Estimation of parameters affecting the uptake of 99mTc-methylenediphosphonate in rat femur with model simulation

The uptake of 99mTc-methylenediphosphonate (MDP) in different parts of rat femur was simulated using a local three-space model for tracer transfer. The model consisted of bone blood, bone ECF-space and space for tracer deposition. The measured 99mTc-MDP concentration in the systemic blood and the local bone blood flow measured by 131I-macroaggregated albumin microspheres were used as input parameters. The measured blood flow values were 6.3, 3.1 and 15.3 ml/100 g/min for proximal, middle and distal femur, respectively. The model parameters that gave the best fit to measured 99mTc-MDP uptake curves in computer simulation showed that bone blood flow, volume of ECF-space, permeability surface area product and accretion constant from ECF-space to space for tracer deposition were highest in distal and lowest in middle femur. The values corresponded to peak extraction fractions of 0.38, 0.62, and 0.31 for proximal, middle and distal femur, respectively. We conclude that the simulation gives acceptable model parameters, and indicates applicability of a similar model into clinical quantitative bone scintigraphy.