T(2)-shortening by strongly magnetized spheres: a chemical exchange model.

It is shown that a chemical exchange model can reproduce nuclear magnetic relaxation caused by diffusion of water molecules near strongly magnetized particles. The agreement is based on the similarity (but not equivalence) of the respective "visit-limiting" mechanisms in the echo-limited regime. The model leads to a single equation that predicts relaxation behavior in both the motional-averaging and visit-limited regimes.

On T(2)-shortening by strongly magnetized spheres: a partial refocusing model.

Computer simulations of water transverse relaxation induced by superparamagnetic particles are shown to disagree with the available theories, covering the slow diffusion domain. Understanding these new simulations, not in the slow diffusion domain, thus requires a new theoretical approach. A "partial refocusing model" is introduced for this purpose; it is based on a spatial division between an inner region where the gradients are too strong for the refocusing pulses to be efficient and an outer region where they are efficient.