Differences in Skeletal Kinetics Between Vertebral and Humeral Bone Measured by 18 F‐Fluoride Positron Emission Tomography in Postmenopausal Women

We have sought to investigate regional differences in skeletal kinetics between lumbar vertebrae and the humerus of postmenopausal women with 18 F‐fluoride positron emission tomography (PET). Twenty‐six women, mean age 62 years, had dynamic PET scans of the lumbar spine and lower humerus after the injection of 180 MBq 18 F‐fluoride ion. Plasma arterial input functions (IFs) were calculated from a mean IF measured arterially from 10 women and scaled according to late individual venous activity. Vertebral and humeral time activity curves were measured by placing regions of interest (ROI) over lumbar vertebrae and the humeral shaft. Using a three‐compartmental model and nonlinear regression analysis the macroconstant Ki , representing plasma clearance of fluoride to bone mineral, and the individual rate constants K 1 (related to regional skeletal blood flow) and k 2 to k 4 describing transport between plasma, an extracellular fluid compartment and a bone mineral compartment, were measured. Mean vertebral Ki (3.47 × 10 −2 ml · min −1 · ml −1 ) and K 1 (1.08 × 10 −1 ml · min −1 · ml −1 ) were found to be significantly greater than humeral Ki (1.64 × 10 2 ml min −1 ml −1 ; P < 0.0001) and K 1 (3.90 × 10 −2 ml · min −1 · ml −1 ; P < 0.0001) but no significant differences were found in k 2, k 3, and k 4. These findings confirm differences in regional skeletal kinetics between lumbar vertebrae and the lower humerus. These observations may help increase our understanding of the regional differences in pathophysiology and response to treatment that have been observed in sites consisting predominantly of either trabecular or cortical bone. 18 F‐fluoride PET may prove to be a valuable technique in the noninvasive measurement of regional skeletal metabolism.