Aqueous urea as a model system for bi-exponential relaxation.

OBJECT: To evaluate the utility of aqueous urea, doped inner- and outer-sphere relaxation agents, as an adjustable two-component model system. MATERIALS AND METHODS: T2 was measured from 12 molal urea mixtures at pH 7.0 with varying amounts of the MnCl2 and FeO(1.44) (Feridex, Berlex Inc, Montville, NJ). RESULTS: Bi-exponential relaxation was observed, with rates that were bilinearly related to [MnCl2] and [FeO(1.44)]. FeO(1.44) had comparable relaxivities on both urea and water, while MnCl2 relaxivity was > 15x larger for water than for urea. CONCLUSION: Aqueous urea, doped with inner- and outer-sphere contrast agents, is a two-compartment model system, which can be exhibit a wide range of different T2s and signal fractions.