This auxiliary model simply divides the tissue TAC at each frame mid-time by the interpolated plasma activity at the same time and displays it as a curve called Tissue/Input curve Ratio.
Under equilibrium conditions this ratio is equal to the total distribution volume of tracer in tissue. Therefore, the plot of the Tissue/Input curve Ratio against time is a quick means to check whether the system has equilibrated (plot becomes constant) or not.
Note that an explicit blood volume correction is available via the vB input parameter. In the case of vB=0 (default setting), no correction is performed. For vB>0, the tissue TAC is corrected by the scaled whole blood activity before the actual analysis as follows.