168 Part II Chapter 6 very labor-intensive in representative FD populations. Our method of only including FD-attributable uptake does effectively exclude OA in nearly all cases. In FD-lesions involving joints or bone directly next to the joint, however, some measured uptake caused by OA could not be ruled out entirely. With respect to volume of distribution, we consider our finding that SUV was better normalized when compared to SUL somewhat surprising, as the salt Na18F is watersoluble and not lipid-soluble and LBM is a better representation of body water content than BW [25]. Both the established SBS on planar bone scintigraphy and SUVpeak and TLF using Na[18F]F-PET/CT are useful adjuncts to quantify baseline FD-burden. The weak correlation between the well-known SBS and TLF as well as FTV on Na18F-PET that we found, suggest a (partially) different underlying mechanism quantified by these parameters. As reported in previous studies, SBS was correlated to FGF-23 in our current cohort [2, 5]. Since FGF-23 is produced by FD lesions and thus reflects presence of FD, its relation to a static biomarker of disease burden, such as SBS, is not surprising. We observed a clear correlation between TLF and FGF-23 and quantitative imaging biomarkers were strongly related to dynamic serum biomarkers of bone turnover, ALP and P1NP. These highest associations were found for TLF by Na[18F]F-PET/CT. In our study, neither planar SBS or Na[18F]F-PET/CT correlated with the bone resorption marker CTX, with two probable explanations. First, SBS and Na18F-PET depict osteoblastic instead of osteoclastic activity, as both Na18F and 99mTc-HDP bind young osteoid [25]. Second, eighteen out of twenty patients used bisphosphonates, thus inhibiting the activity of osteoclasts and possibly influencing our results [5]. Generally, FD localization and its volume seems more important than its activity, as SBS and TLF both strongly correlate to FGF-23 and TLF strongly correlates to ALP and P1NP, whereas SUVpeak only shows a relatively weak correlation to ALP. Moreover, FTV and TLF (= FTV x mean SUV) showed near-perfect correlation, underlining the marginal influence of the Na18F (mean) SUV in the FD-lesions in relation to volume. There was no significant correlation between SUVpeak and SBS. We observed no correlation between functional imaging and general pain scores, as measured with the BPI. We did observe a clear correlation between higher BPI-scores on the domain interference with normal work with the SBS and TLF on Na[18F]F-PET/CT, suggesting that higher skeletal burden or higher TLF in the individual patient possibly does not result in significantly higher pain-scores but does affect daily working routine since patients are disabled. Furthermore, we found