Corrected equations for susceptibility-induced T2-shortening.

Local field gradients created by paramagnetic entities shorten water proton relaxation times, particularly T2. This "novel" relaxation mechanism, now known as susceptibility-induced relaxation, was described twenty years ago by Gueron, and later extended by others to superparamagnetic particles which have a much larger magnetization. Unfortunately, because of subtle but significant errors, those results are valid only in the strict zero-field limit. These errors are corrected in the present article, and new versions of the relaxation equations are presented. The correction is shown to be significant, not only for transverse and longitudinal relaxation in aqueous superparamagnetic colloids, where the "Gueron" effect is known to be important, but even in some cases for transverse paramagnetic relaxation.