Figure 6.

Theoretical analyses of the dependences of APEXw and APTw on exchange rate (k) (A), labile proton concentration (B), R₂ (C), and R₁ (D). The labile protons were assumed to have a resonance frequency of 3.0 ppm (amine) or 3.5 ppm downfield from water (amide). In all panels, the parameter values were k = 8000 s⁻¹ for amine and k = 30 s⁻¹ for amide, p_b = 0.003, R₂ = 25 s⁻¹, and R₁ = 0.5 s⁻¹, except for the parameter on the x axis. The shaded regions denote the ranges of the amide (10 – 30 s⁻¹) and amine proton exchange rates (700 – 10000 s⁻¹) (1,31). The red and black arrows denote the directions of APEXw and APTw changes, respectively, with decreasing pH under ischemia. APEXw refers to MTR_asym at 2.5 ppm measured with the APEX-weighted spin-locking pulse sequence, and APTw refers to MTR_asym at 3.5 ppm measured with the APT-weighted CEST technique. The thin gray curves indicate the contribution of amine–water proton exchange effect to APTw signals.