Relationship of 99mTc-MDP uptake to regional osseous circulation in skeletally immature and mature dogs

Uptake of intravenously injected ^99mTc-MDP in multiple regions of healthy skeletally immature and mature dogs was correlated with regional chondro-osseous blood flow determined by radioactively labeled microspheres. Compared to the microsphere distribution, the distribution of ^99mTc-MDP indicated that blood flow is an important, but not exclusive, factor in the uptake of bone-seeking tracers. Other factors such as the affinity of the tracer for the various types of chondro-osseous tissues must also affect tracer uptake. A measure of the relative affinity of ^99mTc-MDP for bone was derived by the ratio of the percentage of tracer uptake to the percentage of blood flow. The juxta-ephyseal region demonstrated the greatest affinity for the tracer, followed in decreasing affinity, by the cortical bone, epiphyseal cartilage, trabecular bone of the metaphysis and secondary ossification center, and marrow space. Within the spongiosa, affinity generally increased as the proportion of osseous trabeculae relative to marrow space increased. ^99mTc-MDP uptake is disproportionately increased in areas of active bone growth and remodeling where the surface area of hydroxyapatite crystals available for adsorption is probably highest.