AN INVESTIGATION OF THE PHYSICAL CHARACTERISTICS OF GA-66 AS AN ISOTOPE FOR PET IMAGING AND QUANTIFICATION

Isotopes commonly used for PET imaging and quantification have a strai ghtforward decay scheme involving ''pure'' positron (beta(+)) emission , i.e., 95%-100% beta(+) abundance, with no additional gamma rays. Ga- 66 (E(max)=4.2 MeV, T-1/2=9.5 h) is a member of a category of isotopes with a lower abundance of beta(+,)s (57%) and a more complicated spec trum involving combinations of gamma rays that are emitted in cascade. These additional gamma rays tend to cause a higher singles rate, resu lting in more random coincidence events. The most abundant positron (5 1.5%) in the spectrum has one of the highest energies considered for P ET imaging. For the purposes of monoclonal antibody dosimetry using Ga -66, it is important to verify the quantification in phantoms prior to initiating human studies. A series of quantitative phantom measuremen ts were performed on the PC4600, a head-optimized EGO based scanner wi th multiple detector rings. Count rate linearity was verified over con centrations ranging from 4.0 kBq/cc to 37 kBq/cc (0.11-1.0 mu Ci/cc); resolution averaged 16 mm full width half-maximum in the x and y direc tions in both the direct and cross planes. Axial resolution was 14 mm. The range of the energetic positrons (up to 4.153 MeV, range 7.6 mm i n tissue) was verified as a primary source of resolution degradation. Within the limits outlined above, Ga-66 is a suitable isotope for use as Ga-66 citrate or with monoclonal antibodies in the detection and st aging of tumors and other lesions. In addition, the energetic positron s have possible therapeutic applications when used as a monoclonal ant ibody label. (C) 1997 American Association of Physicists in Medicine.