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Figure 1. (a) A point representing a sample of tissue in the p:q plane. The x axis corresponds to the isotropic component of diffusion (p) and the y axis the anisotropic component of diffusion (q). Any tensor can be decomposed into its p and q components po and qo, which will correspond to a point in the p:q plane. (b) Starting from a point in the p:q plane, we can deduce the standard anisotropy measures RA and FA using simple geometry. Both of these measures will be proportional to the angle 
, in fact RA is proportional to the tangent and FA proportional to the sine. (c) Two tissues will, in general, have different p and q components. Thus a tissue A with components pA and qA, will have a different location from a tissue B with components pB and qB. (d) A tissue A in general will have different p and q components at different times. By plotting these different components in the p:q plane we can obtain a trajectory that illustrates the evolution of tissue in time. In this example we see a trajectory demonstrating three time points for tissue A.
