Fig. 3. Simulated and experimental CEST versus CESL contrast.

(A–D) MTR_asym and SLR_asym, as a function of irradiation duration t_irrad, were simulated with CEAPA solutions for selected ω₁ values. Two different R₂ values were used: 3 s⁻¹ (A–B) and 25 s⁻¹ (C–D). SLR_asym (solid lines in A and C) is always larger than MTR_asym (dashed lines), but the difference is more significant at higher ω₁ and shorter t_irrad values. The ratio of SLR_asym to MTR_asym (B and D) ranges between 1/cos²θ at very short t_irrad and 1/cosθ at long t_irrad, approaching the steady state. Other parameters used were R₁ = 0.35 s⁻¹, P_S = 0.001, and δ = 400 Hz. (E–F) MTR_asym (open symbols) and SLR_asym (filled symbols) of 50 mM myo-inositol (Ins) in PBS were measured at 1 ppm with ω₁ = 100, 200, and 400 Hz at varying irradiation durations (E), and their ratios were obtained (F). For the highest ω₁ of 400 Hz, we only acquired t_irrad up to 2 s to minimize RF heating in the samples. MTR_asym and SLR_asym peaks shift to shorter t_irrad at higher ω₁. The ratio between SLR_asym and MTR_asym increases with ω₁ and is larger at shorter irradiation times (F). To verify the experimental data with CEAPA models, on-resonance spin-locking experiments as a function of ω₁ were performed, and chemical exchange parameters were obtained (see texts). Simulated MTR_asym and SLR_asym data (lines in E) using these chemical exchange parameters match well with the experimental data (symbols in E). t_irad: off-resonance irradiation time.