We have optimized the relevant experimental parameters for all four protocols shown in Figure . One can see that using off-resonant excitation with a small value one can achieve higher Δ values, see Figure . Although the theoretical treatment 𝜀 suggests that the efficiency of the scheme should be the same or even higher for resonant excitation ( = 0); this experiment Δ is more difficult to optimize, in particular, when chemical exchange is constantly going on, as is the case in SABRE. The enhancements obtained by these two methods are moderate, for resonant excitation and for off-(?) resonance 𝜀 ≈ 30 𝜀 ≈ 150 excitation, for the experimental conditions used here. We attribute this to efficient -conversion, rendering small. 𝑆 𝑇 0 𝛿𝑃 Since this population imbalance is the source of non-thermal spin order, the signal enhancement factors become low. A simple way to re-introduce the population imbalance is to 𝛿𝑃 exploit the difference in populations between the and |𝑇 0 〉 |𝑇 ± 〉 states. This can be done in different ways . Here we investigate the simple method of using a single 90 pulse on the proton channel. In this situation, the state populations change as follows (we assume that before applying the pulse 𝑃 𝑇 + = 𝑃 𝑇 - ):