Approximated analytical characterization of the steady-state chemical exchange saturation transfer (CEST) signals

CEST MRI can indirectly detect low-concentrated molecules via their proton exchange with the bulk water and is widely measured by a sensitivity index, the asymmetry of magnetization transfer ratio (MTR_asym ). Because CEST applications are often limited by their low sensitivity or specificity, it is important to characterize MTR_asym analytically to optimize its sensitivity or specifity.

Approximated analytical solutions of the MTR_asym spectrum were derived based on a 2-pool chemical exchange model for slow-to-intermediate exchanges. The optimal saturation pulse power for maximizing the MTR_asym or tuning MTR_asym to a specific exchange rate and the peak position and linewidth of a MTR_asym spectrum were also derived. These approximated analytical solutions were compared with the solutions from the Bloch-McConnell equations using computer simulations.

The approximated analytical solutions of the MTR_asym spectra, the optimizing parameters, and the peak and linewidth of MTR_asym matched well with the solutions of Bloch-McConnell equations in the slow or slow-to-intermediate exchange regimes.

These approximate analytical solutions can provide insights to the understanding of CEST signal property and help the optimization of saturation parameters and the interpretation of CEST data.