Aims

To study the effect of reduced injected dose of the tau PET tracer [¹⁸F]RO948 on standardised uptake value ratios (SUVR) to determine a lowest injected dose that allows for accurate in vivo quantification of tau burden.

Methods

Participants were examined on a Siemens Biograph mCT flow scanner for a 20-minute list-mode acquisition, performed 70 minutes post-injection of 370 MBq [¹⁸F]RO948. Injected dose was manipulated from the list-mode data using 75%, 50%, 25% and 10% of the total injected dose. Reconstructions were performed with scatter, attenuation, random and dead-time corrections.

The inferior cerebellum grey-matter was used to calculate SUVR with each subject yielding SUVR for composite region-of-interests (ROIs) corresponding to Braak regions of tau pathology.

Results

(1) Relative change as function of injected dose

(2) Images at different radioactive dose

Dose-dependent changes were greatest in the Braak I/II ROI in which our participants showed highest tracer retention. Initial results show on average less than two percent change in SUVR when reducing dose from 100% to 50%. A more significant change was observed at 1mCi. The variance increased when lowering the dose. Images remained visually comparable at the 50% injected dose to 100% suggesting these images could still be visually assessed in a clinical setting.

Conclusions

Our findings suggest that a 50% lower injected tracer-dose yields robust tracer retention signal. To further validate these results, we will replicate our analyses in data from Alzheimer’s disease patients from the Swedish BioFinder study in order to compare the diagnostic value of reduced-dose and full-dose [18F]RO948 PET scans.