Studies of skeletal tracer kinetics. IV. Optimum time delay for Tc-99m(Sn) methylene disphosphonate bone imaging

The kinetics of the skeletal tracer Tc-99m(Sn)methylene diphosphonate (Tc-MDP) were studied in four older patients without significant bony disease by means of compartmental analysis. Solution of the model was obtained by digital computer techniques. Tracer concentrations for "target" and "background" areas (to reflect "bone" and "soft-tissue" regions) as time functions were generated for three different imaging criteria: counting rate, a figure of merit, and contrast. The figure of merit reached a maximum early in the second hour after dose administration, but contrast was found to attain a plateau at 6 hr. Either of these criteria is in accord with recommendations in the literature for optimal imaging delay time with Tc-MDP. Bone biopsy in one patient was in good agreement with the model's prediction. Compartmental analysis provides an objective method for determining the optimal imaging delay time, depending on the criterion chosen. The method can be used for any radiopharmaceutical if the compartmental model is known and can be solved.