PBS phantom and three protamine phantoms (denoted in red in A) with pH = 6.1, 6.4, and 6.7 were used for pH sensitivity. A SE-EPI image is used as S0 (A). MTRasym maps of amide-CEST at 3.7 ppm (B) and the guanidyl-CEST at 2 ppm (C), with 38 Hz and 21 Hz saturation pulse power, respectively, show opposite pH-contrasts between different protamine phantoms. (D) With pHenh, the contrast between protamine phantoms with different pH phantoms is larger than both the MTRasym maps at 3.7 and 2 ppm.