Transient decay of longitudinal magnetization in heterogeneous spin systems under selective saturation

The transient behavior of the longitudinal magnetization of the mobile protons in aqueous heterogeneous materials is investigated both theoretically and experimentally when selective saturation is applied by off resonance, RF irradiation to the homogeneously broadened, immobile protons which are coupled to the mobile protons through cross relaxation. Analytical solution of this problem is obtained by a generalization of H. C. Torrey's solution to the Bloch equations. Progressive (but indirect) saturation of the magnetization of the mobile protons under this RF irradiation is dynamically monitored using very-small-tip-angle sampling pulses. The apparent relaxation rate of heat-denatured egg albumin was measured as a function of frequency offset of the saturating RF with different amplitudes using a new dispersion method modified from a broadband technique recently invented for the rapid acquisition of a cross-relaxation z spectrum.