Comparison of six quantitative methods for the measurement of bone turnover at the hip and lumbar spine using ¹⁸F-fluoride PET-CT

Aim
The aim of this study was to evaluate the relationship between different quantification methods used for the measurement of bone plasma clearance (K_i) using ^18F-PET at the hip and lumbar spine.

Methods
Twelve healthy postmenopausal women aged 52–71 years were recruited. Each participant underwent 60-min dynamic ¹⁸F-PET scans at the lumbar spine and hip on two separate occasions with an injected activity of 90 and 180 MBq, respectively. Image-derived input functions were obtained at the aorta from the lumbar spine scans. K_i was evaluated using a three-compartment four-parameter model (K_i-4k), three-compartment three-parameter model (K_i-3k), Patlak analysis (Ki-Pat), spectral analysis (K_i-Spec) and deconvolution (K_i-Decon). Standardized uptake values (SUVs) were also measured.

Results
The Pearson correlation between K_i-4k and K_i-3k, K_i-Pat, K_i-Spec, K_i-Decon and SUV were 0.91, 0.97, 0.94, 0.95 and 0.93, respectively, with a significance of P less than 0.0001. The differences between the correlations measured using Fisher’s Z-test were not significant (P>0.05). Bland–Altman analysis showed that the limits of agreement for K_i measured as the SD of the differences were 0.0082 (25.9%), 0.0062 (11.7%), 0.0098 (20.1%) and 0.0056 (25.5%) ml/min/ml, respectively, and the biases were −0.0081 (−23.8%), −0.0075 (−23.7%), −0.0107 (−29.5%) and −0.0015 (0.8%) ml/min/ml, respectively.

Conclusion
All five methods of quantification (K_i-3k, K_i-Pat, K_i-Spec, K_i-Decon and SUV) strongly correlated with K_i-4k. Although systematic differences of up to 29% were found between K_i-4k and the other methods (K_i-3k, K_i-Pat, K_i-Spec and K_i-Decon), these should not affect the conclusions of clinical studies, provided the methods are applied consistently. However, care should be taken when comparing reports that use different methods of quantification.