Bone metabolism at the spine and hip measured using 18F-PET

Introduction: The imaging technique of 18F-fluoride positron emission tomography (18F-PET) allows the non-invasive assessment of regional bone metabolism at clinically important sites such as the hip and lumbar spine which is not possible using conventional techniques. The aim of the study was to compare the differences between regional bone metabolism (Ki) and standardized uptake values (SUV) (a semi-quantitative index reflecting bone metabolism) at the hip and lumbar spine.

Methods: Twelve healthy postmenopausal women aged between 52 and 70 years, bone mineral density (BMD) T-score ranging from -2.9 to 1.4 at the lumbar spine and from -2.4 to 1.7 at the hip. None had any history of metabolic bone disorder (except untreated osteoporosis in the spine) or had taken any medications that affected skeletal metabolism. Dynamic 18F-PET scans at the lumbar spine and hip were performed on two separate occasions within a 14 days period. The Ki values obtained using the Hawkins 3-compartment model and SUV were compared using Wilcoxon
paired signed rank test.

Results: The Ki values at the lumber spine (mean of L1-L4), total hip outer (including the trabecular and cortical bone), total hip inner (including only the trabecular bone), femoral neck, and cortical shaft (including only cortical bone) were 0.042±0.008, 0.013±0.005, 0.015±0.006, 0.015±0.005, and 0.009±0.002 ml/min/ml respectively. The SUVs were 6.0±1.5, 2.3±0.7, 2.6±1.0, 2.7±1.1 and 1.8±0.4 g/ml respectively. The differences for Ki and SUV at spine and any other hip region were significant (p0.05).

Conclusion: This is the first study directly comparing regional bone metabolism at the spine and hip. We conclude that the bone turnover and SUVs are higher in trabecular bone than in cortical bone. This study showed that regional bone metabolism is on average three times greater at the lumbar spine compared to the hips. The skeletal heterogeneity in terms of bone metabolism is likely to be due (but not limited to) variations in proportion of trabecular and cortical bone, bone perfusion, loading patterns between these sites and the amount of characteristics of haematopoietic tissue.