¹⁸F-Sodium Fluoride (NaF) PET/CT in obese patients on LYSO PET/CT system: Patient dosimetry, optimization of injected activity and acquisition time

Abstract

Purpose: ¹⁸F-Sodium fluoride (NaF) PET/CT has a rapid single-pass extraction and fast clearance from soft tissues resulting in a better target to background ratio. This study aims to establish the optimum acquisition time and dosimetry of ¹⁸F-NaF PET/CT to evaluate bone metastases in obese patients. The secondary objective was to evaluate the impact of acquisition time on image quality, lesion detection rate, noise level, and radiation burden in this patient group. Material and Methods: A total of sixty patients were included in the study (20 patients with body mass index (BMI) 30-35 kg/m2, 20 patients with BMI 35-40 kg/m2, and 20 patients with BMI >40 kg/m2). Images were acquired after intravenous (IV) injection of 0.06 mCi/kg (2.2 MBq/kg) ¹⁸F-NaF. Data was acquired in list mode using ordered subset expectation maximization (OSEM) reconstruction. The raw data could be re-binned to simulate scans with acquisition times of 2, 2.5, and 3-minutes per bed position. Scans were visually analyzed by two observers and scored by rank against a panel of parameters (overall image quality [IQ], noise level, background soft tissue, and lesion detectability), and the contrast-to-noise ratio (CNR) was calculated. Results: Mean CNR for OSEM with 2min/bed is 20.19 (±8.39), 2.5min/bed 21.03 (±8.35) and for 3.0min/bed 22.16 (±8.37). There were no statistically significant differences in CNR between different OSEM acquisitions durations (P>0.05). Lesion delineation was excellent and independent of the duration of acquisition. All relevant lesions could be identified with three acquisition settings tested in this study. Patients were injected a mean activity of 215.4±31.3 MBq with estimated mean effective absorbed doses of 4.09±0.59mSv for ¹⁸F-NaF PET and 7.88±1.66 mSv for CT alone. Conclusion: ¹⁸F-NaF PET/CT can be beneficial in obese patients due to its better pharmacokinetics. Optimal osseous staging can be achieved with relatively low doses and radiation burden. Lesion delineation was excellent regardless of the various acquisition times assessed. However, it is recommended to do 3min per bed position acquisition in patients with BMI >40.