Optimizing saturation time for maximum CEST contrast

To determine the optimal saturation time (T_sat) that maximizes the CEST contrast between a reference and a target signal, such as normal versus pathological tissues or pre- versus post-contrast conditions.

The optimal T_sat (T_{sat, opt}) was determined from T₁ and a single measurement of the normalized saturated signal (S_sat/S₀). We evaluated the analytically derived T_{sat, opt} using simulation, phantom, and in vivo experiments.

Theoretical analysis showed that the S_sat/S₀ measured at T_{sat, opt} is always higher than 0.37. When the repetition time (TR) is fixed or is very long (i.e., >5 T₁), the T_{sat, opt} is independent of TR and can be derived from T₁ and S_sat/S₀ measured at any T_sat. When the steady state S_sat/S₀ is 0.5 or 1/3, the T_{sat, opt} equals to T₁ or T₁/2, respectively. The analytically derived T_{sat, opt} were validated by simulation, phantom and in vivo experiment results, and can be extended to include multiple CEST and non-CEST effects (excluding changes in T₁). The relative error of the T_sat,opt is <10-15% for typical CEST contrasts of <5%. For very large contrast (e.g., >10% of S₀), more accurate estimation of T_{sat, opt} can be obtained from the averaged S_sat/S₀ of the reference and target signals.

T_{sat, opt} can be determined from T₁ and a single measurement of the saturated signal. This approach simplifies optimization of T_sat in CEST experiments, especially when using relatively high saturation powers or resonance frequencies close to water.