3-D Printing of Calibrated Germanium-68 PET Phantoms Without Inactive Walls

We aimed to print a solid-state germanium-68 phantom for positron emission tomography (PET) with excellent control of the activity concentration in the phantom material, with safe handling characteristics, and without inactive walls. To this end, we developed a novel method for incorporating germanium-68 into hydrophobic stereo-lithography monomers. The spontaneous phase transfer process provides complexation and induces phase transfer of aqueous ionic germanium-68 into the hydrophobic monomers, ensures homogenous isotope distribution in the rigid polymer matrix, and prevents leaching in aqueous environments. With this method, we additively manufactured radioactive spheres following the International Electrotechnical Commission design with the addition of a small sphere of 7.7-mm diameter. Extended leaching testing demonstrated near perfect source tightness. The same phantom can be repeatedly used in the quality management of multicenter clinical trials, without preparative errors. Long-lived solid-state phantoms produced with this method will provide consistent and reproducible validation of total body PET/CT systems. PET images with these phantoms acquired within a decaying gallium-68 background at different signal-to-background ratios showed homogenous activity distribution and no fill-related artefacts. The varying background allowed the assessment of trial-specific acquisition protocols that match a given clinical question much better than today’s predefined standard protocol.