Figure 2a:

Diagram illustrates two models used to calculate GFR from MR GBCA (Gd) concentration curves. (a) With MR whole-kidney (WK) model, arterial input function (AIF) (ie, GBCA-concentration in aorta) is convolved by using impulse retention function (IRF) of whole-kidney parenchyma to fit whole-kidney parenchymal renogram. The best fit determines the values of the unknown parameters of the IRF, one of which is the GFR. (b) With MR corticomedullary model, arterial input function is convolved by a cortical (Cx) IRF and a medullary (Med) IRF, which are then fitted to the cortex and medulla renograms, respectively.