Figure 4. Quantitative proton exchange parameter maps of ammonium chloride (NH₄Cl) phantoms and a healthy human volunteer obtained using dictionary-free CEST-MRF..⁶¹.

a. CEST phantom validation experiments. MTR_asym (2.5 ppm) maps with RF saturation powers of 1, 1.5, 2, 2.5, and 3 μT. A phantom with four compartments: (1) pH 4.5, 0.5M NH₄Cl + 1% agarose + PBS, (2) pH 5.0, 0.5 M NH₄Cl + 1% agarose + PBS, (3) pH 4.6, 1 M NH₄Cl +1% agarose + PBS, and (4) pH 7.0, 1% agarose + PBS. RF saturation power dependencies of the direct water saturation, semisolid MTC, and CEST signals can be seen clearly in the MTR_asym (2.5 ppm) maps. b. CEST exchange rate (k_sw) and concentration (M0_s) maps of the phantom from the dictionary-free CEST-MRF. c. Quantitative semisolid MT exchange rate (k_mw) and concentration (M0_m), and amide proton exchange rate (k_sw) and concentration (M0_s) maps of a healthy volunteer human brain. Reproduced and modified with permission from Heo et al. Neuroimage. 2019;189:202-213.⁶¹