MicroPET imaging with nonconventional isotopes

The utilization of new positron emitting isotopes for position emission tomography (PET) imaging raises several questions about their ability to provide images of good quality and to perform accurate quantification. This issue is even more pertinent when using high-resolution scanners designed for the imaging of small animals. At Washington University, we are currently producing a whole array of positron emitters; some of them, like Ga-66 and Br-76, emit high-energy positrons and prompt gamma rays that affect spatial resolution and increase the random coincidence contribution. We have now started to evaluate these isotopes in terms of their ability to perform high-quality imaging. Spatial resolution measurements were evaluated using the Concorde MicroSystem Inc. microPET-R4 camera. Electron transport calculations have been performed and compared with experimental data. They revealed that for this camera, the detector size is still the limiting factor on resolution for isotopes emitting low-energy positrons like F-18 and Cu-64. The transaxial resolution was measured to be around 2 mm at the center of the field of view (FOV) for these isotopes. The dominant factor becomes the positron range for other isotopes like Cu-60 and Tc-94 m, with transaxial resolution of 3.5 and 4.3 mm, respectively. Due to the long tail of the positron range distribution, a strong contrast reduction is observed. In this paper, experimental data on spatial resolution will be presented for a number of nonconventional PET isotopes, and consequences on image quality will be discussed.