From the relaxivity of Gd(DTPA)2- to everything else.

We resolve the observed magnetic field-dependent relaxivity of Gd(DTPA)2- into inner and outer sphere contributions by comparison with theory. After noting that the outer sphere part, which contributes comparable to both 1/T1 and 1/T2, is identical to the "susceptibility" effect, we demonstrate the transition from the outer sphere relaxivity of small complexes, e.g., Gd(DTPA)2-, to that of large magnetic particulates, e.g., magnetite, used as T2 agents. This transition is expressed as a shift from a quantum to a classical mechanical description of relaxivity. We next show how to extrapolate the inner sphere contribution of Gd(DTPA)2- to similar small agents with greater structural symmetry, e.g., Gd(NOTA)-, by lengthening the electronic relaxation time and to ternary macromolecular complexes of small agents with protein by lengthening the rotational relaxation time. Finally, we indicate how relatively large lipid vesicles containing either small paramagnetic complexes or magnetized particulates may be considered analogs of Gd(DTPA)2-, with analogous inner and outer sphere contributions.