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FIGURE 1. (A) Semilogarithmic plot of voxel 13NN activity vs. time in voxel exhibiting uniform (1 compartment) behavior. Initial activity A0 is proportional to regional perfusion 
0. Washout time constant 
corresponds to inverse of regional specific ventilation (s
r). Thus, area under curve is given by A0· = k·0/sr—that is, proportional to ratio of perfusion to specific ventilation. (B) Semilogarithmic plot of voxel 13NN activity vs. time for voxel exhibiting 2-compartment behavior. Tracer washout shows fast compartment with initial activity A0f proportional to its respective initial perfusion fast and slow compartment with initial activity A0s proportional to its initial perfusion slow. Hatched and dotted areas indicate values of k·/sr corresponding to each of those compartments.
