Fig. 8. In vivo amine-water proton exchange (APEX) signals measured with CEST and CESL.

CEST and CESL MRI of rat brains were measured with RF offsets of ±2.5 ppm and ω₁ of 500 Hz. (A–B) Data of a single pixel (inset in A) were examined with different flip angle error λ (=1 for CEST and 0 for CESL). The CEST signal intensity at −2.5 ppm and MTR_asym (black lines) oscillate for short t_irrad. Similar to the simulation and phantom studies, the in vivo results with flip angle error λ = 0.3 are already close to those of ideal spin-lock with λ = 0 and have higher sensitivity than saturation transfer with λ = 1. (C) In ischemic rats, the MTR_asym and SLR_asym maps with t_irrad = 0.15 s show similar ischemic contrast, but the latter has about 20% higher sensitivity. While SLR_asym maps are calculated from the difference of two normalized maps of S/S₀ at ± 2.5 ppm, this ischemic contrast mostly originates from the APEX-weighted signal at 2.5 ppm. Note the difference gray scales of the maps. (D) The averaged MTR_asym and SLR_asym (n = 4) as a function of t_irrad were obtained from the 2×2 mm² ROIs in the ischemic ipsilateral (blue) and normal contralateral sides (red box). The CESL SLR_asym signal (solid) has higher sensitivity and higher ischemic contrast (red vs. blue) than CEST MTR_asym (dashed).