Straightforward PET phantom preparation using ⁶⁸Gallium diluted with acid

The present study aimed to establish a user-friendly practical way to prepare phantoms for routine quantitative ⁶⁸Gallium (Ga) PET.

We produced gallium ions in 0.05 M HCl from a ⁶⁸Ge/⁶⁸Ga generator and prepared a phantom in five ways. We directly diluted [⁶⁸Ga]Ga³⁺ in water (i), neutralized [⁶⁸ Ga]Ga³⁺ in NaOH then diluted it with water (ii), labeled prostate-specific membrane antigen (PSMA)-617 with [⁶⁸Ga]Ga³⁺ then added it to water (iii), and directly added [⁶⁸Ga]Ga³⁺ to 0.05 M HCl (iv) and 0.1 M citric acid (v). A cylindrical phantom was scanned for 60 min in list mode, and images were reconstructed into consecutive 5-min frames. Temporal changes in spatial uniformity and image noise were evaluated based on the standard deviations (SD) of inter-ROI differences, coefficients of variation (CV), SUV_mean, and the fraction of pixels outside the SUV range of 0.95-1.05.

The visible distribution of activity in the phantom prepared using method (ii) was not uniform with peripheral accumulation, and spatial uniformity progressively deteriorated over time. The SD increased from 0.0206 to 0.0361 at 5 and 60 min, respectively. In contrast, the phantoms prepared using the other four methods yielded visually homogeneous images and the methods (iii), (iv), and (v) maintained relatively stable SDs throughout image acquisition. The CV increased modestly over time under all conditions, and was consistent with decreasing count statistics, whereas SUV_mean remained stable with only small deviations from unity. Pixel-wise analysis showed higher fractions of pixels outside SUV 0.95-1.05 for methods (i) and (ii).

Diluting [⁶⁸Ga]Ga³⁺ with 0.05 M HCl or 0.1 M citric acid enabled reliable preparation of a uniform phantom with spatial uniformity comparable to that of radiolabeling. Direct dilution in water showed no apparent peripheral accumulation on visual assessment, but quantitative analyses suggested less stable uniformity than acidic dilution. These approaches are practical for quality assurance and cross-calibration in quantitative ⁶⁸Ga PET.