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

Abstract

Purpose: 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 (MTRasym). Because CEST applications are often limited by their low sensitivity or specificity, it is important to characterize MTRasym analytically to optimize its sensitivity or specifity. Methods: Approximated analytical solutions of the MTRasym spectrum were derived based on a 2‐pool chemical exchange model for slow‐to‐intermediate exchanges. The optimal saturation pulse power for maximizing the MTRasym or tuning MTRasym to a specific exchange rate and the peak position and linewidth of a MTRasym spectrum were also derived. These approximated analytical solutions were compared with the solutions from the Bloch‐McConnell equations using computer simulations. Results: The approximated analytical solutions of the MTRasym spectra, the optimizing parameters, and the peak and linewidth of MTRasym matched well with the solutions of Bloch‐McConnell equations in the slow or slow‐to‐intermediate exchange regimes. Conclusion: 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. © 2019 International Society for Magnetic Resonance