An investigation of the physical characteristics of ⁶⁶Ga as an isotope for PET imaging and quantification

Isotopes commonly used for PET imaging and quantification have a straightforward decay scheme involving 'pure' positron (β⁺) emission, i.e., 95%-100% β⁺ abundance, with no additional gamma rays. ⁶⁶Ga (E(max) =4.2 MeV, T( 1/4 ) =9.5 h) is a member of a category of isotopes with a lower abundance of β⁺'s (57%) and a more complicated spectrum involving combinations of gamma rays that are emitted in cascade. These additional gamma rays tend to cause a higher singles rate, resulting in more random coincidence events. The most abundant positron (51.5%) in the spectrum has one of the highest energies considered for PET imaging. For the purposes of monoclonal antibody dosimetry using ⁶⁶Ga, it is important to verify the quantification in phantoms prior to initiating human studies. A series of quantitative phantom measurements were performed on the PC4600, a head- optimized BGO based scanner with multiple detector rings. Count rate linearity was verified over concentrations ranging from 4.0 kBq/cc to 37 kBq/cc (0.11-1.0 μCi/cc); resolution averaged 16 mm full width half-maximum in the x and y directions 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 in tissue) was verified as a primary source of resolution degradation. Within the limits outlined above, ⁶⁶Ga is a suitable isotope for use as ⁶⁶Ga citrate or with monoclonal antibodies in the detection and staging of tumors and other lesions. In addition, the energetic positrons have possible therapeutic applications when used as a monoclonal antibody label.