Figure 2.

(A) Excitation profiles for FLEX labeling pulses applied at several off-resonance frequencies (blue, green, magenta curves) and on-resonance (red curve). The selectivity profiles of the off-resonance pulse pairs vary as a function of pulse duration and, in order to null the water signal, the different pulse durations have different offset frequencies (o1). These different offset frequencies lead to different efficiency of excitation for the exchangeable protons, e.g. the imino protons indicated here. The use of a short non-selective excitation pulse (red line) is shown to give much higher λ, but also excites water, which may cause complications (ignored in this plot). (B) On-resonance labeling efficiency for different pulse durations and exchange rates. Example slices taken through the surface plot are projected onto the vertical planes (i.e., curves for k_sw = 30 and 1000 s⁻¹ shown on the labeling efficiency/pulse duration plane and curves for pulse duration = 0.02, 0.1, and 0.2 ms shown in blue, green, and magenta respectively plotted on the labeling efficiency/exchange rate plane).