Figure 1.

(A) Example of fitting the Z-spectrum (blue line) consisting of two contributions (R_guan and $Z_{\mathit{background}}^{ss}$) simulated using Eqs. 7,8,10–11. The corresponding $Z_{\mathit{guan}}^{ss}$ calculated from the R_guan using Eq. 7 with R1=0.5 s⁻¹ was also plotted (black line). In the fitting, $R_{\mathit{guan}}^{\mathit{max}}$= 0.2 s⁻¹, w = 0.250 ppm, and ${\mathrm{\Delta }}_{\text{guan}}=$ 1.95 ppm were used for the Lorentzian line-shape (R_guan), while C0 =0.55, C1=-0.2 were applied for the broad background signal ( $Z_{\mathit{background}}^{ss}$). Other polynomial terms were assumed to be zero in $Z_{\mathit{background}}^{ss}$). ΔZ is the observed pure guanidinium CEST signal. (B) One typical Z-spectrum recorded in WT mouse with 1 μT saturation power and two-second saturation length (Open Square) was plotted together with the $Z_{\mathit{background}}^{ss}$ fitting using several polynomial functions with different orders (solid lines) defined by Eq. 11.