV gamma photons can be detected, which are formed when an electron and a positron meet and annihilate. The photons are emitted at the place of the annihilation at the same time and at (almost accurately) 180° to one another. Since the photons appear simultaneously, they can be detected in coincidence, which means that it is not necessary to use collimators to indicate the line along which the decay can occur, as it is obvious that the decay could only happen along the line connecting the two detectors in coincidence. This is the ideal situation, of course, as a number of other processes affect detection, these will be further discussed in subsequent chapters.
One of the greatest difficulty when applying positron emission tomography is the fact that isotopes with short half-lives are used during examinations. The most common PET-radiopharmaceutical, FDG (fluorodeoxyglucose) is traced by F-18, the half-life of which is 109.5 minutes, and which can only be produced in a cyclotron. Therefore, producing the isotope and conducting the examination in the same place is the most favourable. In Hungary, two out of the four PET devices can operate this way: the one in the PET centre in Budafoki street, Budapest, and the one in Debrecen. No cyclotron belongs to the devices is Kecskemét and in Amerikai street, Budapest, thus they have to use one of the external cyclotrons in order to conduct FDG examinations.
Medical cyclotrons first appeared in hospitals at the beginning of the 1960s (London, St. Louis). By the 1970s Anger’s positron emission cameras were replaced by multidetector devices, but before the early 1980s computers were not sufficiently fast so that PET could be used routinely as a diagnostic device.
Important dates: 1978: the first gated PET image, 1979: the first brain image, in 1996 there were 160 PET devices altogether in the world, in 2000 the PET-CT device was introduced (Townsend and Nutt). New detector materials appeared that to some extent facilitated the production of PET devices (LYSO, LSO), by 2005 more than 300 devices were known about around the world. In 2008 the first MR-compatible PET device appeared (it was suitable for examining small animals, University of California). In 2009 a PET-MRI suitable for conducting human brain examinations was constructed in Jülich. The total dose load of a complete PET-CT examination is ~20mSv (the annual dose limit for radiation workers), which is approximately eight times higher than that of the annual background radiation, so it cannot be neglected. By now the sensitivity of PET has reached an outstanding level, which is illustrated by the image below.

Whole-body PET-CT scan

 
The most important PET isotope, as mentioned above, is FDG. For examining myocardial perfusion C-11 bound to acetate (half-life: 20.3 minutes) and Rb-82 (half-life: 1.25 minutes) are used. O-15 (half-life: 124 s) and N-13 (half-life: 10 minutes) are also commonly used PET isotopes. The magnetic field in cyclotrons used in practice (a device originally described by a Hungarian physicist, Sándor Gaál in 1929; however, Lawrence is considered to have invented it because he patented it) is 1.5 T, their diameter can reach 1.5 m and they can accelerate protons to energies as high as 15 MeV. Cyclotrons are very expensive with considerable radiation protection and radiation shielding.