Dual tracer single acquisition PET/CT scans utilizing 18F-NaF and 18F-FDG for oncologic applications

Objectives Dual tracer single acquisition (DTSA) PET/CT scans utilizing 18F-NaF and 18F-FDG in oncology have been utilized to simultaneously image for skeletal and soft tissue metastases. The method involves the injection of 18F-NaF plus 18F-FDG in a single syringe followed by a single PET/CT acquisition. The FDG-to-NaF ratio can be subjective, based upon the clinical indication. What is not well known is the optimal FDG-to-NaF ratio in terms of visual analysis, and the effect of DTSA upon the reference background liver SUV.

Methods We retrospectively analyzed patients referred for clinical reasons for DTSA PET/CT imaging. Head-to-toe imaging utilizing a total dose of 10 mCi was used, with 3D acquisition at 2.5 min/bed after a 90-minute uptake time. Using a qualitative visual estimate of scan quality, we determined the optimum FDG-to-NaF ratio. The average and maximum liver SUV was obtained, calculated based upon the injected FDG dose. The DTSA liver SUV was compared to both an unmatched group and a group matched for age, uptake time, and body mass index of patients who had undergone FDG only PET/CT.

Results There were 14 DTSA patients with FDG-to-NaF ratios ranging from 4.1:1 to 0.7:1. Visually, the optimum ratio was 3:1. The average liver SUV in DTSA patients was 1.85 +/- 0.31 versus 1.6 +/- 0.33 for the unmatched group (p < 0.02, n=114) versus 1.61 +/- 0.35 for the matched group (p=0.11, n=14). The SUV-max in DTSA patients was 2.28 +/- 0.39 versus 1.95 +/- 0.38 in the unmatched (p < 0.01, n=108) versus 1.98 +/- 0.40 in the matched group (p < 0.05).

Conclusions An FDG-to-NaF ratio of 3:1 was determined to be optimal for DTSA PET/CT. The reference background liver SUV appears to be slightly higher in DTSA compared to FDG-only scans