Figure 5.

Logarithmic plots of the fitting parameters for the Lorentzians in Figure 4 as a function of B_1,rms for both discrete and CW saturation pulses. (A) Total GP depolarization rate measured for four different saturation frequencies: Δ_TP, f_TP, f_RBC, Δ_RBC. For low B_1,rms the GP depolarization rate for both saturation pulse types increases in parallel, but diverges beyond ~10 μT, at which point the depolarization rate for the discrete pulses begin to quickly outpace those of the CW pulses, at both on- and off-resonance frequencies. (B) Low-power region of the plot in panel (A). (C) FWHM of the z-spectra lines associated with the TP and RBC compartments. As B_1,rms increases, both lines broaden and the compartment selectivity of the saturation decreases, first gradually and then more rapidly for B_1,rms > ~10 μT. In the high B_1,rms regime, discrete pulses, particularly for the TP fit, exhibit greater widths than their CW counterparts. The low FWHM for the discrete saturation with a B_1,rms of 27.5 μT is an artefact of the unreliable fitting of the disappearing RBC peak at high saturation power. (D) Relative contribution of the TP compartment to the total GP depolarization at four different saturation frequencies. For all saturation powers, almost the entire GP depolarization is associated with the exchange of saturated TP magnetization with the GP at both f_TP and Δ_TP. On the other hand, while the TP contribution at f_RBC and Δ_RBC is low at low power, it begins to rapidly increase, and eventually dominate, at higher RF saturation power.