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Fig 5. A basic model of the imaging voxel for permeability studies. The plasma volume, vp, is small compared with the EES (shown in gray) and the intracellular space, from which contrast is excluded. Observed relaxivity changes are a weighted average of the plasma and EES contrast concentration. The large black arrows signify isodirectional flux of contrast between these 2 spaces, with a rate constant equal to Ktrans. This flux is essentially zero in the presence of an intact BBB but is nonzero in many pathophysiologic conditions.
