Toward small, stable, and physically robust radiotracers for positron imaging

• Alginate-based ionic hydrogels were fabricated into novel tracer materials using a simple and customisable method. • The type of alginate and gelling cation was used to vary the physico-chemical, mechanical, and radiochemical properties of tracer materials. • A unique evaporative radiolabelling procedure enhanced labelling efficiency and optimised radiochemical performance. • The tracers enabled successful positron emission particle tracking in a representative experimental set-up, demonstrating their practical utility. Positron-based nuclear imaging plays a critical role in both nuclear medicine and flow visualization studies of engineering systems. Common techniques such as Positron Emission Particle Tracking (PEPT) and Positron Emission Tomography (PET) depend on the reliable radiolabelling of materials with suitable radioisotopes, achieving high activation efficiency and desirable tracer characteristics. However, the radiolabelling of solid particulates presents significant challenges. This study presents the first stage in the formulation, fabrication, and radiolabelling of novel “designer” tracers aimed at overcoming these limitations. It was found that the use of a porous alginate-based hydrogel combined with an evaporative radiolabelling technique enables the fabrication of small, mechanically robust tracers with high specific activity and radiochemical stability. The facile synthesis procedures employed also allow for customization of the formulation and production method to achieve tailored tracer characteristics, while evaporative radiolabelling minimises the handling issues associated with small particulate precursor materials. The best-performing tracer, a yttrium-alginate hydrogel formulated from a low M/G ratio source of sodium alginate, was highly mechanically and chemically stable. It also exhibited favourable properties when radiolabelled through evaporative methods, including an average radiochemical yield and radiochemical stability of 82.8%, and 97.1%, respectively, and a maximum single-tracer activity of 151 MBq on a 683 μ m particle. The ability of this novel formulation to provide high quality PEPT data was demonstrated and compared to conventional tracer materials.