FIGURE 2.  (Top) Simplified 2-tissue-compartment model for labeled leucine, with k₂ representing effective loss of ¹¹C tracer from tissue and k₃ representing effective incorporation of L-[1-¹¹C]-leucine into protein. Tracer concentration in combined free and metabolic compartment is denoted as C_f+m, and tracer concentration in protein pool is denoted as C_b. (Bottom) Simplified 2-tissue-compartment model for unlabeled leucine, with k_rec representing recycled leucine as product of brain protein breakdown. Models describing L-[1-¹¹C]-leucine and unlabeled leucine differ only with respect to k_rec, which is zero for L-[1-¹¹C]-leucine.