Symposium on Positron Emission Tomography

There are two distinct advantages when using positron emitting radionuclides for radiotracer studies. The first is that the commonly used PET tracers are all biogenic elements with low atomic numbers and short half-lives: O = 123 s, N = 9.8 min, C = 20 min, F = 110 min. These radionuclides provide an ideal starting point for the production of a variety of tracers that are either natural substrates, such as methionine or glucose, or analogues, such as fluorodeoxyglucose. All of these lowZ nuclides require a cyclotron for their production, which, due to the short half-lives involved, needs to be situated close to where the imaging studies are to be performed. The second advantage results from the physics of the imaging process. The PET scanner detects two anti-parallel 511 keV photons that result from the annihilation of the positron with a nearby electron (annihilation coincidence detection). If both photons are detected within a short time of one another, it is assumed that they originated from the same nuclear disintegration, and that the original disintegration occurred somewhere along the line joining the two detection points. The images that can be obtained using positron emitters are in general of higher quality than can be obtained from single photon emitting radionuclides. The combination of low-Z radionuclides and highquality imaging makes possible quantitative measurements of physiological and biochemical processes in vivo, in absolute units. For example, blood flow in ml×min×g or metabolic rate in mol×min×g. Until fairly recently, such measurements have been the endpoint for most studies performed worldwide, usually for research purposes. For most clinical PET applications, however, studies are almost always simply an image of tracer uptake and are in most cases performed with Flabelled deoxyglucose (FDG). The annihilation coincidence detection principle was first suggested by Brownell and Sweet in 1953 [1], and the first tomographic PET scanner was described by TerPogossian et al. in 1975 [2]. In 1979, FDG PET was described by Reivich et al. [3], who developed the work of Sokoloff et al. [4] with C-labelled deoxyglucose. The reason for the very long time it has taken for PET, and in particular clinical PET, to become more widely distributed is largely due to the considerable infrastructure required to support it. To perform PET scanning, even with a very limited number of tracers, requires a cyclotron, a radiochemistry laboratory, a scanner and computing facilities, and a contingent of qualified staff to operate the facility. The total capital outlay is therefore a minimum of £1±2 million plus considerable running costs. More recently, as proven clinical applications of PET based on FDG have emerged, different arrangements have become feasible. For example, FDG can be produced by a regional distribution centre and transported to scanning centres within a 2 h radius. This allows simple clinical FDG scans to be performed in a conventional nuclear medicine department with little additional infrastructure other than the PET scanner itself. In response to this development, manufacturers are developing a variety of low-cost dedicated PET and hybrid PET/SPET scanners in an effort to make PET scanning available at a cost comparable to that of a standard gamma camera.